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'[EE] Soldering Thermocouple Eire?'
2006\06\15@015910 by Rich Graziano

picon face
There is, IMHO, some real value to this list that makes it somewhat unique. The level of academic vigilance is excellent, and that is a wonderful thing, especially for the junior members.  

I was intending only to discuss the affects of Sn-Pb extrinsicity, I did not expect to discuss the physics of thermocouples, seeback effect or peltier phenomenon.  So I will not. I may have been a little loose or under scientific in my language.  But all the above mentioned phenomenon are actually related.  

First, recognize that if you heat one end of any metal and cool the other end, electrons will migrate from the hot end to the cold end.  A potential difference (Delta V) will develop across the conductor as a result of the temperature difference (Delta T).  That potential is referred to as the seeback effect and is usually represented by capital S, where S = ds/dT.  Suppose we choose a conductor, say aluminum, for example.  If you connect Al conductors (wires) at each end and you measure across the ends of that conductor which is H on one end and C on the other, you will not expect a voltage because there are no dissimilar junctions.  However, if you connect, say Ni or Cu or Cr to the ends you will be able to measure the net voltage difference because the Ni, Cu, or Cr or whatever will have a different Seeback coefficient.  The voltage across the two points, say a and b, may be expressed mathematically simply as V(ab) = Integral of S(ab) dT.  This is pretty simple. The volta!
ge, moreover, will not be linear with respect to temperature, as becomes apparent.   However, the classical physics model fails to fully explain the electron migration.  There are scattering consideration and arguments in QM also.  Things do become much less trivial here.

The bottom line is this:   without the junction of dissimilar metals having different Seeback coefficients no potential difference will be realized.  What you do measure is across the junction.  I will not carry this out but anyone who has an interest in a mathematical analysis or wants to talk about phonon scattering and other principles of thermocouple effect, including applications, may talk to me off list because the amount of text will exceed that allowable limit for the list.  

Also, If you remember your physics courses, you will have discussed such things as mean free path and mean free velocity of electrons, and collisions with lattice elements when you studied electrical resistance.  This is also germane to understanding thermocouples.  The deeper you dig the more gold you will find!

Regards
Rich

2006\06\15@071506 by olin piclist

face picon face
Rich Graziano wrote:
> The bottom line is this:   without the junction of dissimilar metals
> having different Seeback coefficients no potential difference will be
> realized.

Again, no.  The potential difference caused by the temperature gradient in
the material is always there.  If you want to extract power from that
potential back at a fixed temperature, you need a return path.  If that
return path is made of the same material, then it's potential back to the
fixed temperature will cancell the potential for the other leg.  Hence a
different material is used that produces a different potential at the same
temperature difference.

However the potential difference in each single leg is there and can be
proven to exist in isolation.  For example, the potential can be used to
deflect or regulate the flow of electrons in a vaccuum.  This also provides
an absolute measure of the effect, whereas a thermocouple can only measure
the relative effect of two materials against each other.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\06\15@101859 by Gerhard Fiedler

picon face
Olin Lathrop wrote:

> Rich Graziano wrote:
>> The bottom line is this:   without the junction of dissimilar metals
>> having different Seeback coefficients no potential difference will be
>> realized.
>
> Again, no.  The potential difference caused by the temperature gradient in
> the material is always there.  

You probably can create a capacitor that has its both platters connected;
all (platters and connection) of the same material, and a dielectric that's
a good temperature isolator. Then you heat up one side of the capacitor,
and when it's hot, open the connection between the platters. There should
be a voltage across that cap that you can measure when the hot platter has
cooled down (so that you can connect your measurement wires across the cap
without creating secondary thermoelements).

Not sure how realistic that experiment is, though :)

Gerhard

2006\06\15@113648 by Rich Graziano

picon face
Thank you for the reply.  It is interesting, I am sure, to others as well as
myself.  The Temperature gradient, as I stated, causes a potential across
the conductor which is hot at one end and cold at the other; that is true.
But you cannot measure it without a dissimilar junction.  Look at the
example I gave of the aluminum rod.  If you heat one end and cool the other
you will establish a temperature gradient.  If you then connect a meter
across the hot end and the cold end with aluminum wire you will be unable to
measure any potential difference because the Seebeck coefficients are the
same.

Yes, you are correct about the temperature gradient and its effect.  It is a
well known principle that the temperature gradient causes a potential
difference across the crystalline material; which expressed by the
differential equation I included. That potential difference is because of
the migration of electrons toward the cold end, which is introduced by the
delta T.

I think we may be saying the same thing in general. You appear to be
defining the origin of the potential difference and I am referring to the
measurement of that potential.  I am not inclined to think that you believe
you can realize a thermocouple effect without a dissimilar junction.   If
you do, I would be interested in knowing why you believe that because that
would obviate the role of the Seebeck coefficient.  The Seebeck phenomenon,
the thermocouple phenomenon and the peltier phenomenon are all related
phenomena. These phenomena apply to semiconductors as well.  It is the basis
for the Thermoelectric Cooling/Heating devices.

Thank you for the opportunity to discuss this matter in an academic tone.
These ideas may be of interest to others on the list.  These discussions, I
believe, contribute to the unique power of this list.  Personally, I find
this list to be outstanding.
Rich


{Original Message removed}

2006\06\15@120141 by Wouter van Ooijen

face picon face
> Thank you for the reply.  It is interesting, I am sure, to
> others as well as
> myself.  The Temperature gradient, as I stated, causes a
> potential across
> the conductor which is hot at one end and cold at the other;
> that is true.
> But you cannot measure it without a dissimilar junction.

can't you measure it using electrostatic effects?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\06\15@123259 by olin piclist

face picon face
Rich Graziano wrote:
> The Temperature gradient, as I stated, causes a
> potential across the conductor which is hot at one end and cold at the
> other; that is true. But you cannot measure it without a dissimilar
> junction.

