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'[EE]: Temperature sensor for my oven?'
2004\10\14@102113 by No Religion

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face

Hello folks!
Could you help me please? I'd need an advice for a
temperature sensor for my oven.

# it should be easily interfaceable with a PIC.

# it should be able to measure realiably (and without
 getting damaged) up to at least 250C.

# it should be cheap and easy to find, and it would be a
 plus if one could order a bunch of free samples online.

Am I asking way too much?

Thanks!
N.R.

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2004\10\14@124017 by Mark Scoville

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Hello,

I don't know if thermistors will survive at 250C (No experience). I know
RTD's will go that high - but they are usually a pain to interface. I like
thermocouples myself.

Should you decide on a thermocouple take a look at the MAXIM MAX6675 chip.
This chip makes it virtually painless to interface to a K type thermocouple.
Connect up 5V to the MAX6675 and connect up the thermocouple to the MAX6675
and get temperature data via SPI. We use the MAX6675 chip and a K type
thermocouple to measure 800C, so 250C would not be a problem. MAXIM is
pretty good about samples as well.

As far as getting a sample of a K type thermocouple... don't know of anyone
giving those away. I don't think they are very expensive though.

-Mark

{Quote hidden}

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2004\10\14@134714 by Roland

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At 04:20 PM 14/10/2004 +0100, you wrote:
>
>Hello folks!
>Could you help me please? I'd need an advice for a
>temperature sensor for my oven.

Hi

Have a look at these microchip docs. They're on my 2003/04 technical
library CD rom, otherwise should be on the net. If not I can post direct to
you. I just happened to be looking at them last night.

\Content\download\appnote\analog\adc\

00679a.pdf  AN679  Temperature Sensing Technologies
00684a.pdf  AN684  Single Supply Temperature Sensing with Thermocouples
00685b.pdf  AN685  Thermistors in Single Supply Temperature Sensing Circuits

># it should be easily interfaceable with a PIC.

you're gonna need to do A/D, otherwise easy to do.

># it should be able to measure realiably (and without
>  getting damaged) up to at least 250C.

anything over about 150C is going to need a junction device. J and K type
thermocouples (760C and 1260C) are very common in industry, and quite
cheap. (R120, ~USD20?)

># it should be cheap and easy to find, and it would be a

scrapyard;-), on any scrap oven

>  plus if one could order a bunch of free samples online.

don't know.



Regards
Roland Jollivet

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2004\10\14@142511 by Robert Rolf

picon face
Thermistor and spot welded (or screw clamped) leads
with teflon wire?
Thermocouple and instrumentation amp/cold junction comp?

Check out
http://www.omega.com/
for all you could possibly want by way of
COTS temp sensors.

No Religion wrote:

{Quote hidden}

Yes.

R

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2004\10\15@053753 by Alan B. Pearce

face picon face
>As far as getting a sample of a K type thermocouple...
>don't know of anyone giving those away. I don't think
>they are very expensive though.

The biggest hassle with them is making sure you get one with suitable
insulation on the wires. Many of the ones that seem to be available locally
are rated to a lower temperature than I expect, and I think it is because
the wire insulation will not take higher temps.

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2004\10\15@155904 by Peter L. Peres

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On Thu, 14 Oct 2004, Mark Scoville wrote:

> Hello,
>
> I don't know if thermistors will survive at 250C (No experience). I know
> RTD's will go that high - but they are usually a pain to interface. I like
> thermocouples myself.

Why are rtd's hard to interface ? It's just a handy valued resistor (100
ohms at 25C is common). All you need to read it is an ohmmeter and a table
(supplied). Your trusty dvm will work fine if you don't need wire
compensation, and unlike thermocouples they do not need special chips and
outstanding high temperature insulated wiring skills (you can use the
chassis/oven wall etc as return).

> Should you decide on a thermocouple take a look at the MAXIM MAX6675 chip.
> This chip makes it virtually painless to interface to a K type thermocouple.
> Connect up 5V to the MAX6675 and connect up the thermocouple to the MAX6675
> and get temperature data via SPI. We use the MAX6675 chip and a K type
> thermocouple to measure 800C, so 250C would not be a problem. MAXIM is
> pretty good about samples as well.
>
> As far as getting a sample of a K type thermocouple... don't know of anyone
> giving those away. I don't think they are very expensive though.

