Author Topic: EEVblog #419 - Thermocouple Tutorial  (Read 15847 times)

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Online EEVblog

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EEVblog #419 - Thermocouple Tutorial
« on: February 03, 2013, 09:01:53 am »
Everything you need to know about how Thermocouples work.
K type thermocouples, the Seebeck effect, the Seebeck coefficient, and cold junction compensation.
Along with some practical measurements with a multimeter to demonstrate the effect.



Dave.
« Last Edit: February 03, 2013, 09:07:48 am by EEVblog »
 

Offline nessatse

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #1 on: February 03, 2013, 10:27:26 am »
Dave,


Your explanation of the NIST tables is unfortunately incorrect.  The horizontal axis represents the units of the junction temperature (0-10°C), to  be added to the vertical temperature which increments in 10's.  It has nothing to do with the cold junction temperature  which is assumed to be 0°C.
 

jucole

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #2 on: February 03, 2013, 10:36:45 am »
doh you beat me to it! - yes the tables provide all the voltages for each temperature from 0 to -270 using a cold junction of 0c
 

Online EEVblog

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #3 on: February 03, 2013, 10:39:12 am »
Yes, oops, have already corrected that in annotation.

Dave.
 

Offline Salas

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #4 on: February 03, 2013, 12:48:36 pm »
F87V reads just OPEn without the their brown Tcouple inserted. Any idea if it has an internal sensor to compare or it works otherwise?
 

Offline Mark Bolton

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #5 on: February 03, 2013, 07:31:05 pm »
I only just stumbled on this blog a few days ago and it is fortuitous that the subject of thermocouples comes up now. I am building an airplane and have a bunch of thermo couples and old gauges that i am hoping to put into service.  Cylinder head / Oil temp  - 400 / 200 deg F. 
 

Online EEVblog

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #6 on: February 03, 2013, 07:43:07 pm »
F87V reads just OPEn without the their brown Tcouple inserted. Any idea if it has an internal sensor to compare or it works otherwise?

Certainly would have an internal temp sensor, it's just not displaying it and has an open detection mechanism. It could have simply always displayed the compensated temp all the time, but then if the probe is broken and you get only the ambient temp displayed, that could be dangerous. i.e not detecting high temps. So better to display OPEN.

Dave.
 

Offline Jon Chandler

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #7 on: February 03, 2013, 08:11:03 pm »
I've come up with an inexpensive thermocouple circuit based on the TI TMP512/TMP513 Temperature and Power Supply SystemMonitor.  The chip has a local temperature sensor for cold junction compensation, and the 40 mV range of the ADC (for measuring shunt voltage) provides good resolution of the thermocouple output.  It's a lower cost alternative to the dedicated chips from Maxim and AD.

The article describes the software written in Swordfish Basic fro PIC18F series micros.  I use a lookup table for linear results.

A Thermocouple Measurement Circuit With Swordfish Software
 

Offline Ketturi

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #8 on: February 03, 2013, 08:24:02 pm »
This was very interesting, keep up Dave! Although table was bit goof-up :P

Is there way to fix those connections in thermocouples? I broke mine but was able to fix wires together by arc welding them, but now it's showing temperature couple degrees off.
Ketturi electronics: http://ketturi.kapsi.fi
 

Offline Psi

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #9 on: February 03, 2013, 09:28:31 pm »
I thought it would be cool to show a bit more energy being produced from the effect.
So i soldered a lamp to a peltier block and took a blowtorch to it :P

Here's the result.


I didn't think to measure the voltage and current.
If anyone really wants to know i can do it again.
Maybe put the steel block into the freezer overnight, that should max it out.



« Last Edit: February 03, 2013, 09:30:57 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline HKJ

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #10 on: February 03, 2013, 09:33:36 pm »
F87V reads just OPEn without the their brown Tcouple inserted. Any idea if it has an internal sensor to compare or it works otherwise?

It does, just short the input terminals with a wire and you can read the internal sensor.
 

Offline Salas

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #11 on: February 04, 2013, 01:48:12 am »
F87V reads just OPEn without the their brown Tcouple inserted. Any idea if it has an internal sensor to compare or it works otherwise?

It does, just short the input terminals with a wire and you can read the internal sensor.

Indeed. :-+ Just a link and we got ourselves a quick room temp indicator.  8)
 

Offline SLJ

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #12 on: February 04, 2013, 02:36:45 am »
I found this a few months ago.  You don't need a temp option on your DMM or analog meter.  Just convert the ohms reading on your meter to the temp (F or C) listed on the conversion chart on the side of the probe.  It does work and it's pretty close.  Cost in 1973 was $12.50 U.S..


