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'[EE]: Transient response of a thermistor...'
2001\04\10@100406 by Jose S. Samonte Jr.

picon face
Hello! =)

Does a thermistor have a transient response?
How fast does a thermistor change its resistance with respect to temperature?
Can it be computed?

Thank you so much.
Best regards.
Have a nice day to all!

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2001\04\10@110939 by Fernando Teixeira

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Hello
The transient response depends on the sensor and on the thermal
interface with
ambien around the sensor.
The same  thermistor in air has a slower thermal response than in water,
or if it
touches some body.
You can find the curves in the suppliers web sites.
Regards
Fernando


"Jose S. Samonte Jr." wrote:

{Quote hidden}

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2001\04\10@113631 by Bruce Kizerian

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The very tiny ones are usually the fastest. Thermistors used for medical
applications often have fast thermal response times. There is a bunch of
good info at http://www.thermometrics.com/

Bruce Kizerian

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2001\04\10@124741 by Roman Black

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Bruce Kizerian wrote:
>
> The very tiny ones are usually the fastest. Thermistors used for medical
> applications often have fast thermal response times. There is a bunch of
> good info at http://www.thermometrics.com/

Yep, and removing the plastic casing from around
those cheap electronic thermometer sensors can
speed up response from 30 seconds to less than
1 second. I have one of those in my incubator.
Those cheap thermometers also usually have a "trim"
resistor inside you can change the value to make
them quite accurate, as they are often sold with
error of 2 degrees C or worse. :o)
-Roman

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2001\04\11@052731 by Peter L. Peres

picon face
You can treat a thermistor as a thermal mass coupled to your system
through a heat connection of limited capacity. If the pins connect to
another system or board at a different temperature then it also has
heat losses and functions as a heatflow meter of sorts. If there is self
heating then some heat is generated in the thermistor and adds to the heat
flow from other sources.

The response speed depends on the specific heat coefficient of the
thermistor and the inflow/outflow capacities of the thermal coupling and
of the losses.

In practice one measures the time required for the whole assembly
including the heatsing to heat or cool as the thermistor's transient
response is very fast compared to it.

Peter

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2001\04\11@073814 by Bob Ammerman

picon face
Convert the heat model to an electrical one:

Masses == capacitors

Connections between masses == resistors

Temperators == voltages

Heat flow == currents

It can help you understand what is going on.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


{Original Message removed}

2001\04\11@090828 by Jose S. Samonte Jr.

picon face
I didn't get what you mean sir Bob...=(


Bob Ammerman <RAMMERMANspamspam_OUTPRODIGY.NET> wrote:
Convert the heat model to an electrical one:

Masses == capacitors

Connections between masses == resistors

Temperators == voltages

Heat flow == currents

It can help you understand what is going on.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


{Original Message removed}

2001\04\11@101929 by Bob Ammerman

picon face
Thermal 'circuits' are very similar to electrical circuits. You can use a
lot of the same analysis tools (like ohms law, kirchoffs law, etc) by just
changing the terminology:

Masses(things that store heat) == capacitors

Connections between masses == resistors

Temperatures == voltages

Heat flows == currents

So, for example if you know the thermal mass of the thermistor (its
pseudo-capacitance) and the thermal resistance of its connection to the item
being measured (the pseudo-resistance) you can compute the corresponding
time constant RC (just like in an electrical circuit).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\04\12@063229 by Jose S. Samonte Jr.

picon face
Hello, sir Peres! =)

The current is 200uA. I try to display the temperature data on an LCD, but the
reading takes more than 10 minutes to be stable, meaning the change is very
little. Why is this so?


"Peter L. Peres" <KILLspamplpKILLspamspamACTCOM.CO.IL> wrote:
What is the current in the thermistor, what is the dissipated power in it,
how do you measure it, and how do you define 'stabilize' ?

Peter

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2001\04\12@070353 by Alan B. Pearce

face picon face
>The current is 200uA. I try to display the temperature data on an LCD, but
the
>reading takes more than 10 minutes to be stable, meaning the change is very
>little. Why is this so?

How big are the variations you are getting? remember that if you could do a
step change of the temperature the resistance of the thermistor is going to
follow a curve similar to the charge or discharge curve of a capacitor
because of thermal inertia, that is to say the thermistor will not
instantaneously change temperature, but will take a certain time to reach
the new temperature.

