EEVblog #419 - Thermocouple Tutorial

[xDR1TeK]

Hey Skimask, cool story man. Just to prove, pen and paper can stretch a long way.
Very moving, you didn't need your peers' approval, it is gratifying alone to know that you can do it. It was all you. I don't normally read long posts, but yours was very well written.

Dave, you are the man, thanks for the video. Love it. Many thumbs up.

Oh, just remembered, the book you mentioned, The Art of Electronics, fantastic book.
My instructor advised me to get it. Old book and awfully nice.

[Paul Moir]

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.

You can also silver hard solder them if you need a reliable & small connection but don't have the welding gear.  A propane torch will do.  I used to do them that way until I got a TIG welder.

One thing Dave touched on but didn't really go much into is that thermocouple wire isn't all that accurate.  Special grade K wire for example is +-1.1degC out of the box.  You can compensate for that with calibration, but that can get tricky.

[Wytnucls]

This is how temperature measurement is implemented on the Fluke 17B:
I don't pretend to understand it, but someone might.

[AlfBaz]

I wonder what the voltages for VDD, VDDA and VSS are?
Also whats the res of the cct hanging off pin 3 on U6?

My guess is U6 is a diff amp that is subtracting the temp measured by TMP35 at the junction from the thermocouple temperature

[Wytnucls]

Yes . I think so too. Voltage differential.
Here is TMP35 datasheet:
http://www.analog.com/static/imported-files/data_sheets/TMP35_36_37.pdf
And here is the whole circuit:

[EEVblog]

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?

Yep, that's the military, they have reams of procedures and check lists for everything, and they never change them.
That's why companies like Fluke have to offer for example, an average responding version of the Fluke 28-II, instead of the True RMS one. Because all the procedures and values are based around old average responding meters.

Dave.

[Jon Chandler]

I think their test procedure might have been more valid if the guy making the readings was calling them out, rather than simply agreeing with every number called out...but that's just me.

[Skimask]

Yep, that's the military, they have reams of procedures and check lists for everything, and they never change them.
That's why companies like Fluke have to offer for example, an average responding version of the Fluke 28-II, instead of the True RMS one. Because all the procedures and values are based around old average responding meters.

Dave.

That's no joke...
20+ years of working aircraft directly, plenty of times troubleshooting a problem when I knew something was wrong, but the "book" said it was correct.  And not a thing I could do about it at that particular moment because the paperwork to make the changes had to go up the chain and get back down again, and if I was to go with my gut instinct and something went wrong, it was literally my ass in a wringer.  On the upside, a person could call the problem worse than it actually was and ground the aircraft, if anything just to buy some time to get the phone calls in and get authorization to do whatever it was that needed to be done correctly.
It's a bit better these days in the U.S.A.F.  A few years ago, they put all of the tech orders on ruggedized laptops, each one with the relevant tech info needed to do the job.  Updates come daily thru the network when the laptops are put in "the rack".  Changes can take literally hours vs. weeks/months like it used to.

I digress...A tech rep for Panasonic came to the shop one day to show us their design and how good it was.  He had three of them laid out on the desk for us to play with and ask questions.  I sat down in front of one, played with it, and pushed it off the table, ~3ft drop to the carpeted floor.  Screen cracked, hard drive crashed (heard the click of death when we picked it up), and a hinge cracked.  Got "the look" from the tech rep and from some of my supervision.  I look, laughed, and walked away as did a few other fellow maintainers.
So much for ruggedized...

On the Fluke thing.  Back in the day, the books only called for the PSM-37 meters to be used for this particular job of checking the E.G.T. wiring.  Old dinosaur of a meter, but at the same time, it didn't care if it was cold, hot, wet, whatever.  When the PSM-37 went out of style, they switched over to the 8025A, which didn't like to work below 20F ambient temp, which is a problem half the year up here in North Dakota.  Had to rewrite the books 3 times for that, once for the meter switch, again for the cautions about temperature, and again when we got the 8025B, which worked when it was cold.
Then we switched over to the Fluke 87 series...and all was well...