Not true.  There have already been two examples posted how to do exactly
that.

I mentioned that the potential could be used to deflect an electron beam in
the same post you are replying to.  Gerhard mentioned charging up a
capacitor with the potential, disconnecting the source, bringing both plates
to the same temperature, then measuring with ordinary means.  Neither method
is easy to do on your desk, but both look theoretically valid to me and
should be workable with the proper laboratory setup.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\06\15@123549 by olin piclist

face picon face
Wouter van Ooijen wrote:
>> But you cannot measure it without a dissimilar junction.
>
> can't you measure it using electrostatic effects?

Theoretically yes.  That would now be a third method mentioned here of
measuring the potential without a junction of dissimilar metals required.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\06\15@140431 by Vasile Surducan

face picon face
On 6/15/06, Olin Lathrop <spam_OUTolin_piclistTakeThisOuTspamembedinc.com> wrote:
> Rich Graziano wrote:
> > The bottom line is this:   without the junction of dissimilar metals
> > having different Seeback coefficients no potential difference will be
> > realized.
>
> Again, no.  The potential difference caused by the temperature gradient in
> the material is always there.


 This is fascinating. I'll not come with formulas and explanation
because my lack of english will do it imposible (and really have no
time for this). But a simple example will probably be better.
On old days (when I lived, being there) when there wasn't available
any voltage to current transducers, old automation guys used a pair of
wire with the same nature of the termocouple, but having higher cross
section. IE for an iron-constantan termocouple, the wires carrying
signal (on long distance) where made by iron and constantan.
Amaizing, if you could believed that. So Olin's theory is academic but
not practical. There is no potential difference at the thermocouple
ends even the thermocouple  terminals are both near a huge gas flame
oven at more than 50C.

greetings,
Vasile

2006\06\15@180302 by John Ferrell

face picon face
I must not understand the question.
In less complicated times we measured the junction with a balanced bridge
(L&N K2 Potentiometer/galvanometer and made our connections in melting ice
baths.
We made our thermocouples from wire by melting a bead on the tip with an
acetylene torch. They were not so accurate as to escape individual
calibration in the intended range.

John Ferrell    W8CCW
"My Competition is not my enemy"
http://DixieNC.US

{Original Message removed}

2006\06\15@215226 by Rich Graziano

picon face
Hi Wouter:

You are correct, Wouter.  It is true that an electric field will be produced
because electrons are in motion along the aluminum rod in the hypothetical
experiment I posed.

Therefore, you are correct; you can measure it with sensitive electrometer.
The very high input impedance of the MPF102 makes it ideal for a gross
"go/no go" detector.  Just put an antenna on the gate and a LED in one leg
and battery power it.  That will detect it.  But there are much more
sophisticated devices that actually perform a quantitative measurement.  It
just dosen't see practical for a thermocouple application.  Besides, the TC
junction is what is placed in the thermal energy.  I have actually taped the
iron and the constantan separately to a nickel substrate. Since the error
canceled I was able to make a measurement of the substrate temperature
profile.
Regards
Rich


{Original Message removed}

2006\06\15@223221 by Rich Graziano

picon face
Thank you again, my friend.  Yes, of course. I am saying the same thing
about the causes of the potential difference. I explained that in the
hypothetical experiment I gave with the aluminum rod which was heated at one
end and cooled at the other.  It is well known the temperature gradient
across the conductor is what causes the potential difference across the
conductor.  The simple mathematical expression for the Seebeck effect is S =
dV/dT, although it may also be given by more explicit relationships.

For example, the Average energy per electron in a metal of some density of
state, g(E) is an exponential function of the Fermi energy a zero K.  That
exact relationship, which I cannot construct here, leads to the conclusion
that energy of the electron is greater at the hot end.  Which is what we
would expect.  We also see that the Fermi energy contributes largely to the
average energy of the electron.  The potential difference increases to where
further diffusion approaches a limit.

My original point was that it is that iron and the constantan that are
connected to the meter or DAS that measures the millivolt output that
obtains from the temperature gradient.  As I stated earlier, only one metal
is required for a potential difference to appear across the temperature
gradient, but it could not be measured without introducing conductors with
different Seebeck coefficients.  Wouter, then reminded me, correctly so,
that an electrometer could be used to measure the field.

So I am not disagreeing with you.  I am simply addressing a different
question; that of acquiring the thermocouple measurement.  Thank you again.
I thoroughly enjoy your very well informed perspective,

Regards
Rich


{Original Message removed}

2006\06\15@231628 by Rich Graziano

picon face
I am somewhat skeptical about the capacitor charge technique, for several
reasons.  But now I am curious as to what you are actually measuring when
you heat one end of a conductor and cool or hold constant the other end, if
not the current or field of the experimental rod.  Are we still referring to
a practical measurement of some thermal environment as a thermocouple,
thermistor, RTD, silicon junction, etc. would lend itself to?  It seems that
I am still thinking thermocouple measurement but the discussion has moved to
temperature gradient and measurement of its effects apart from the
application of temperature measurement.  I am a bit puzzled by that but it
is an interesting academic discussion.

I thought I had explained the temperature gradient and its relationship to
the potential difference and the reason why dissimilar metals are used, in
the practical sense. Perhaps, I was not very clear.  I apologize if that is
the case.  There are several persons here who have a deeper understanding of
the physics, perhaps much more that I, who may have something to say that I
am not understanding.  I apologize for that.

But if there is some debate as to the ability to measure the potential
difference across the hypothetical conductor that is characterized by a
temperature gradient, then the question that comes to mind is whether or not
we are still speaking of some practical temperature measurement or some
academic exercise about a lab experiment?

If we are not referring to a practical measurement but something in a lab,
then there is the possibility of measuring the field before the diffusion is
inhibited, and several other viable techniques also come to mind.  But that
is still related to the hypothetical experiment I posed with the Aluminum
Rod. I remain, however, skeptical about the capacitor charge technique for
several reasons.