It would be good to check though. Cheapest I know is about $20 and the
wires are too short for any normal use in an oven (you need special wire
and special crimping tubes to prolong them). The special chip and the
female connector you need to put on your board is not included in this
price. Also if it ever misreads for any reason you will have no way to
check it. I used to check thermocouples by putting them against a hot
soldering iron and reading ~2-3mV on a DVM. Some go/no go test. I don't do
this anymore.

Were the wing temperature readings on the doomed shuttle coming from
thermocouples ? They went off scale *low* as the wing exceeded temperature
probably cooking the wiring and shorting the signals.

Peter
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2004\10\17@030217 by hilip Stortz

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i've seen lower prices for the very basic bare junction type for
multimeters.  also check omega.com, lots of thermocouples and wire (and
you can make your' own thermocouple from the wire by welding it together
with a capacitor discharge, ideally but not necessarily in argon
depending on your accuracy needs, or even soldering though it's not
ideal).  for a single device, ebay is of course often helpful.  the nice
thing about thermocouples is that they are nearly linear, an rtd is a
terribly non-linear thing.  then again thermocouples have their down
side as well, but i like them for reasonably high temperatures.  an rtd
(or thermistor) is best for control around a single point, then you can
set up a bridge etc. with the appropriate value resistors, otherwise you
need that pesky lookup table which adds code and makes a pure analog
solution difficult.  i believe national and/or ti also have chips
specifically for interfacing with thermocouples.  

also, as long as you keep both wires at the same temperature (i.e. close
together and insulated from the outside) you can use ordinary crimps and
wires to extend thermocouples, though the proper crimp and/or wire is
better, but where it connects to the control circuitry you usually have
a terminal strip on a pcb, it's important to provide thermal insulation
for the terminal strip if possible and keep it away from heat sources.
you can also put several junctions in series if you need a higher
voltage (i've seen this done on a datalogger before to get enough
voltage to get the needed resolution), as well as it being trivial to
hook 2 in series to measure temperature differential directly.

"Peter L. Peres" wrote:
-------
> It would be good to check though. Cheapest I know is about $20 and the
> wires are too short for any normal use in an oven (you need special wire
> and special crimping tubes to prolong them). The special chip and the
-------

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2004\10\18@120300 by Mark Scoville

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> > I don't know if thermistors will survive at 250C (No experience). I know
> > RTD's will go that high - but they are usually a pain to
> interface. I like
> > thermocouples myself.
>
> Why are rtd's hard to interface ? It's just a handy valued resistor (100
> ohms at 25C is common). All you need to read it is an ohmmeter
> and a table
> (supplied). Your trusty dvm will work fine if you don't need wire
> compensation, and unlike thermocouples they do not need special chips and
> outstanding high temperature insulated wiring skills (you can use the
> chassis/oven wall etc as return).
>

Hello Peter,

I only meant that RTDs are a pain to interface in comparison to a 1-chip
solution. Recall the original poster was looking for a solution that was
easy to interface to a PIC. I have interfaced both RTD's and thermocouples
in industrial products. I have used the MAX6675 - it does the job and is
only 1 part between the thermocouple and the processor (PIC in my case).
When using RTD's we always end up using many more parts (more board space,
more costly to assemble, more opportunities for assembly mistakes). For me
the MAX6675 is an elegant solution.

If you are aware of a simple way (1 chip) to interface RTD's to the SPI bus
(including open/shorted RTD indication) please tell me what it is. I am in
the design phase right now of another product which has both RTD and K
thermocouple interfaces (customer picks which one they want to use). Right
now the RTD interface we use has op-amps and generates a voltage that is
read on a analog input of the PIC. Not very elegant if you ask me... but it
works. If you know of a better way (less parts, direct digital to SPI etc.)
lay it on me - I'm all ears.

>
> Were the wing temperature readings on the doomed shuttle coming from
> thermocouples ? They went off scale *low* as the wing exceeded
> temperature
> probably cooking the wiring and shorting the signals.
>

Don't know. Interesting to think about though. Maybe they use thermocouples.
However, I don't think that shorting together thermocouple wires will cause
a low reading. Maybe if the wires shorted to something else funny things
will happen. But if you short the two wires of a thermocouple together you
just get another thermocouple which measures the temperature at the point of
the short - don't you?