On another side-side note, before the PSM-37, we had to use an actual Wheatstone Bridge circuit to take readings on the E.G.T. wiring...bah!  Who came up with that bright idea?

[SeanB]

When I was an appy I had to go and read through all the STI's on the base...... found a gem there though, a STI about us instrument mechs and our shoes, and that we were prohibited from polishing them. drove the RSM nuts for about 3 months coming onto parade with dirty unpolished shoes, and pointing him to the relevant order, which he had actually signed for. My solution for when they got too scruffy was the one item we had plenty of, matt black spray cans......... Ever resprayed an entire aircraft using cans of olive drab and desert brown? Did that after going over the outside with a box of scotchbrite pads. paint shop was busy with another, and we needed the chopper fast with a fresh paint job.

[jesuscf]

Not long ago I wrote this little program to generate the NIST table for the k type thermocouple.  It may be useful for somebody...

Code: [Select]

/* This program generates a table similar to the one at   http://srdata.nist.gov/its90/download/type_k.tab */
 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

// if( t < 0ºC)
//     E = coeff[0][0]+coeff[1][0]*t+coeff[2][0]*t^2+...+coeff[10][0]*t^10
// else
//     E = coeff[0][0]+coeff[1][0]*t+coeff[2][0]*t^2+...+coeff[9][0]*t^9+a0*exp(a1*(t - a2)*(t - a2)).
//
// coefficients for E=f(t), E is in mV and t is in oC.

const float coeff[11][2] =
{
	{  0.000000000000E+00, -0.176004136860E-01 },
	{  0.394501280250E-01,  0.389212049750E-01 },
	{  0.236223735980E-04,  0.185587700320E-04 },
	{ -0.328589067840E-06, -0.994575928740E-07 },
	{ -0.499048287770E-08,  0.318409457190E-09 },
	{ -0.675090591730E-10, -0.560728448890E-12 },
	{ -0.574103274280E-12,  0.560750590590E-15 },
	{ -0.310888728940E-14, -0.320207200030E-18 },
	{ -0.104516093650E-16,  0.971511471520E-22 },
	{ -0.198892668780E-19, -0.121047212750E-25 },
	{ -0.163226974860E-22,  0.000000000000E+00 }
};

// Coefficients for exponential portion of equation above
const float a[] = { 0.1185976E+00, -0.1183432E-03, 0.1269686E+03 };

// Ranges for coefficients above
const float range[] = { -270.000 ,  0.000, 1372.00 };

float C_to_mv (float t)
{
	int k;
	float mv=0.0;
	
	if(t<range[1])
	{
		for(k=10; k>0; k--)	mv=(mv+coeff[k][0])*t;
		// mv+=coeff[0][0]; // Not needed because coeff[0][0]=0.0
	}
	else
	{
		for(k=9; k>0; k--) mv=(mv+coeff[k][1])*t;
		mv+=coeff[0][1]+a[0]*exp(a[1]*(t-a[2])*(t-a[2]));
	}
	return mv;
}

void Create_Table (void)
{
	float t;
	char i;
	
	printf("\n oC      0     -1     -2     -3     -4     -5     -6     -7     -8     -9    -10\n");
	printf("\n%4.0f %6.3f", range[0], C_to_mv(range[0]));
	
	for(t=range[0]+10.0; t<=range[1]; t+=10.0)
	{
		printf("\n%4.0f", t);
		for(i=0; i>=-10; i--)
		{
			printf(" %6.3f", C_to_mv(t+i));
		}
	}

	printf("\n");
	printf("\n oC      0      1      2      3      4      5      6      7      8      9     10\n");
	for(t=range[1]; t<=range[2]; t+=10.0)
	{
		printf("\n%4.0f", t);
		for(i=0; i<=10; i++)
		{
			if((t+i)<(range[2]+0.1))
				printf(" %6.3f", C_to_mv(t+i));
			else
				return;
		}
	}
}

void main (void)
{
	Create_Table();
}

[xDR1TeK]

Holy Moly!!

[Orpheus]

Dave, I first saw this tutorial when just after you announced it on Twitter, but you already seemed annoyed with the YouTYube critics, so I didn't post anything here or there, knowing you'd already heard from others, and clearly knew enough to see their logic.