I hope I have not created some misunderstanding as to inability to connect
to a meter or measuring device, as I had stated it, if one connects to the
meter or measuring device by wires having the same Seebeck coefficient as
the hypothetical aluminum rod.  If anyone can prove that to be incorrect, I
am very anxious to know it because it is still such a widely accepted
principle.  I look forward to such an explanation.
Regards
Rich







{Original Message removed}

2006\06\15@232144 by Rich Graziano

picon face
I also remember those times, John.  But it is refreshing to discuss the
physics of it.  I always learn something from this list.  I usually learn
much more from my dumb mistakes that anything else.  And, there are people
on this list that are much smarter than I so I look forward to being
corrected.
Rich


{Original Message removed}

2006\06\16@003747 by Vasile Surducan

face picon face
On 6/16/06, Rich Graziano <.....rgrazia1KILLspamspam@spam@rochester.rr.com> wrote:
> Hi Wouter:
>
> You are correct, Wouter.  It is true that an electric field will be produced
> because electrons are in motion along the aluminum rod in the hypothetical
> experiment I posed.
>
> Therefore, you are correct; you can measure it with sensitive electrometer.
> The very high input impedance of the MPF102 makes it ideal for a gross
> "go/no go" detector.  Just put an antenna on the gate and a LED in one leg
> and battery power it.  That will detect it.  But there are much more
> sophisticated devices that actually perform a quantitative measurement.  It
> just dosen't see practical for a thermocouple application.


 Because it really does not exist. As the hypotetical problem itself.
The theory of measuring temperature with thermocoples is clear. Even
you're compensating the cold junction with various methodes, measuring
temperature accurate with thermocouple is possible only for a short
time range because of the thermocouple eaging. The errors made by
parasytical  two thermocouples made on the cold sides could be
neglected if is used a trully methode of making contacts (no
soldering, no silver, but mechanical contact or welding).

greetings,
Vasile

2006\06\16@072605 by Gerhard Fiedler

picon face
Rich Graziano wrote:

> I am somewhat skeptical about the capacitor charge technique, for several
> reasons.  

Since I brought this one up, I am interested in your reasons. I am too,
more in a generic way though :)


> But now I am curious as to what you are actually measuring when you heat
> one end of a conductor and cool or hold constant the other end, if not
> the current or field of the experimental rod.

I thought that we were talking about the voltage generated by the Seebeck
effect. A thermocouple is one way to measure (indirectly, because
differentially) this effect. The three other methods are other ways to more
directly (but less suited for most practical situations) measure this same
effect.


> Are we still referring to a practical measurement of some thermal
> environment as a thermocouple, thermistor, RTD, silicon junction, etc.
> would lend itself to?  

No. We are talking about the principle, about what generates the effect we
measure with a thermocouple.

> It seems that I am still thinking thermocouple measurement but the
> discussion has moved to temperature gradient and measurement of its
> effects apart from the application of temperature measurement.  I am a
> bit puzzled by that but it is an interesting academic discussion.

I'm not sure why you are puzzled. The history of this thread and its
predecessor ("Soldering thermocouple wire?" started by Picdude) makes it
pretty clear how this went. A few reminders:

Citing a message from Olin:
> Rich Graziano wrote:
>> Yes they are related, but the EMF produced by the
>> junction is a thermocouple effect.
>
> Once again, it's not the junction that produces the EMF.

Several people wrongly stated that the junction generates the voltage we
are measuring with a thermocouple. It doesn't; Olin corrected that. A
junction may be necessary to measure the Seebeck effect in a practical way,
but the junction itself doesn't seem to generate anything of value.

Then you wrote in the very first message of this thread:

> The bottom line is this: without the junction
> of dissimilar metals having different Seeback
> coefficients no potential difference will be
> realized.

I don't know what exactly you meant with "realized" (ever since we don't
know anymore what "is" means I'm unsure about even simple things :) , but
it seems that this phrase pretty much contradicts the findings of Seebeck
and everybody who confirmed them. It doesn't seem that a junction is
necessary for the effect to happen. Several methods have been proposed (as
thought experiments, so far) that don't include a junction. I would be
surprised if we were the first ones to propose these things, but I don't
know how to find out whether such experiments have actually been conducted.


> I hope I have not created some misunderstanding as to inability to
> connect to a meter or measuring device, as I had stated it, if one
> connects to the meter or measuring device by wires having the same
> Seebeck coefficient as the hypothetical aluminum rod.  

It does seem to be impossible to create a steady current created by the
voltage that has been caused by the Seebeck effect without introducing a
second material and therefore a junction -- because of what you described:
any attempt to connect the two ends will cancel the different effects.

But an electrostatic meter or a cathode ray beam is a "measuring device"
and doesn't need a steady current, so it should be possible to measure that
effect your proposed aluminum rod. And with a capacitor construction, you
keep the voltage stored that was created at the time when a temperature
difference existed until a time when there is no temperature difference
anymore and therefore you can connect other materials to measure in a more
traditional way without introducing Seebeck effects.


> If anyone can prove that to be incorrect, I am very anxious to know it
> because it is still such a widely accepted principle.  

I'm not sure what you mean by "prove" (I think the thought experiments
sound plausible, but of course they are not "proof" until executed), or
"that" (possibly the assertion that a second material, a junction, is
necessary to be able to measure anything?), or "it" (the conception that a
junction is necessary?). If I was correct in my interpretations, then that
is not a widely accepted principle, it is more a widely employed technique
(the use of a junction). Nobody said it wasn't, or that it wasn't a useful
one.

Gerhard

2006\06\16@072619 by olin piclist

face picon face
Rich Graziano wrote:
> I am somewhat skeptical about the capacitor charge technique, for
> several reasons.

You said this twice but never listed any.  While it might be difficult to
actually perform the experiment given the small voltages, necessity for very
low leakage, the logistics of heating and cooling while not allowing the
charge to leak away, and possible changes to dielectric properties over
temperature, I see no theoretical objection to Gerhard's idea.