-Mark

Doesn't matter what your business card says... We're all in the results
business.


> Peter
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2004\10\18@122821 by Mark Scoville

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> -----Original Message-----
> From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu]On Behalf
> Of Philip Stortz
> Sent: Sunday, October 17, 2004 3:05 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE]: Temperature sensor for my oven?
>
>
> i've seen lower prices for the very basic bare junction type for
> multimeters.  also check omega.com, lots of thermocouples and wire (and
> you can make your' own thermocouple from the wire by welding it together
> with a capacitor discharge, ideally but not necessarily in argon
> depending on your accuracy needs, or even soldering though it's not
> ideal).  for a single device, ebay is of course often helpful.  the nice

Yes, Omega has lots of stuff. Good tutorial section on thermocouples in
their catalog too.

When desperate I have in a pinch twisted the thermocouple wires together
tightly using a bench vise and pliers - no welding or soldering. I've never
checked to see how *Bad* this is. I know it's not ideal, but when you don't
have anything else it's better than nothing.

> thing about thermocouples is that they are nearly linear, an rtd is a
> terribly non-linear thing.  then again thermocouples have their down
> side as well, but i like them for reasonably high temperatures.  an rtd
> (or thermistor) is best for control around a single point, then you can
> set up a bridge etc. with the appropriate value resistors, otherwise you
> need that pesky lookup table which adds code and makes a pure analog
> solution difficult.  i believe national and/or ti also have chips
> specifically for interfacing with thermocouples.

FWIW, Linear Technology makes thermocouple interface chips too (We use the
LTKA00 & LT1025 chipset for J type applications)

-Mark




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2004\10\18@145422 by Peter L. Peres

picon face

On Mon, 18 Oct 2004, Mark Scoville wrote:

>>> I don't know if thermistors will survive at 250C (No experience). I know
>>> RTD's will go that high - but they are usually a pain to
>> interface. I like
>>> thermocouples myself.
>>
>> Why are rtd's hard to interface ? It's just a handy valued resistor (100
>> ohms at 25C is common). All you need to read it is an ohmmeter
>> and a table
>> (supplied). Your trusty dvm will work fine if you don't need wire
>> compensation, and unlike thermocouples they do not need special chips and
>> outstanding high temperature insulated wiring skills (you can use the
>> chassis/oven wall etc as return).
>>
>
> Hello Peter,
>
> I only meant that RTDs are a pain to interface in comparison to a 1-chip
> solution. Recall the original poster was looking for a solution that was
> easy to interface to a PIC. I have interfaced both RTD's and thermocouples

The op was looking for an easy interface to a one off homemade smd oven.
For one off, unless you buy a ready made dvm with thermocouple option and
a K thermocouple supplied in the box you are better off with a rtd imho.

> in industrial products. I have used the MAX6675 - it does the job and is
> only 1 part between the thermocouple and the processor (PIC in my case).
> When using RTD's we always end up using many more parts (more board space,
> more costly to assemble, more opportunities for assembly mistakes). For me
> the MAX6675 is an elegant solution.

I agree 100% for industrial product use. Also for home one-off iff one
already has the thermocouple paraphernalia lying around. The funny
connector, high temperature cable, crimping set for prolonging same, and a
sample chip.

> If you are aware of a simple way (1 chip) to interface RTD's to the SPI bus
> (including open/shorted RTD indication) please tell me what it is. I am in
> the design phase right now of another product which has both RTD and K
> thermocouple interfaces (customer picks which one they want to use). Right
> now the RTD interface we use has op-amps and generates a voltage that is
> read on a analog input of the PIC. Not very elegant if you ask me... but it
> works. If you know of a better way (less parts, direct digital to SPI etc.)
> lay it on me - I'm all ears.

The opamp solution is the one I know. short/open sensing should be done by
same imho (too high or too low input reading). The advantage I see is that
you can make a rtd interface with a section of a lm324 and 4-5 resistors
and it will work and you can check it out against a LM35 at room
temperature with your dvm. Low cost rtds for room temperature can be f.ex.
low cost inductors (several ohms of dc resistance for f.ex. 10mH).

It's the lack of need to calibrate between units when changing or
installing the sensor at the customer that makes rtds a good idea for
production imho.

