How does a thermocouple sensor work??

Submitted: Tuesday, Apr 08, 2008 at 12:38
ThreadID: 56428 Views:3468 Replies:6 FollowUps:6
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I fitted a EGT gauge to the vehicle about 4 months ago. It worked fine til about 3 weeks ago- now does not indicate any reading.
I'm trying to establish if its the gauge or the thermo sensor that's died.
The sensor is showing full scale resistance (when at operating temp)- is this normal??
Anyone out there can give me a fault finding guide ??
Cheers

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Reply By: Member - joc45 (WA) - Tuesday, Apr 08, 2008 at 12:54

Tuesday, Apr 08, 2008 at 12:54
As I understand it, most pyro guages are thermocouples.
If it's a true thermocouple, it is made of two dissimilar metals, and generates a small voltage when there is a temperature difference between the two metals. The guage end measures this voltage and displays it as a temperature.
So if measuring resistance of the sender, the resistance of the thermocouple will not change much with temperature. If you have a meter which measures millivolts, you should be able to measure a voltage across the thermocouple when it is heated.
Conversely, the guage should respond to a small voltage across it. But I wouldn't recommend using a battery to check the guage, as it might overload the meter (the output of the thermocouple is usually only millivolts)
If the sender is a thermistor, the resistance varies with temperature, and the guage measures this resistance. You should be able to see the resistance change with temperature when measured with an ohm meter.
Gerry
AnswerID: 297369

Reply By: Dazmit - Tuesday, Apr 08, 2008 at 13:15

Tuesday, Apr 08, 2008 at 13:15
From memory 25 C at the thermocouple tip equates to 1.0 mV dc for a Type K thermocouple therefore if EGT is 250 C you should be measuring approx 10 mV across the 2 wires of the Thermocouple.

Cheers

Darryl
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Follow Up By: Member - John and Val W (ACT) - Tuesday, Apr 08, 2008 at 14:05

Tuesday, Apr 08, 2008 at 14:05
Nope..... a thermocouple develops a small voltage at the junction of 2 dissimilar metals (or other conductors or semiconductors). There will actually be two junctions, one at each end of one of the wires. The net voltage generated is dependent on the temperature difference between these 2 junctions, not their actual temperature. From rusty memory a Type k uses copper and constantan (an alloy) and produces about 47 microvolts per degree C difference between the two junctions. Usual practice is to assume we know the temperature of one of the junctions, (say 20 deg C) and by measuring the small voltage determine the temperature difference, hence roughly the unknown temperature. Unless its a very big difference the voltage will be far too small to measure on your average voltmeter.

All that said, I suspect the original question did not actually relate to a thermocouple, but to some other type of sensor, probably a thermistor which is a very different beast. With these their resistance varies with temperature, some rise, some fall as temperature rises, and they are very nonlinear. I doubt that we can offer much help on this one.

John
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Follow Up By: Ircon - Tuesday, Apr 08, 2008 at 15:47

Tuesday, Apr 08, 2008 at 15:47
Nit picking I know but Type K is Chromel vs Alumel. Type T is Copper vs Constantan. Copper/con is often used for extension wire for Chromel/Alumel Thermocouples as it has a very similar temp vs millivolt output at lower (around ambient) temperatures and is traditionally cheaper.

For reference junction temperature of 25 Deg C the thermoelectric voltage for a Type K thermocouple is 4.095 mV at 100 Deg C; 8.137 mV at 200 Deg C and 12.207 mV at 300 Deg C.

This means that if the point at which your meter is connected is 25 Deg C (cold junction) and the hot junction is at 100 deg c you will measure 4.095 mV or .0049 V.

Strictly speaking the temp vs mV relationship is not linear.

But I agree it may not be a thermocouple at all

From an old ex Instrument fitter!

Regards,

Rosscoe
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Follow Up By: Member - John and Val W (ACT) - Tuesday, Apr 08, 2008 at 16:41

Tuesday, Apr 08, 2008 at 16:41
Thanks Rosscoe. As I said - rusty memory!! I don't disagree with your facts. The central issue though is that a thermocouple, be it chromal or coppercon, will deliver such a small voltage that it will barely show on most meters. The term thermocouple is loosely used to mean temperature sensor, and I think this has led us astray. Could be this device uses a thermopile ( a lot of thermocouples in series) to produce a meaningful voltage, but it won't be helpful to extrapolate on conjecture!

