How to test thermocouples?

[Lazy Flame]

I couldn't find a topic that covered this everyday problem. Several threads involve it but I was wanting to see a more comprehensive source for info, a "Thermocouples for Dummies"

I have a type K I'm working with and it plugs into a printed control board. I'm not familiar w/that arrangment.

I aasume it shuts down the auger when the exhaust gets to hot?

I heard I can get a millivolt tester and do some math based on a given range (on a table) and temperature and figure it out. Or use a lighter on the TC and get a test reading that should be in that range. I prefer Farenheit over Celsius whenever possible. http://www.pyromation.com/downloads/data/emfj_f.pdf

Some testers do the math. Can it be done without that tool?

I have a thread-in pop-up tester I used to use on mobile home water heaters and downflow gas furnaces. Are all Tc's the same thread size? I paid $13 for the tester 15yrs ago, I don't see it for any less than $30 now. http://www.dan-marcrvparts.com/rvthte.html

Thanx for any experienced input

EDIT- what's the temp range on these? 32 - 1400F doesn't sound quite right. http://www.omega.com/temperature/Z/pdf/z203.pdf

EDIT- They may go from -340 to 1650F I think what I want to know is what is the range used by my stove. I was wondering what the max temp was going thru the vent.

 

[basod]

The gas turbines I work on use type K tc's. I'm not sure what your trying to test on them? TC's either work or they don't, occasionally they begin to fail (open) and will come and go in reading. I'll see if I can find some table info in my old text books. I haven't used them in years as I use calibrated Fluke instruments when I loop check these circuits. The Fluke model 744 has input circuit and selectable type (R,K, J etc.) but at about $7000 probably not a good investment for home troubleshooting. I know Ametek sells a simple loop calibrator for tc's/RTD's. Check out Transcat.com, either way when dealing with mV signals you'll require an accurate meter to produce the resolution with a type K tc

 

[Lazy Flame]

The gas turbines I work on use type K tc's. I'm not sure what your trying to test on them? TC's either work or they don't, occasionally they begin to fail (open) and will come and go in reading. I'll see if I can find some table info in my old text books. I haven't used them in years as I use calibrated Fluke instruments when I loop check these circuits. The Fluke model 744 has input circuit and selectable type (R,K, J etc.) but at about $7000 probably not a good investment for home troubleshooting. I know Ametek sells a simple loop calibrator for tc's/RTD's. Check out Transcat.com, either way when dealing with mV signals you'll require an accurate meter to produce the resolution with a type K tc

Type k seems to be common in pellet stoves. I have read 3 cases of erradic burns and operation due to suspected thermocouplae failure or inconsistencies. That led me to believe that they could be inaccurate without failing completely. The ones I messed w/were as u said. They either work or don't. Ppl were wanting to know if the part was bad before sinkning $70 into a questinoable part.

I'll post or link the threads together, unless of course I get lazier than now, or forget.

As mentioned I had a tool that was super cheap and tested Tc's. I didn't need a meter much less a $7000 one, lol. I was asking to see it there were any more gadgets or easy tricks. I used to have a good Tc onboard my truck and I just plugged in and out of appliances to check for operation, but my stoves Tc plugs into a control board via a 2 srand molex. Water heater Tc's terminate in a standard thread which looks completley different. That difference makes my cheapo testers useless.

This is all based on my completely uneducated guesswork. I may be horrifically mistaken. Again.

I'll see if I can't borrow a meter. Your right, so far they're way to expensive for a household tool. Then all I need to know is what readings are in range for my stove.

Thanx for the help!

 

[UncleAnthony]

You are in right direction on omega.com . They are the t/c experts, and actually have a good thermocouple
introduction book.
Yes , type k t/c's have an extensive temperature range, kinda accurate.

Can use omega type k chart and and accurate millivoltmeter and get the temperature accurately.

BASOD is correct in buying an inexpensive meter as below for this purpose, much easier.

Also as BASOD states, and my experience also, about 99.9%, t/c's either work or are open.
Normally they are arc welded with a clean borox solution, making good contact.
I have seen a few t/c's ( out of a couple thousand ) that actually do not read correctly.
(I worked in military calibration lab for a few years, actually calibrating thousands of these thermocouples ).

here is a digital thermometer for $26 . Probably good enough accuracy for this. Extech also sells a good meter for $88.

Best of luck!

http://www.virtualvillage.com/digit...lebase&utm_medium=shcomp&utm_campaign=Digital Thermometer for K Type Thermocouples

 

[Lazy Flame]

Re: Is there a way to test a thermocouple?

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The easiest way to check a k-type thermocouple is to measure the voltage output with a DVM or DMM. A k-type thermocouple will output roughly 40uV per degree C and a flame from a candle or bic style lighter will burn at about 1000°C. When you connect your thermocouple to the DMM at room temperature you should have about 0v, when the thermocouple tip goes into the lighter flame you will have about 40mV.

40uV/°C * 1000°C = 40mV

Now depending on the type of probe you have on the end of your thermocouple you may not be able to get it up to the full 1000°C so your voltage my fluctuate between 35mV and 40mV.

This is some thermocouple testing data from Omega, it will probably give you more information that you will even want to know about thermocouples and the differences between them. If you want more information just let me know.

http://www.omega.com/temperature/Z/pdf/z021-032.pdf

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Electrical Engineering - Thermocouple Test

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Expert: Dave Nyce - 1/21/2007

Question
A few days back, i saw an instrument engineer use a meter to check a thermocouple.
I was told the parameter to be checked is the temperature but i was wondering
how can there be a temperature since the system was shutdown. At first I thought
he measured the continuity of the thermocouple.Questions are:

1.How to check and verify the thermocouple is in a good condition.
2.What test can be done and what parameter to check?
3.I saw he used a meter Fluke brand. Any idea what is the meter?

Answer
1. A complete thermocouple connection includes two junctions: one is the measuring junction, and the other is the "cold junction". There are various types of thermocouples, like J, K, T, etc., depending on the types of metals used. The charts that show thermocouple output voltages (as in the Omega catalog) assume that the cold junction is at 0 degrees C. The thermocouple that goes to your process (the measurement junction) is only one junction. Usually, the cold junction is simulated inside the measuring circuit. You can measure the process temperature by using a standard volt meter. You would meausre the millivolts of the thermocouple, and then look up the room temperature mV for that type of thermocouple. For example, if your room temperature (where your meter is located) is 20C, and the thermocouple chart shows 5mV for 20C, then subtract 5mV from your measured reading, and then look up the temperature for that voltage in the chart.

But it is easier to just use a voltmeter that has the cold junction compensation included inside the voltmeter. Then you can just read the temperature directly from the meter when the meter is connected across the two wires of the thermocouple.

2. You read the voltage in millivolts (mV) across the two thermocouple wires.

3. The Fluke model 116 has a built-in function for measuring temperature if you are using a type K thermocouple (which is the most common type). So you can connect the meter directly across the leads of a k-type thermocouple and the meter will indicate the temperature (without having to look up anything in tables).

Hope this helps!

Dave