{Quote hidden}

Yes but it depends on the wires. I assume that nasa knows what they are
doing and would not allow something failing to cause a major misreading
(in the wrong direction too) on an instrument (or on several) without
providing some check.

Peter
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2004\10\18@204952 by Bill & Pookie

picon face
How would one of those thermal couples that is used in gas heaters and water
heaters to keep the pilot light on and power the thermostat work as a
sensing device?

Bill

----- Original Message -----
From: "Mark Scoville" <mscovillespamKILLspamunicontrolinc.com>
To: <.....piclistKILLspamspam.....mit.edu>
Sent: Monday, October 18, 2004 9:31 AM
Subject: RE: [EE]: Temperature sensor for my oven?


> > {Original Message removed}

2004\10\19@000612 by Mike Singer

picon face
Interesting device is coming.

220_31780.pdf
D14ES1B0804.pdf
--------------------------------------------------
Infrared Thermosensor ES1B

Achieve Low-cost Measurements with an Infrared Thermosensor.
&#8226; Non-contact measurement.
&#8226; The ES1B has an electromotive output as high as that of a thermocouple,
thus connecting directly to the thermocouple input terminal
of the Temperature Controller is possible.
&#8226; Four temperature ranges are available to cover a wide range of
temperature measurement needs, including those in the food processing,
packaging, molding, and electronics industries.
&#8226; High-accuracy temperature measurement is ensured by a highspeed
response of 300 ms (for a 63% response) and an indication
reproducibility of &#177;1% PV.
&#8226; Unlike thermocouples, the Thermosensor does not deteriorate.
Therefore, stable, real-time temperature control can be maintained.
---------------------------------------------------

Mike.

___________________________________________

2004\10\19@020151 by Jinx

face picon face
www.omron.com.au/product_info/es1b/index.asp

If you Google for

es1b thermosensor pdf

there's a oeiwcsnts.omron pdf link

I saw a brochure for these last week when getting relays from
Omron's store for a customer's environment-measuring modules.
Whether to wear the NZ$300+ price tag fortunately isn't my decision,
although I don't think my customer's modules call for something
this fancy anyway

____________________________________________

2004\10\19@102733 by Peter L. Peres

picon face

On Mon, 18 Oct 2004, Bill & Pookie wrote:

> How would one of those thermal couples that is used in gas heaters and water
> heaters to keep the pilot light on and power the thermostat work as a
> sensing device?

There are 2 kinds: mechanical and electrical. The mechanical kind is not a
thermocouple, it's a closed tube liquid thermometer just like the one in a
fridge thermostat. The other end holds an expandable bellows that latches
the main (inlet) gas valve open when it expands enough (after the pilot is
lit and the tube heats up).

The electrical kind uses a thermocouple (or several in series). Once upon
a time it would operate a simple d'Arsonval meter-like device that would
lock the main gas valve open as above, Or simply power a solenoid that
would hold a plate and lock the valve open as above.

The thermostat usually has a second thermocouple, bimetal device, or
liquid thermometer as above, that is independent from the pilot light
device. The pilot light device is a safety device that prevents gas leaks
if the pilot light goes out and the main thermostat opens the main flame
valve.

Peter
____________________________________________

2004\10\19@102749 by Peter L. Peres

picon face

On Mon, 18 Oct 2004, Peter L. Peres wrote:

> It's the lack of need to calibrate between units when changing or installing
> the sensor at the customer that makes rtds a good idea for production imho.

That should of course read, "the lack of need to calibrate ... makes
thermocouples a good idea for production imho"

Peter
____________________________________________

2004\10\19@140638 by Mike Singer

picon face
There was a discussion on using incandescent micro-lamps as
temperature sensors two years ago with Wagner Lipnharski and Roman
Black.

I experimented a little with 60V-50mA lamp - small glass tube with two wires:
- 120 Ohm at 25C;
- 155 Ohm in boiling water;
- At 250C, I guess, its resistance would be aprox 200 Ohm;

At 10ma current the voltage range would be ~ 1mv&#8211;2mv &#8211; pretty enough
to get it amplified by some Op Amp to, say, 2v&#8211;4v suitable for PIC's
ADC.

Glass tube provides better stability than exposed element, I think.
Time constant of temperature drop was about 10s.

That's just an option. Dedicated device is better, of course.