> But now I am curious as to what you are actually
> measuring when you heat one end of a conductor and cool or hold
> constant the other end, if not the current or field of the experimental
> rod.

The electric field generated by the temperature gradient in the rod.

> Are we still referring to a practical measurement of some thermal
> environment as a thermocouple, thermistor, RTD, silicon junction, etc.
> would lend itself to?

I was objecting to your statements just because they were incorrect.  This
has nothing to do with whether the technology is to be used for practical
temperature measurement or is considered more from a theoretical standpoint.
There is enough general confusion out there, so I try to speak up when
someone is playing fast and loose with the physics.  Statements like "the
EMF in a thermocouple is generated at the junction" are just plain wrong and
need to be squashed before anyone mislearns some physics.  I would not have
objected to "a junction of dissimilar metals is required in a traditional
wired thermocouple to present the temperature-dependent voltage to the
electronic measuring circuit", but that's not what you said.

> But if there is some debate as to the ability to measure the potential
> difference across the hypothetical conductor that is characterized by a
> temperature gradient, then the question that comes to mind is whether
> or not we are still speaking of some practical temperature measurement
> or some academic exercise about a lab experiment?

What's the difference, since the same physics applies to both?  Today's
laboratory experiments are tomorrows practical applications.  Such a general
distinction is just more vague handwaving without specific examples.

> I remain, however, skeptical about the
> capacitor charge technique for several reasons.

Although I note you list none.

> I hope I have not created some misunderstanding as to inability to
> connect to a meter or measuring device, as I had stated it, if one
> connects to the meter or measuring device by wires having the same
> Seebeck coefficient as the hypothetical aluminum rod.

This is the sort of inaccurate use of words that leads to confusion and
therefore discussions like this.  It appears now that you maybe do
understand how thermocouples work, but vague physics and sloppy wording
resulted in outright wrong statements.  I think I understand what you are
trying to say above, but taken directly it makes little sense.  The Seebeck
coefficient of the wires has nothing to do with the ability or not to
connect them to a rod with a temperature gradient.  It does have a lot to do
with the ability to measure a voltage resulting from the temperature
difference, and I believe that's what you're trying to say, but note that
you didn't say anything like that.  Neatness, clarity, and precision are
important.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\06\16@115451 by Rich Graziano

picon face
Hi Gerhard:
Thank you for the opportunity to explain my skepticism.  I certainly did not
intend to impugn you in any way and I hope you did not take it that way.  My
apologies if it came across that way.  I did say that I was skeptical of the
technique, I did not say that it was impossible.  Given the charge rate time
of the capacitor and the rate of change in electron migration in the rod,
the unstable dielectric circumstances and other factors, the mathematics
would have to reconcile these and result in some mathematical expression
that predicts, or at least approximates the potential across the rod versus
temperature, within reason.

I am a bit shy to use the term temperature, now, because it is really energy
that effects the migration of the electrons.  I have been made aware that my
inaccurate expression is troublesome.  I appreciate that criticism.  The
Seebeck coefficient is really a physical property that depends on
temperature.  The simple concept can be expressed as S = S(T), although it
is in the general.  The actual expression cannot be expressed here but it is
quite simple.

To characterize the charge on the capacitor, even if the dielectric is
stable, you are concerned with migration of electrons from the H to the C,
where the electrons will collect to establish the potential difference
across the rod.

To validate your measurement you will need to compare calculated values with
measured values.   This also refers to Wouter's idea about the electrometer
because the migration of the electrons is characterized by the average
energy per electron as it relates to the density of states of the given
metal; g(E).

This is actually given in terms of E(T) which demonstrates that the
Fermi-Dirac distribution extends to considerably higher energies as
"temperature" increases.  The migration therefore is a nonlinear function
that may produce only a small expansion of the electric field because the
process is typically slow.  The electric field produced will be weak.  I
apologize again if this expanation is convoluted.

I believe Olin is correct in providing some level of academic vigilance on
the statements of basic physics presented on the List.  In the past ten
years or so I have had little to say on the list but I do enjoy the
discussions.  However, I found this topic to be of particular interest
because I have encountered a paucity of understanding by engineers as to the
actual physics of the Seebeck phenomena, especially at the quantum level.

I am not sure, but I do not think anyone has disagreed with the simplified
analogy of the aluminum rod because it is more or less a universal classroom
analogy.   Most, if not all, engineers have at one time or another studied
electrical resistance in conductors and the mean free path of electrons and
the migration of electrons.  That theoretical edifice applies here.  The
mathematics to explain it, while a bit extensive, is reasonable enough; but
not the simple function expressions I gave here.

I am happy to be criticized.  I am happy that bogus as well as valid ideas
are scrutinized very carefully.  Olin is Right.

Regards
Rich






{Original Message removed}

2006\06\16@122803 by Rich Graziano

picon face
Thank you, Olin.

You are correct.  I spoke ambiguously about the junction.  My thinking was
more about the connection to a measurement system and not about physics at
that time.  But I became interested in the physics as questions were raised.
Whatever we think we know, we can be sure that someone knows more. :-)  So I
never claim to be the last authority on anything.  But I did believe that I
had some understanding of the physics involved with regards to Seebeck and
the properties of materials.

The more one explores materials the more one is certain that we are not
really certain.  But that is a major value of scientific inquiry. The study
of scattering as it relates to the energy levels and transition is exciting
and it can be directly applied to understanding the Seebeck phenomenon with
respect also to the Density of States of various materials and even to the
current density of the field.  Actually, the entire exploration can be
complex but very exciting.

My understanding of thermocouples is simply put that the Seebeck effect can
be utilized with two different metals where one such junction of the two
metals is held constant and the other is placed in some thermal environment
to be measured.  The voltage across each of the different metal components
depends uniquely on its own Seebeck coefficient.  The potential difference
then develops between these two circuit components (the wires) and is the
measure of the variable element that measures the temperature.