Also if the thermistor is not thermally bonded to a (relatively) large mass
any draft of air around it will cause it to change temperature (typically
cool off) which may show on your readout if it is sensitive enough. This
will easily change the temperature 0.1C and often up to 0.5C just by waving
your hand to disturb the air around it.

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2001\04\12@070802 by Chris Carr

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face
I have only been half following this thread so my apologies if this
information has already appeared but what is the manufacturer and part
number of the thermistor or what does it look like, size, packaging etc.

Regards
Chris

Hello, sir Peres! =)

The current is 200uA. I try to display the temperature data on an LCD, but
the
reading takes more than 10 minutes to be stable, meaning the change is very
little. Why is this so?


"Peter L. Peres" <RemoveMEplpTakeThisOuTspamACTCOM.CO.IL> wrote:
What is the current in the thermistor, what is the dissipated power in it,
how do you measure it, and how do you define 'stabilize' ?

Peter

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2001\04\12@071221 by Jose S. Samonte Jr.

picon face
Hello sir Alan! =)
For example, sir, without touching first the thermistor, I get a 30.27
reading. When I put it in my armpit, the reading goes up rapidly before it
slows down to about 36.65, but still goes up slowly and for a very very long
time by 0.01 degree increments. Why is this so? Any explanation and solution
to this problem?

Thank you so much.
Best regards.


"Alan B. Pearce" <spamBeGoneA.B.PearcespamBeGonespamRL.AC.UK> wrote:
>The current is 200uA. I try to display the temperature data on an LCD, but
the
>reading takes more than 10 minutes to be stable, meaning the change is very
>little. Why is this so?

How big are the variations you are getting? remember that if you could do a
step change of the temperature the resistance of the thermistor is going to
follow a curve similar to the charge or discharge curve of a capacitor
because of thermal inertia, that is to say the thermistor will not
instantaneously change temperature, but will take a certain time to reach
the new temperature.

Also if the thermistor is not thermally bonded to a (relatively) large mass
any draft of air around it will cause it to change temperature (typically
cool off) which may show on your readout if it is sensitive enough. This
will easily change the temperature 0.1C and often up to 0.5C just by waving
your hand to disturb the air around it.

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2001\04\12@071835 by Jose S. Samonte Jr.

picon face
I only got the thermistor probes as samples, for my school project.
They are from YSI. They are skin thermistor probes, with part number, 409AC.

Thank you so much sir Chris.
I also hope that you would help me with my problem.
Best regards.

Chris Carr <TakeThisOuTnyedEraseMEspamspam_OUTBTINTERNET.COM> wrote:
I have only been half following this thread so my apologies if this
information has already appeared but what is the manufacturer and part
number of the thermistor or what does it look like, size, packaging etc.

Regards
Chris

Hello, sir Peres! =)

The current is 200uA. I try to display the temperature data on an LCD, but
the
reading takes more than 10 minutes to be stable, meaning the change is very
little. Why is this so?


"Peter L. Peres" <RemoveMEplpspamTakeThisOuTACTCOM.CO.IL> wrote:
What is the current in the thermistor, what is the dissipated power in it,
how do you measure it, and how do you define 'stabilize' ?

Peter

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2001\04\12@080128 by Alan B. Pearce

face picon face
>When I put it in my armpit, the reading goes up rapidly before it
>slows down to about 36.65, but still goes up slowly and for a very very
long
>time by 0.01 degree increments. Why is this so? Any explanation and
solution
>to this problem?

Well the problem is as I explained before - the probe has a thermal mass
that has to be heated. This takes time to do and will follow an exponential
curve like the voltage across a charging capacitor. If you are really trying
to take measurements to 0.01C then every measurement will see a change, even
when you enclose it under your armpit, as your body temperature will also go
up and down, depending on how active you are. With a sensor that sensitive
get to a point where you are sitting down and let the temperature
stabilise - may take several minutes. Now stand up suddenly and see what the
temperature does.

The solution is to decide if you really need to measure to 0.01C. I suspect
that measuring to 0.1C is really accurate enough, especially for the human
body, if that is what your instrument is to measure. The only alternative if
you have to measure more accurately is to decide if you can stand the time
delay for the probe to settle to the accuracy you want. If not you will have
to find a probe with lower thermal mass.

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2001\04\12@121353 by Chris Cox

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face
Jose:
RE Creeping thermistor end curves...