Regards,
Mike.

___________________________________________

2004\10\19@173133 by Russell McMahon

face
flavicon
face
> There was a discussion on using incandescent micro-lamps as
> temperature sensors two years ago with Wagner Lipnharski and Roman
> Black.

> I experimented a little with 60V-50mA lamp - small glass tube with two
> wires:
> - 120 Ohm at 25C;
> - 155 Ohm in boiling water;
> - At 250C, I guess, its resistance would be aprox 200 Ohm;
>
> At 10ma current the voltage range would be ~ 1mv&#8211;2mv &#8211; pretty enough
> to get it amplified by some Op Amp to, say, 2v&#8211;4v suitable for PIC's
> ADC.

> Glass tube provides better stability than exposed element, I think.
> Time constant of temperature drop was about 10s.

That sounds like an excellent idea. But the delta V will be far larger than that at constant current.
At 200 ohms, 10 mA V = 0.01 x 200 = 2 volts. At 25c and 120 ohms it's 1.2 volts.
Presumably you meant 10 microamps, where your figure would be correct.
I imagine you want to keep the current so low that there is no significant self heating.
At 10 mA you get about 20 mW which is not vast, but probably noticeable.
It would be interesting to know how well they work at mA level currents as the A2D can then be used directly.
Some processors have an eg 20x internal ADC gain giving a 0.128V full scale range which would allow about 0.5 mA excitation.


       Russell McMahon

___________________________________________

2004\10\19@191411 by Bill & Pookie

picon face
In my old system, the thermocouple provides power if the pilot light is
burning.  This is the power that the thermostat supplies to the main burners
when the thermostat's points close.

Just thinking that this power could be measured.

Bill

{Original Message removed}

2004\10\20@104518 by Peter L. Peres

picon face


On Tue, 19 Oct 2004, Bill & Pookie wrote:

> In my old system, the thermocouple provides power if the pilot light is
> burning.  This is the power that the thermostat supplies to the main burners
> when the thermostat's points close.
>
> Just thinking that this power could be measured.

You can measure it using a low drop ammeter (the easiest way) if you do
not care too much about precision. It should read on the 10A scale of a
dvm with really short wires to the thermocouple. The current will likely
be under 1 A.

Peter
____________________________________________

2004\10\20@104523 by Peter L. Peres

picon face
part 1 2072 bytes content-type:TEXT/PLAIN; charset=UTF-8; format=flowed (decoded quoted-printable)



On Wed, 20 Oct 2004, Russell McMahon wrote:

{Quote hidden}

I wrote about this about 2 years ago. I used a lightbulb for 24V 40mA and *collapsed* the glass on the filament by holding it in a gas flame (glass melts and is sucked in by internal vacuum forming a glass bead with the insulated filament at the center.

This reduces the time constant and reduces effects by self heating. I wrote about this too.

My rtd was about 15 ohms at room temperature, about 25 ohms at soldering iron temperature (~300C, poor thermal contact). I calibrated it later. This is from ~2001 or earlier.

Peter

part 2 79 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

____________________________________________

2004\10\20@104526 by Peter L. Peres

picon face

On Tue, 19 Oct 2004, Bill & Pookie wrote:

> In my old system, the thermocouple provides power if the pilot light is
> burning.  This is the power that the thermostat supplies to the main burners
> when the thermostat's points close.
>
> Just thinking that this power could be measured.

Yes but typically a thermocouple at 1000C will produce 20mV. To get any
kind of current from that you need a really low resistance circuit and
magnet coils with high sensitivity. Those 20mV in a 20mohm circuit will
give an amp. You can do a number of things with 1 amp but the circuit
needs to be *very* well built. 20mohm is a very low circuit resistance
(including the thermocouple wires!!!)

Peter
____________________________________________

2004\10\20@120437 by Bill & Pookie

picon face
Not to argue the point, but here I go, arguing the point.

All I know about voltage and amps is the EGBDF thingie.  But the
thermocouple sure appears to be the only source of electrical power in the
system and it will turn on the main gas jets if the pilot light has heated
it up enuff.  In fact I have used a battery on the old heater at the
thermostat to start the thing heating.  "A penny for your fuse box."

Never measured on so don't know the facts.  Just going by I have seen work.

Bill

{Original Message removed}

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