Offline LaurenceW

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #13 on: February 04, 2013, 02:45:52 am »
SLJ, I think what you have there is "just" a Thermistor - a temperature dependant resistor element. It produces a change in resistance, not a voltage difference, when heated. the relationship between the resistance and temperature is either non-linear, or certainly not "convenient" (i.e 1 ohm per volt or similar), so it needs a scale to cross-refer the resistance to a temperature.
If you don't measure, you don't get.
 

Offline SLJ

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #14 on: February 04, 2013, 03:14:11 am »
Yes,  as I stated there is a conversion chart on the side of the probe (ohms to F or C) and a larger paper chart in the case.  Not as accurate as my 87V and it does not have the temp range but I found it to be an interesting item.

Here's the chart:  http://www.stevenjohnson.com/data/thermy.pdf

Offline Jeff1946

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #15 on: February 04, 2013, 07:57:18 am »
Couple of comments, Type K is often used because the metals are oxidation resistant.  A simple way to make a junction is twist the wires together than "fuse" them by striking with a hammer against a hard surface.   In the lab we melted them together using a hydrogen oxygen torch with a slightly reducing flame.
 

Offline bitwelder

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #16 on: February 04, 2013, 09:23:18 am »
Question: let's say that the tip of a thermocouple probe got spoiled/damaged. Would it be possible just to cut away a few cm of the wires and join them together on the tip, to have the probe working again?
 

Offline SeanB

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #17 on: February 04, 2013, 09:34:23 am »
Yes, either you spot weld them or arc or gas weld them together, or just quick and dirty wind them together tightly. Some were stir welded together.

A common use for them is in gas valves, where they keep gas flowing to the pilot light, but switch it off if the pilot goes out for any reason. May only be millivolts, but a big one can supply a lot of current, enough to keep a valve open.
 

Offline nicknails

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #18 on: February 04, 2013, 10:37:04 am »
Analog Devices also has a nice whiteboard series about thermocouples.

 

Online AlfBaz

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #19 on: February 04, 2013, 02:46:26 pm »
I remember looking into thermocouples years and years ago, and I wrote some C code to calculate the voltage produced by the thermocouple of choice given the temperature.

It can also create a table of all the nist values for the temperature range specified and saves it to a file. Rename the file to csv, txt or what ever and it will look pretty much the same as the NIST tables Dave showed.

In the attached zip you have an exe which requires no dll's other than what comes with a bog standard install of windows and just runs without any install bs. It worked in windows 98 and It still works on windows7. The source code is attached also and all the math is done in a function at the end of the file

Thought it may be of interest
 

Offline Skimask

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #20 on: February 04, 2013, 06:25:10 pm »
Ahhh...memories....
Great video...

Short story made long...

I used to work B-52's, specifically instruments/autopilot.
Way back when I was a young technician with not a lot of credibility, we had a problem concerning an engine on a particular spot that was overheating.  The jet would land, we'd inspect it the engine, find no evidence of overheating, but ended up changing out a few engines (at great cost, a few million $$$ each) because that's what the book said to do.

I didn't come into the picture until the 3rd engine had been changed.  Previously the wiring had been checked (numerous times or so I was told), gauges cal'd, engines ran up...  Everything checked good....On the ground.

The problem only seemed to happen while it was flying at altitude, with an ambient OAT of...well...we'll say about -50C.  AND the engines were overheating by just about double that amount.  Normal max temp was about 485C and the engines would get up to about 600C before they shut them down.

In this particular case, the book told us to hook up a Fluke 87 in resistance mode, take a reading in "one direction" (e.g. + to +, - to -), then take another reading in "the other direction" (e.g. + to -, - to +), and take the average of those 2 readings to get an overall resistance value for the wiring.  That would tell us if any of the terminals needed cleaning, repair, etc.  I knew why this particular method worked (e.g. the way it was connected, the Fluke was either fighting or going along with the current being generated in the thermocouples).

Well, short story made long (again), I ended up sitting down one night with a chart of wire resistances for the various wire types (which I found in the back of my brand new just bought a week before book "The Art of Electronics"), along with a wiring diagram for the aircraft.
I calculated a fair estimate of each leg's resistance for each wire's type, wrote it down, graphed it, etc.  A lot of math extending way out to the right of the decimal point and a lot of math that nobody else could begin to understand.

At the end of the night, I came to the conclusion that the wiring was swapped (e.g. + for -) where the engine wiring meets the wing.  Problem was to get to that wiring meant a lot of work for a lot of people to pull panels from the aircraft, technicians turning screwdrivers, metal technicians drilling out stripped out screws, and so on.