 Plot the curve from your armpit temp(a remarkably stable temperature
found most anywhere people gather, provided they're not having
religious/political discussions) and see how close it matches an RC
charging curve with various values. The curve "shape" could represent an
infinite number of "mathematical" resistor/capacitor values, or maybe
just one or two. Is the temp shape congruent with/very-close-to, some
charging curve? Does it mimic an RC time constant? After reading your
question and concern about the time it takes, I'm getting suspicious
that it may. Which brings up some interesting questions for QED
students.

 Of course, the stable arm pit temp is only guaranteed after you have
kept your unclothed arm compressed against your body for at least ten
minutes. The minute you open your arm to place the probe all bets are
off and your curve now represents two variables: A somewhat cooled
armpit, now creeping back up to temp, and the probe response being
masked by it. If you wish to use your armpit as a constant, rig a probe
attached to a piece of string and slip ONLY the string between your arm
and bod, NOT the wire leads to the probe. They transmit heat. (Yes,
we're being persnickety but this IS an experiment, right?) After about
ten minutes probe and body should have stabilized as much as you can
expect. Keep the probe on about 20 centimeters of string out BEHIND you
so you don't breathe on it, and away from the body. The rest of the
string is in front to pull the probe with. Take your initial measurement
and then pull the probe into your armpit area and start plotting.

 Now, on a more practical note. As long as a skin temp probe indicates
about 80-90 percent of it's final value (that's creeping very slowly by
now) in say 30 seconds, the med types will be very happy. How long do
your 409AC's take? The docs don't really care that a probe is all the
way to it's last itsy-bitsy Planck value. An RC curve doesn't REALLY
continue to charge for ever and ever. It gets bogged down in all the QED
noise at the end and the final value has to start following ambient
conditions. What the meds want to know 30 seconds from that hastily
placed skin probe is: is that temp 102F or 105??????? 102.4 slowly going
to 103 may be interesting. 105 will have everybody jumping...

Chris Cox (Another Chris, don't confuse the two of us.)

"Jose S. Samonte Jr." wrote:
{Quote hidden}

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2001\04\12@173700 by David Cary

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Dear "Jose S. Samonte Jr.",

Chris Cox may be on to something.

Chris Cox <RemoveMEcvhcoxEraseMEspamEraseMEBELLSOUTH.NET> on 2001-04-12 11:01:25 AM wrote:
> armpit temp(a remarkably stable temperature
...
> The curve "shape"
...
> Does it mimic an RC time constant?

Yes, take a bunch of data look at the graph.

It always takes about 7 time constants to settle within 1 part in 1000 --
because ln(1000) is roughly 7. In particular, if, hypothetically, the human
armpit had a time constant of 1 minute, then when it is cooled by a 10 K
transient, it will take about 7 minutes to settle to within 0.01 K of the final
value.

Do you see the same response when you warm your probe up slightly above body
temperature before measuring ?

Since human body temperatures normally change throughout the day (Is that the
``circadian rhythm'' ?), I would much rather use a fixed reference. I know
several people who test their thermocouples by dropping them in icewater, which
is always very close to 0 C.
Maybe you could try sticking your probes to the outside of a metal can of
icewater. (Or would plastic more closely emulate a human body ?)

--
David Cary

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2001\04\13@020348 by Chris Carr

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face
>I only got the thermistor probes as samples, for my school project.
>They are from YSI. They are skin thermistor probes, with part number,
>409AC.

OK you are using a suitable packaging. I also note from the manufacturers
web site that the accuracy you are attempting to achieve is greater than
that of the manufacturers own instrument built specifically to interface
with this family of thermistors. This should tell you something.

Are you sure the drift problem lies with the themistor ? What about all the
other components you are using ? Every one has a temperature coefficient
which can produce a drift in your readings at the level of accuracy that you
are working at (including the wire connecting the thermistor to the
instrument).
Are you letting the instrument stabilise before doing your measurements? Or
are you just switching it on and using it. Is it in a box ?

Chris

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2001\04\13@022943 by Jose S. Samonte Jr.

picon face
Hello! =)
I just switch on and use it.
Should I let the thermistor stabilize first before switching on my project? By
how long?

Thank you sir Chris! =)

Chris Carr <RemoveMEnyedspam_OUTspamKILLspamBTINTERNET.COM> wrote:
>I only got the thermistor probes as samples, for my school project.
>They are from YSI. They are skin thermistor probes, with part number,
>409AC.