That voltage may be mathematically expressed as the integral of the Seebeck
difference; (Sa-Sb) dT, over the boundaries of integration, which is the
difference between the measured temperature and the reference temperature.
If this is incorrect or ambiguous I look forward to your correction.
Regards
Rich


{Original Message removed}

2006\06\16@134448 by Peter

picon face

On Thu, 15 Jun 2006, Olin Lathrop wrote:

> Rich Graziano wrote:
>> The Temperature gradient, as I stated, causes a
>> potential across the conductor which is hot at one end and cold at the
>> other; that is true. But you cannot measure it without a dissimilar
>> junction.
>
> Not true.  There have already been two examples posted how to do exactly
> that.
>
> I mentioned that the potential could be used to deflect an electron beam in
> the same post you are replying to.  Gerhard mentioned charging up a
> capacitor with the potential, disconnecting the source, bringing both plates
> to the same temperature, then measuring with ordinary means.  Neither method
> is easy to do on your desk, but both look theoretically valid to me and
> should be workable with the proper laboratory setup.

I do not see how the potential can be measured when the bar is free
standing in a vacuum. The only thing that can be measured is total
charge, since it is a conductor. The thermoelectrical potential appears
as a voltage only when a junction exists, as the potential must be
materialised somehow. With a free-standing bar the electrons would like
to diffuse somewhere but there is nowhere to, (if the thermal energy is
under the first excitation energy level), so they don't go anywhere.

Thermoelectrical potentials are always measured with reference to
something else (Platinum usually) afaik. The absolute values for the
thermoelectric potentials are calculated afaik, with good concordance
with measured values.

Peter

2006\06\16@135834 by Peter

picon face

I think that this thread is really about terminology. Potential and
voltage are one and the same in theory but in practice it is more
complicated. In the case of thermoelectric potentials the absolute t.
potential describes what the electrons would do if they could (the
definition of the word 'potential' is roughly 'it would do x if it would
be allowed to do it'). But by the time they get an opportunity to show
what they want to do other things interfere. So the absolute t.
potential is a calculated (absolute) value, which corresponds very well
with measured (relative) values in practice. The t. scale gives voltages
in mV/K at a specified temperature (more precise: table over
temperature), for a conductor X vs a reference electrode made of Y
(usually Pt). I don't know if this paper was quoted here, it has the
goods:

http://materials.usask.ca/samples/Thermoelectric-Seebeck.pdf

Peter

2006\06\16@140819 by Rich Graziano

picon face
Yes, Peter.  See my latest post. I explain why I say that.  Of course there
are laboratory methods that could be applied to measurement if that is what
your goal is.  I can contribute several others. But that was not what "I"
was talking about.  If you want to connect it to a meter, which is the
condition I posed, you cannot measure it with connects of the same seeback
coefficient.  This is not said with respect to the ability to determine the
measurement by some laboratory or esoteric methods, it was referring to
connecting to a meter of data acquisition system.  You must read my
clarification that is posted on the list so I do not have to write it again.
Regards
Rich
{Original Message removed}

2006\06\16@142111 by Rich Graziano

picon face
Thank you, Peter.  I think you are correct about the terminology, and as
Olin pointed out mine could have been better.  Please review my posts on how
the thermocouple measurement proceeds and how the potential of the "rod" is
obtained.  Also, review my reply to Gerhard. If there are errors in the
theory I will greatly appreciate your clarification.  I appreciate the
feedback it is good for me as well as others.

Regards
Rich


{Original Message removed}

2006\06\16@142526 by olin piclist

face picon face
Peter wrote:
> I do not see how the potential can be measured when the bar is free
> standing in a vacuum.

It is not free standing.  One end is held at a known or fixed voltage
relative to the electron beam electronics.  The voltage at the other end
will vary depending on the temperature difference between the two ends of
the bar.

> The thermoelectrical potential appears
> as a voltage only when a junction exists,

No, it does not.  That is the point.  The previous few posts on this topic
discussed this at length.


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2006\06\16@143216 by Rich Graziano

picon face
Thank you again.  I have not seen this particular information but my
understanding is consistent with it in general.  Recently I have had an
opportunity to work on developing an instrument that required an explanation
of these principles at a different level and in addition to the scattering
phenomenon at the quantum level.  The objective is a measurement technique
application in spectroscopy.  The Seebeck is only tangentially related but
the physics that gives rise to it is related to my project.  Also, as Vasile
said it would be good to present the math.  But this medium has no equation
editor.  I apologize for my convoluted expressions.  I should have thought
more about what was actually being addressed before I said anything.

Regards
Rich


{Original Message removed}

2006\06\16@152240 by Gerhard Fiedler

picon face

Peter wrote:

> Potential and voltage are one and the same in theory but in practice it
> is more complicated.

I'm not sure this is on firm ground. I'm no ace in physics terminology, but
I think "voltage" is a term for the value of an electric field potential,
both usually relative to something. If there is a voltage between two
points, there is a potential difference. If there is a potential
difference, there's a voltage. I think both in theory and in practice it is
as simple as that. (It may be technically challenging to measure the
voltage aka potential difference between two specific points, but that's
another matter.)

At least the paper you cite seems to use the terms voltage and potential in
this sense.


> In the case of thermoelectric potentials the absolute t. potential
> describes what the electrons would do if they could (the definition of
> the word 'potential' is roughly 'it would do x if it would be allowed to
> do it').

How do you get to this? I'm not sure there is a difference -- as far as the
potential goes -- between a thermoelectric potential (like between the two
ends of that obviously famous aluminum rod), a chemoelectric potential
(like between the two poles of a lead-acid battery), an electrostatic
potential (as when rubbing an isolator) or an electric potential created
through other means (electromagnetic fields etc). It's always an excess (or
a lack) of charges, compared to a referential.