At the morning meeting (with a lot of heavy hitters and much higher supervision than I cared to deal with since I was basically a young punk), I showed my supervision all of my math (not that one person in the room understood any of it), and tried to explain what I thought the problem was.
At the end of this hours long (seemed like, actually maybe 20 minutes) explanation of what was going on, the big boss (the Wing Commander) directed the flightline expeditor (my boss) to assembled a team of screwdriver turners and screw-taker-outers to assist me....All the while I was getting these looks like I'm a big time dumbass wasting other peoples time.  At this point, I'd been on shift for about 13 hours and was getting anxious.

We got a half dozen folks out there, started turning screws, removing panels, etc.  I was up there pulling screws out with my paperwork showing how I thought wire #1 and wire #2 were misplaced on terminal #2 and terminal #1 respectively.
After about an hour, we got all the panels off and I was the first to stick my head in there.
and...
Came back out with a grin on my face.
I motioned for the flightline expeditor (my boss) to come over and stick his head in there take a look see.
He came out with a look of "W...T...F..." on his face.
30 seconds later, the wires were swapped.

The worst part about this whole thing....
3 days later was this aircraft's next and final flight to the "Boneyard" at Davis-Monthan A.F.B. in Tucson, Az.
You can see that exact aircraft, 61-027 sitting in the "Boneyard", across the street, right at the junction of Quijota St. and S. Wilmot Rd, circled, with an arrow pointing to the troublesome engine #5 and the panel is under the wing right at the pod root.

Like I said, short story made long.


EDIT:  Forgot one small detail...
We figured out later on that when that aircraft had been in depot maintenance, that particular dual engine pod had been removed entirely to facilitate corrosion work on the leading edge and those particular wires that were swapped were 2 of 12 wires NOT in some sort of cannon plug or multi-pin connector...terminal lugs all the way for that bunch.
« Last Edit: February 04, 2013, 07:47:13 pm by Skimask »
I didn't take it apart.
I turned it on.

The only stupid question is, well, most of them...

Save a fuse...Blow an electrician.
 

Offline Wytnucls

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #21 on: February 04, 2013, 06:40:45 pm »
Great story Skimask.
It wouldn't be the first time that electrical wiring was connected the wrong way on an aircraft, nor the last.
I remember an incident where a Lufthansa A-320 got airborne with the Captain's flight controls connections reversed. A sharp First Officer took over, after take-off, saving the day.
 

Offline Jon Chandler

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #22 on: February 04, 2013, 07:35:18 pm »
I used to work in a naval shipyard and remember a couple weapons systems technicians checking out some wiring on a submarine one day.  One guy had the meter, while the second guy was reading the test procedure and recording the results.

"Measure from terminal 1 to terminal 7.  Five volts, right?"

"Yep."

"Measure from terminal 3 to terminal 12. Zero volts, right?"

"Yep."

Amazingly, all of the measurements I heard were exactly right.  Imagine that?  Perhaps these guys helped the Air Force sometime? :)
 

Offline Skimask

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #23 on: February 04, 2013, 07:51:49 pm »
Amazingly, all of the measurements I heard were exactly right.  Imagine that?  Perhaps these guys helped the Air Force sometime? :)
Normally, I'd say, yep, should've been an obvious fix, except that the problem didn't present itself until there was a marked ambient temperature change.  I watched the guys do the calibrations, did one myself, watched the engine run ops checks, we did everything by the book, just like we're supposed to do to keep from getting somebody killed.  This is one of those cases where you expected the guys weeks/months before you to do the right work as well.  And I was the only one with enough balls (e.g. brains) to think outside the box.
And what did I get for a great fix?  An "Atta-boy" from my shop chief at the time.  Didn't hear squat from the higher ups...as usual.  Such is the life of a maintainer...
I didn't take it apart.
I turned it on.

The only stupid question is, well, most of them...

Save a fuse...Blow an electrician.
 

Offline SeanB

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Re: EEVblog #419 - Thermocouple Tutorial
« Reply #24 on: February 05, 2013, 03:12:28 am »
Similar, i got a batch of computers all from the same plane, all marked as fail system test. Politely phoned and asked what the problem was, then had them do the very complex swap out of the big air data computer ( needs autopilot removed, fuel system drained so you can get the tank on top of it out then remove a whole lot of panels held in by rivets, and not blind ones but solid ones, and then take it out , replace and put the big pile back together then test fly it) that was out of tolerance on the one output transformer. They did not like me for that.

Best still was the computer that came in from the training flight of the OC, written on the sheet "First bomb fell on target". I had to phone him ( big guy, but nice enough) and congratulate him, then ask for more detail. He selected a spread of 5 practise bombs, but they were released early, with no1 instead of no3 landing on target. yet another call to my old trainer to run another self check and change something else, but this was much easier, just 4 screws and 3 plugs inside the cockpit on the right.
 


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