OK you are using a suitable packaging. I also note from the manufacturers
web site that the accuracy you are attempting to achieve is greater than
that of the manufacturers own instrument built specifically to interface
with this family of thermistors. This should tell you something.

Are you sure the drift problem lies with the themistor ? What about all the
other components you are using ? Every one has a temperature coefficient
which can produce a drift in your readings at the level of accuracy that you
are working at (including the wire connecting the thermistor to the
instrument).
Are you letting the instrument stabilise before doing your measurements? Or
are you just switching it on and using it. Is it in a box ?

Chris

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2001\04\13@025944 by Chris Carr

flavicon
face
Switch everything on for 24 hours before using it, then see what results you
get. Read again what I wrote below. Have you eliminated the rest of the
circuitry as the cause of the drift ? Experiment, read the data sheets for
each component looking in particular at temperature co-efficients, calculate
the possible effect that this could have on your overall accuracy. Are you
sure that there is a single cause, is it temperature and not supply voltages
settling down after switch on.

I would suggest you investigate the basic procedures followed by a National
Calibration Laboratory when using tracable standards to calibrate
instruments and ask yourself why they follow those procedures. This should
give you idea of the procedures you should be following.

Chris Carr (not Cox  8-)  )

Hello! =)
I just switch on and use it.
Should I let the thermistor stabilize first before switching on my project?
By
how long?

Thank you sir Chris! =)

Chris Carr <RemoveMEnyedTakeThisOuTspamspamBTINTERNET.COM> wrote:
>I only got the thermistor probes as samples, for my school project.
>They are from YSI. They are skin thermistor probes, with part number,
>409AC.

OK you are using a suitable packaging. I also note from the manufacturers
web site that the accuracy you are attempting to achieve is greater than
that of the manufacturers own instrument built specifically to interface
with this family of thermistors. This should tell you something.

Are you sure the drift problem lies with the themistor ? What about all the
other components you are using ? Every one has a temperature coefficient
which can produce a drift in your readings at the level of accuracy that you
are working at (including the wire connecting the thermistor to the
instrument).
Are you letting the instrument stabilise before doing your measurements? Or
are you just switching it on and using it. Is it in a box ?

Chris

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2001\04\13@030224 by Chris Carr

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face
Sorry forgot to modify the Subject line and cull the bottom of the message.


>
> Switch everything on for 24 hours before using it, then see what results
you
> get. Read again what I wrote below. Have you eliminated the rest of the
> circuitry as the cause of the drift ? Experiment, read the data sheets for
> each component looking in particular at temperature co-efficients,
calculate
> the possible effect that this could have on your overall accuracy. Are you
> sure that there is a single cause, is it temperature and not supply
voltages
> settling down after switch on.
>
> I would suggest you investigate the basic procedures followed by a
National
> Calibration Laboratory when using tracable standards to calibrate
> instruments and ask yourself why they follow those procedures. This should
> give you idea of the procedures you should be following.
>
> Chris Carr (not Cox  8-)  )
>
> Hello! =)
> I just switch on and use it.
> Should I let the thermistor stabilize first before switching on my
project?
{Quote hidden}

the
> other components you are using ? Every one has a temperature coefficient
> which can produce a drift in your readings at the level of accuracy that
you
> are working at (including the wire connecting the thermistor to the
> instrument).
> Are you letting the instrument stabilise before doing your measurements?
Or
> are you just switching it on and using it. Is it in a box ?
>
> Chris
>

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2001\04\13@035855 by Jose S. Samonte Jr.

picon face
Basic procedures of a National Calibration Laboratory? =)


Chris Carr <RemoveMEnyedKILLspamspamBTINTERNET.COM> wrote:
Sorry forgot to modify the Subject line and cull the bottom of the message.