Of course once you start to extract current, there are huge differences --
but then we're not anymore talking about potential alone, we're talking
about migration speed of charges, source resistance and other concepts.
Potential doesn't require any current. A current may destroy it, a current
may be needed to measure it, but that are different matters.


> But by the time they get an opportunity to show what they want to do
> other things interfere.

I guess with the "opportunity to show what they want to do" you mean
"current". And that has nothing to do with whether that's a potential or a
voltage.

Take the water pipe analogy (pressure=voltage/potential, flow=current). The
pressure may be there, without any water flowing. There are means to
measure pressure that need a small but ongoing flow, and there are others
that just need a relocation of some water particles, not an ongoing flow.
Both may be influenced in various degrees by the difficulty of providing
the necessary water particles fast enough, but that has not much to do with
the pressure itself, more with a specific way of measuring it (and of
course with the pressure inside the meter).


> I don't know if this paper was quoted here, it has the goods:
>
> http://materials.usask.ca/samples/Thermoelectric-Seebeck.pdf

This paper states very clearly that the Seebeck effect has nothing to do
with any junction of metals:

"A voltage is therefore developed between the hot and cold ends [of the
famous aluminum rod] with the hot end at positive potential. The potential
difference ÄV across a piece of metal due to a temperature difference ÄT is
called the Seebeck effect."

They speak very clearly about "a piece of metal" and not about two pieces
of different metals. They also use the term "potential difference" for ÄV,
and a few sentences below, they call the same ÄV a "voltage difference",
indicating that the authors (like AFAIR everybody else whose works I've
read) use the terms interchangeably.

Gerhard

2006\06\16@162241 by Peter

picon face


On Fri, 16 Jun 2006, Olin Lathrop wrote:

{Quote hidden}

2006\06\16@181327 by Rich Graziano

picon face
Hi Peter:
Did you read my reply to Gerhard?
----- Original Message -----
From: "Peter" <plpspamKILLspamactcom.co.il>
To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam.....mit.edu>
Sent: Friday, June 16, 2006 4:22 PM
Subject: Re: [EE] Soldering Thermocouple Eire?


{Quote hidden}

> --

2006\06\17@132443 by Peter

picon face

On Fri, 16 Jun 2006, Olin Lathrop wrote:

> Peter wrote:
>> I do not see how the potential can be measured when the bar is free
>> standing in a vacuum.
>
> It is not free standing.  One end is held at a known or fixed voltage
> relative to the electron beam electronics.  The voltage at the other end
> will vary depending on the temperature difference between the two ends of
> the bar.

But if it is held (potential wise) then there is at least one junction.
Is this what you mean ? Say, heat the tip of a grounded pin in vacuum
and see what happens to an electron beam that passes very near it ? But
there is already one junction on it, the grounded one. It could be made
of the same material as the pin, but there still is a junction there.

I think that it would be more interesting to have an unevenly heated pin
suspended and scanned not with an electron beam but with a vibrating
reed electrometer. I also think that there will be no measurable
potential *difference* although the potential should be there ;-)

Peter

2006\06\17@133641 by Peter

picon face

On Fri, 16 Jun 2006, Gerhard Fiedler wrote:

> Peter wrote:
>
>> Potential and voltage are one and the same in theory but in practice it
>> is more complicated.
>
> I'm not sure this is on firm ground. I'm no ace in physics terminology, but
> I think "voltage" is a term for the value of an electric field potential,

The definition of potential is 'the energy required to move a punctiform
charge unit from the point being measured to infinity'. All normally
measured voltages are *relative* differences of potential, not absolute.
In this sense a voltage is a potetnial *difference*, but if only a
potential exists then a voltage must not also exist (until someone
*measures* the *difference* between that object's potential and a
reference).

But the definition of the t. voltage is absolute, not relative. Its
measured version, is relative, no matter what (access to infinity is
somewhat difficult). So while the t. potential should be there, there is
no way to measure it as is, it must be measured with reference to
something else. This implies the need for at least *one* contact or
quasi-contact. Even the pin/bar proposed by Olin has one contact.

Peter

2006\06\17@133819 by Peter

picon face

On Fri, 16 Jun 2006, Rich Graziano wrote:

> Hi Peter:
> Did you read my reply to Gerhard?

Yes ... today. I see digests, please be patient with my answers.
Turnaround time is 24 hours + X.

Peter

2006\06\17@154511 by Gerhard Fiedler

picon face
Peter wrote:

>>> Potential and voltage are one and the same in theory but in practice it
>>> is more complicated.
>>
>> I'm not sure this is on firm ground. I'm no ace in physics terminology, but
>> I think "voltage" is a term for the value of an electric field potential,
>
> The definition of potential is 'the energy required to move a punctiform
> charge unit from the point being measured to infinity'. All normally
> measured voltages are *relative* differences of potential, not absolute.

I still don't see how you can make a difference between 'voltage' and
'potential' -- at least not any difference that would be relevant to the
Seebeck effect. (At the very least, you should be more careful with the
papers you cite in a given context: the one you cited uses 'potential
difference' and 'voltage difference' as synonyms :)

Every voltage is relative to something. Every potential is relative to
something. No difference here. (Like in a normal circuit: voltages are
usually given relative to ground. They represent potentials relative to the
generally not known -- in absolute terms -- ground potential.)

Absolute potential is relative to infinity, and has a voltage that is given
relative to infinity. Difficult to measure, as you state, but still
existent as concept. No difference either. (You could say, for example,
that a certain potential has 1 kV -- absolute, i.e. relative to infinity.
That 1 kV would then be its voltage, no?)

> In this sense a voltage is a potetnial *difference*,

A voltage is a difference just as a potential is a difference. Either must
be referenced to something (this 'something' includes infinity) to make any
sense.