Cheers

John
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Reply By: Angler - Tuesday, Apr 08, 2008 at 14:27

Tuesday, Apr 08, 2008 at 14:27
Mine is a WWG pyrometer and puts out 4.04 millivolts at 100 degrees Celsius and virtually doubles each time up to 37.3 millivolts at 900 degrees. I reckon it should read close enough to zero ohms at low temps as it is shorted at the sensor end by a sort of spot weld.

I guess you could heat it up and measure the small voltage across the terminals to check the sensor. (thats why the gauge is so expensive, it reads such very low voltages.)

Hope this helps. At least the sensor should be the the cheapest part.
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Reply By: Member - DOZER- Tuesday, Apr 08, 2008 at 15:36

Tuesday, Apr 08, 2008 at 15:36
The two wires coming out of the thermocouple should not have continuity with the outside shroud...disconnect the gauge from it and measure if there are any ohms resistance between either wire and ground.
Also, joining the two wires from the gauge together should result in ambient temperature being displayed, as these two wires are also discimilar....
If the gauge doesnt read ambient, you have a problem upstream.
However 999 out of 1000 times its the thermocouple that dies...
Andrew
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Follow Up By: Member -Signman - Tuesday, Apr 08, 2008 at 15:55

Tuesday, Apr 08, 2008 at 15:55
Hi Dozer..
just checked now with ohm meter,
BOTH wires coming from the sensor show a short circuit to earth (ie. to the shroud braid & also chassis earth)...and a short circuit between them...
I don't think the gauge would be accurate enough to register ambient. It's not a digital- it's an analog sweep type..
Thanks for your info.




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Reply By: Ircon - Tuesday, Apr 08, 2008 at 16:10

Tuesday, Apr 08, 2008 at 16:10
Hi Signman,

If it is a thermocouple it should read a very LOW resistance because it is effectively a short circuit caused by the connection two wires of different but conductive metals.
A common fault with industrial thermocouple is open circuit caused by mechanical fatigue or corrosion.

As others have said a thermocouple produces a "known" temp vs mV relationship due the difference between the hot junction (sensor end) and the cold junction (meter end).

For a Type K Chromel vs Alumel thermocouple if the meter terminals are at 25 Deg C the reading will be 4.095 mV if the sensor is at 100 Deg C. The mV reading will vary as the cold junction temperature changes even if the sensor remains at 100 Deg C. So lets say for a practical experiment if you were to put the sensor in a container of BOILING water you should measure about 4 mV or 0.004 V DC.

Regards,

Rosscoe
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Follow Up By: geocacher (djcache) - Wednesday, Apr 09, 2008 at 23:25

Wednesday, Apr 09, 2008 at 23:25
Saved me some typing Rosscoe.

Scientific Instrument Maker??

Dave
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Follow Up By: Ircon - Thursday, Apr 10, 2008 at 08:37

Thursday, Apr 10, 2008 at 08:37
Hi Steve,

Yep, that's what it says on my indentured documents.
Long long time ago (1964 to 1968).

Worked for Foxall/Foxboro; Leeds+Northrup; and Honeywell.

Now down a slightly different path. More into factory automation rather than Process Control. Mostly with Siemens PLCs and VSDs

Regards,

Rosscoe
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Reply By: Ian from Thermoguard Instruments - Saturday, Apr 12, 2008 at 13:29

Saturday, Apr 12, 2008 at 13:29
Hi Signman,

Haven't been on here for a while so I've only just noticed your post. Looks like you been given enough info on the theory of thermocouples. As far as I know all EGT gauges use type K TCs. While I can't guarantee this info will apply to all brands of EGT TC, this what have measured on the ones I supply:

Mine are MIMS construction (Mineral Insulated, Metal Sheathed) with the actual thermoelectric junction insulated from the metal sheath (which is 310 stainless-steel, good for use up to 1100 C). So, each wire should be insulated from the sheath (open circuit). My TCs have 2.5m fibreglass insulated, stainless-steel braided leads and typically measure between 11 and 15 Ohms between the leads at ambient temperature.

I've never been sure whether the thermoelectric voltage developed by the TC affects the resistance reading but I've measured them with several different digital multimeters and always get the same range of resistance.

So before despatch I check each TC to ensure open-circuit between each wire and the sheath/braid and 11 to 15 Ohms between the wires. Then I connect them up to the digital indicator and check for 0 C reading in an ice slurry.

Hope this is of some help.
Ian
AnswerID: 298108

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