>
> Switch everything on for 24 hours before using it, then see what results
you
> get. Read again what I wrote below. Have you eliminated the rest of the
> circuitry as the cause of the drift ? Experiment, read the data sheets for
> each component looking in particular at temperature co-efficients,
calculate
> the possible effect that this could have on your overall accuracy. Are you
> sure that there is a single cause, is it temperature and not supply
voltages
> settling down after switch on.
>
> I would suggest you investigate the basic procedures followed by a
National
> Calibration Laboratory when using tracable standards to calibrate
> instruments and ask yourself why they follow those procedures. This should
> give you idea of the procedures you should be following.
>
> Chris Carr (not Cox  8-)  )
>
> Hello! =)
> I just switch on and use it.
> Should I let the thermistor stabilize first before switching on my
project?
{Quote hidden}

the
> other components you are using ? Every one has a temperature coefficient
> which can produce a drift in your readings at the level of accuracy that
you
> are working at (including the wire connecting the thermistor to the
> instrument).
> Are you letting the instrument stabilise before doing your measurements?
Or
> are you just switching it on and using it. Is it in a box ?
>
> Chris
>

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2001\04\13@090319 by Chris Carr

flavicon
face
>Basic procedures of a National Calibration Laboratory? =)
http://www.ilac.org/

The following gives a brief background on tracibility of Measurements and
lists calibration and testing laboratories
Be warned it is a large document so it takes some time to download.
http://www.ilac.org/downloads/Ilac-g2.pdf

I would suggest that you or your tutor contact a laboratory near to you and
arrange a visit. Contact the Quality Manager at a local ISO9000 registered
electronics company or the tutor running courses on Quality at your College.

Go to the Hewlett-Packard site (Damn I keep forgetting they have rebranded)
Agilent I mean Agilent http://www.tm.agilent.com/ and explore they are a
well known source of information on test and measurement. Look at their
34401A digital multimeter and explore. Not all of the information on this
instrument is directly related to your problem but a lot of it is, I will
leave it to you to work out what relates and how. Just this once I will be
kind and give you the links from the front page to that instrument.

Products & Services / Test & Measurement / General Purpose Instruments
Digital Multimeters/ Voltmeters
34401A Digital Multimeter, 6.5 Digit


Chris Carr


>
>> Switch everything on for 24 hours before using it, then see what results
you
> >get. Read again what I wrote below. Have you eliminated the rest of the
>> circuitry as the cause of the drift ? Experiment, read the data sheets
for
>> each component looking in particular at temperature co-efficients,
calculate
>> the possible effect that this could have on your overall accuracy. Are
you
>> sure that there is a single cause, is it temperature and not supply
voltages
>> settling down after switch on.
>>
>> I would suggest you investigate the basic procedures followed by a
National
>> Calibration Laboratory when using tracable standards to calibrate
>> instruments and ask yourself why they follow those procedures. This
should
>> give you idea of the procedures you should be following.
>>
>> Chris Carr (not Cox  8-)  )
>>
> >Hello! =)
>> I just switch on and use it.
>> Should I let the thermistor stabilize first before switching on my
project?
{Quote hidden}

the
>> other components you are using ? Every one has a temperature coefficient
>> which can produce a drift in your readings at the level of accuracy that
you
>> are working at (including the wire connecting the thermistor to the
>> instrument).
>> Are you letting the instrument stabilise before doing your measurements?
Or
>> are you just switching it on and using it. Is it in a box ?
>>
>> Chris
>>

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2001\04\14@164518 by Jose S. Samonte Jr.

picon face
What does being 'cooled off by a 10K transient' mean? =)


David Cary <KILLspamdcaryspamBeGonespamBRUNSWICKOUTDOOR.COM> wrote:
Dear "Jose S. Samonte Jr.",

Chris Cox may be on to something.

Chris Cox <EraseMEcvhcoxspamEraseMEBELLSOUTH.NET> on 2001-04-12 11:01:25 AM wrote:
> armpit temp(a remarkably stable temperature
...
> The curve "shape"
...
> Does it mimic an RC time constant?

Yes, take a bunch of data look at the graph.

It always takes about 7 time constants to settle within 1 part in 1000 --
because ln(1000) is roughly 7. In particular, if, hypothetically, the human
armpit had a time constant of 1 minute, then when it is cooled by a 10 K
transient, it will take about 7 minutes to settle to within 0.01 K of the
final
value.

Do you see the same response when you warm your probe up slightly above body
temperature before measuring ?

Since human body temperatures normally change throughout the day (Is that the
``circadian rhythm'' ?), I would much rather use a fixed reference. I know
several people who test their thermocouples by dropping them in icewater,
which
is always very close to 0 C.
Maybe you could try sticking your probes to the outside of a metal can of
icewater. (Or would plastic more closely emulate a human body ?)

--
David Cary

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