> but if only a potential exists then a voltage must not also exist

Describe a situation where a potential exists but no voltage... For every
point that has a known potential, I can give you its voltage (relative to
infinity); just tell me its potential :)  You seem to not take into account
that the reference that is implicit in the concept of an absolute potential
(infinity) necessarily always exists, and is used as reference point for
expressing the value of this absolute potential (in volt).


> But the definition of the t. voltage is absolute, not relative.

Oops... Didn't you just base your argument regarding the difference between
the concepts of 'voltage' and 'potential' on the statement that potential
is absolute whereas voltage is relative? And here you introduce the concept
of an 'absolute voltage'?

This inconsistency left aside (as it is not really relevant for the
thermoelectric voltage)... is it really absolute? I thought it was
relative. The Seebeck effect seems to talk about the /difference/ of
potential that is caused by a /difference/ in temperature along a
(semi)conductor. I don't see anything absolute here.

Again citing from the paper you mentioned: "Seebeck effect: A temperature
difference between two points in a conductor or semiconductor results in a
voltage difference between these two points." And later: "A voltage is
therefore developed between the hot and cold ends with the hot end at
positive potential." Just in case my explanation is not scientific enough
:)

This is confirmed by the practical ways to measure that potential
difference (in a typical thermocouple setup): you measure a difference of
two Seebeck voltages, both a function of the temperature /difference/
between two points. Every thermocouple measurement needs to reference the
temperature difference you gain from the measured voltage to some absolute
reference (the so called cold junction temperature) -- and this can't be
measured with a thermocouple (alone), never ever. This is not because we
measure the difference of two Seebeck voltages; this is because the Seebeck
voltages themselves are voltage differences based on the temperature
difference.


> So while the t. potential should be there,

Where is it, as an absolute potential? As I see it, it is always,
necessarily, relative to another point -- at least what they call the
Seebeck effect. Of course, you can always reference both the temperature
and the potential of both ends of the rod under test to infinity (absolute
zero of both temperature and potential). But that doesn't seem to add much
value -- compared to looking at it as one point (with a temperature and a
potential) relative to another point (with a temperature and a potential).

I also don't think you can speak of a Seebeck effect if you have only one
single point (with its temperature and its potential). You need two points,
with a temperature difference between them, connected by a conductor, for
the Seebeck effect -- which causes a potential (or voltage) difference
between them. I don't think that the electric potential of a single point
(with uniform temperature) changes while changing its temperature. The
Seebeck effect (aka thermoelectric voltage) needs a temperature
/difference/ between two points on the conductor. Nothing absolute in this.


> ... it must be measured with reference to something else. This implies
> the need for at least *one* contact or quasi-contact. Even the pin/bar
> proposed by Olin has one contact.

No. The "contact" could be (and should be) of the same material as the rod
under test. This may put some restrictions on the range of suitable
materials, but not on the principle itself.

I think you may be mixing up the fact that in order to measure the Seebeck
effect in a closed current loop we need necessarily two different materials
(and therefore measure the difference of the Seebeck effect between the two
materials) with some other things.

A few affirmations (all IMO, of course :)

1- In order to create a closed loop where current can flow (which is the
case of all "normal" ways to measure voltages), it is necessary to use two
different materials in order to see a Seebeck effect. If the loop consisted
of the same material, the Seebeck effects on both sides of the hot spot
would cancel themselves out. That's the thermocouple: the resulting voltage
is a function of the difference of the Seebeck effects in both materials
and the difference in temperature between the hot and the cold end. (This
implies a certain configuration of what is hot and cold, but can be
reformulated for any other configuration.)

2- The Seebeck effect itself does not need a second material; it happens
within one material. It creates an electric field, which implies a
potential difference (aka voltage difference) between points of different
temperature along the conductor. These different potentials can be measured
(without introducing a second, different material) in a number of ways --
but all these methods have one thing in common: they don't rely on a
continuous current flow, therefore they don't need a closed loop, which
makes them conceptually different from the way we commonly measure
thermocouple voltages. However, measuring electric potentials in such ways
is nothing particularly strange (in laboratories that deal with electric
fields, at least).

3- While it may be that a common use of 'voltage' and 'potential' exists
that implies that 'potential' is absolute and 'voltage' is relative, the
author of this distinction doesn't adhere to this use (he earlier suggested
an 'absolute voltage'). I agree with the author's practical sense (here and
in general <g>) and say that both voltage and potential can be relative
(and in most applications are) and absolute. (For example, mechanic
potential energy is pretty much always used in a relative form, and it's
conceptually pretty similar to an electric potential.) I also think that
the value of an absolute electric potential is given in volt, and probably
can therefore be called a 'voltage' (an absolute one, maybe, but still).

If there is a difference between the terms, I'd say the voltage is the
value of the potential (in the sense of "we have here an electric potential
with an unknown voltage" or so)... but then, as long as we talk about
'bigger' and 'smaller' (as in the Seebeck effect), we always talk about the
(relative) values.


Not sure this is of interest to many, but for me it is surprising that we
need to discuss this (and important that we do, at least for me -- I could
be wrong on any or all questions :).

Gerhard

2006\06\17@172349 by Rich Graziano

picon face
Isn't voltage a potential difference?

----- Original Message -----
From: "Gerhard Fiedler" <EraseMElistsspam_OUTspamTakeThisOuTconnectionbrazil.com>
To: <piclistspamspam_OUTmit.edu>
Sent: Saturday, June 17, 2006 3:44 PM
Subject: Re: [EE] Soldering Thermocouple Eire?


{Quote hidden}

> --

2006\06\17@184727 by olin piclist

face picon face
Peter wrote:
> But if it is held (potential wise) then there is at least one
> junction.

It could be made of the same metal as the cathode and tied to it, for
example.

> Is this what you mean ? Say, heat the tip of a grounded pin in vacuum
> and see what happens to an electron beam that passes very near it ?

Basically yes, although I object to saying "grounded" because there is no
ground reference defined yet.

> But there is already one junction on it, the grounded one. It could
> be made of the same material as the pin,

Right.  If the point is to prove that a junction is not necessary to observe
the Seebeck effect.

> but there still is a junction there.

Huh?

This discussion is getting silly.  There have been several posts and
references to papers explaining about the Seebeck effect and how
thermocouples harness it to measure a temperature difference.  Hopefully
nobody is confused anymore.  What is your point?


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2006\06\17@233507 by Gerhard Fiedler

picon face
Rich Graziano wrote:

> Isn't voltage a potential difference?

Not sure. I thought that "voltage" comes from "volt", which is the unit of
measurement for electric potential. A potential, just as well as its value,
can be absolute or relative -- with absolute being nothing more than a
special case of relative, with a special, universally defined but abstract,
reference.

So, given an absolute potential calculated in volt (say one volt), wouldn't
you call that a "voltage"? Or is it possible that "one volt (absolute)" is
/not/ a voltage?

I think this distinction between potential and voltage doesn't make sense.
But let's wait until Peter reads his digest :)

Gerhard

2006\06\18@172429 by Peter

picon face

On Sat, 17 Jun 2006, Gerhard Fiedler wrote:

> Peter wrote:
>
>>>> Potential and voltage are one and the same in theory but in practice it
>>>> is more complicated.
>>>
>>> I'm not sure this is on firm ground. I'm no ace in physics terminology, but
>>> I think "voltage" is a term for the value of an electric field potential,
>>
>> The definition of potential is 'the energy required to move a punctiform
>> charge unit from the point being measured to infinity'. All normally
>> measured voltages are *relative* differences of potential, not absolute.
>
> I still don't see how you can make a difference between 'voltage' and
> 'potential' -- at least not any difference that would be relevant to the
> Seebeck effect. (At the very least, you should be more careful with the
> papers you cite in a given context: the one you cited uses 'potential
> difference' and 'voltage difference' as synonyms :)

To put it in a single phrase: nobody ever saw or measured a potential,
at most, they saw or measured a potential difference, also called a
voltage. Potentials are theoretical things. The were defined as such,
because they involve a Gedankenexperiment (that of moving a charge unit
to infinity, where both the 'unit' and 'infinity' are rather hadr to
get).

>> but if only a potential exists then a voltage must not also exist
>
> Describe a situation where a potential exists but no voltage... For every
> point that has a known potential, I can give you its voltage (relative to
> infinity); just tell me its potential :)  You seem to not take into account
> that the reference that is implicit in the concept of an absolute potential
> (infinity) necessarily always exists, and is used as reference point for
> expressing the value of this absolute potential (in volt).

For example an electron coming out of the (nearest) Big Bang represents
a unit of charge that may or may not have been coming from infinity (and
there is absolutely no way to prove it either way). This electron has a
potential energy, which can be measured (indirectly, to within the
principle of uncertainty's limits, by provoking a collision), which
should be proportional to a hypothetical acceleration voltage that
accelerated it. But good luck finding that voltage, or measuring it
(remember you have to go through the event horizon represented by the
Big Bang to find the 'other' electrode).

As the paper I quoted indicated, there is a very nice calculation for
the t. potential (absolute) for each material and temperature, but the
real life values, are the *measured* differences with Pt as reference.
The fact that the theory is in good concordance with practice may or may
not be a coincidence (achieved by trying really hard to explain the
observed reality, for example - this is just to play the devil's
advocate). Additionally anyone who has worked with precision
thermocouple measurements (like Spehro I think) will tell you that there
are a number of things that are different between theory and practice,
like non-linearities in the curves and such. Of course they can be
explained very conveniently by invoking crystalline form changes in the
materials involved. BUT the fact is that the practice is more
complicated than the theory.

Peter

2006\06\18@175047 by Peter

picon face

On Sat, 17 Jun 2006, Olin Lathrop wrote:

> Peter wrote:
>> but there still is a junction there.
>
> Huh?

Well the simplest piece of 'wire' to be examined for t. voltage would be
one made of two atoms of the same material, forming a crystal (i.e.
tightly coupled and probably sharing electron orbit energy levels
forming a conduction band). If one is at temperature X and the other is
at X+dt then the electrons which share energy levels have a better
probability to be in/nearer the colder atom, barring any energy level
jumps that might be made available by the extra thermal energy (!!). Of
course, if there are energy level jumps available then the electrons
could prefer the hotter atom instead. So the electrons around this
two-atom crystal do whatever they damn well please when one of the atoms
is 'hotter' than the other, and there is no easy way to say what
polarity each end of the bar will take without knowing a lot of details
about the material.

But to actually measure this, one should touch *both* of the atoms at
the same time to be able to measure the voltage (!) corresponding to the
potential (!) difference expected/caculated. And it is really necessary
to 'touch' both at the same time. Otherwise one can never be sure
whether one measured a real potential difference or a fluctuation.
Because of the uncertainty principle, one can be sure only to within
1/2hbar that what was really measured was really there. So the highest
measured 'voltage' between the two atoms is less than e*d but the
highest possible theoretical potential is e*d (where d is the distance
between the two atoms in the 'crystal'). Therefore the highest possible
measured voltage is smaller than the calculated maximum potential
difference.

By mathematical induction, this could be extrapolated to a bar made of a
material and having a temperature gradient across it, and a theoretical
potential difference.

> This discussion is getting silly.  There have been several posts and

I agree. It is just hairsplitting and my argumentation is probably
incorrect.

Peter

2006\06\18@180610 by Peter

picon face

On Sun, 18 Jun 2006, Gerhard Fiedler wrote:

> I think this distinction between potential and voltage doesn't make sense.
> But let's wait until Peter reads his digest :)

They are one and the same but good luck proving that an absolute
potential is really as much as the calculations say it is. And I am NO
physicist. Some people on this list are, however. I hope that they are
healthy and did not become disabled by laughing too hard at my postings
...

Peter


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