But I just checked, and you still have that copper-only loop demo of the Seebeck effect. Now since I KNOW you would never want to do anything that could be used to support the audiophile "directional copper" fallacy, and you did reemphasize a bit later that a thermocouple could be made from a single copper, I feel obligated to raise my voice.

When you apply the soldering iron to the midpoint of the copper loop, it does experience a Seebeck potential, but that is absolutely NOT what you are measuring on the meter. The isothermal plate assures that the two Seebeck reference points (closely linked to the sockets A and B) are at the same temperature. The meter circuit assures that these point are also at the same potential, absent a real external emf. However when you heat the midloop point L, the Seebeck potentials LA and LB are identical, and A =B.

Unless the wire has some preferred direction (for whatever reason), the meter will see no potential difference at the sockets *except* as some effect other than the symmetric Seebeck potential.

You know that there isn't a preferred direction in the wire and the potential drop is exactly the same on the + and - legs, but many Young Players are excessively swayed by the polarized labels on the meter (or in any schematic) and instinctively assume a "right" direction flow. Surely you remember some problem set in Basic Circuit Theory where a classmate made this assumption or railed against the "impossibility" of the problem due to assuming that a labeled polarity dictated a direction of current flow. (e.g. I've often had to explain that the ideal voltage source shouldn't be interpreted as a battery or such, but as a fixed [guaranteed] voltage drop or potential, and further explained that they'd see real examples of fixed [non-proportional, non-ohmic] voltage drops soon enough.)

I hope you don't feel I'm being a pain. I'm just trying to help knock out a minor ding. Maybe you just meant a few of those sentences to mean something other than what I and others interpreted them to mean, and you could do a quick audio patch.

[reagle]

Here are a few pictures from a cheap Chinese Thermocouple thermometer.
They do read local temp for cold junction compensation, but I don't see much thermal mass near the connectors.

[AlfBaz]

I was under the impression that you are compensating with the temperature of the "cold" junction not the ambient temperature. The mass of the pcb, solder connection and terminal block may create a temperature difference to the placement of that thermistor if you take a reading after moving somewhere where the ambient temperature hasn't had time to "soak" the above mentioned elements.
If this is indeed is the case I'd be tempted to put a blob of heatsink compound to help

[Orpheus]

Both are essential: knowing the temperature at the proximal (near) end of the thermocouple AND having both "near" terminals at the same temperature. Well, perhaps not technically "essential" but it's a cheaper and more accurate set-up, so why not

In order to interpret the thermocouple voltage, you NEED to know the "base" voltage at the ner end of the thermocouple, because the thermocouple voltage only tells you the *difference* between the near and far ends, not the absolute temperature. You also need both near ends to be at the same temperature, or the two legs of the thermocouple will be reading "different differences". Sure, you could measure the temperature at the proximal end of each leg separately and temperature compensate each leg separately, based on its metal, but the parts cost and computation/look-up would be dissuasive -- and you wouldn't necessarily be as accurate, because the standard NIST tables weren't generated with that assumption.

Actually, now that I think about it, if the near ends are at different temperatures, it won't even always be clear what the "exact correct" reading should be: the two "near" temperatures would be measurements, and we all know that measurements almost inevitably disagree a few digits out. Since the thermocouple legs generate Seebeck voltages in the microvolt or millivolt range, the independent variation/uncertainty in the two temperature sensors *plus* the various contact/thermocouple uncertainties might create errors that can't be calibrated away anywhere as readily as a "one temperature" system. A man with two watches, and all that...

[jaqkar]

Hi, I am new here

Just watched the video and some things are not clear to me yet. Hopefully someone can help with clarification.

1. The nist tables work on assumed reference junction temp of 0, if so how do you compensate for ambient temp. Substract temp from final reading or do you take the diff in mv from ambient and measured?

2. If you use for example type J cable which I understand is Iron/Constantan to a point and from there you use a copper connector and copper wires to a temperature controller it will mess up the actual readings because of the 3rd different metal you introduce?


 

[retrolefty]

Hi, I am new here

Just watched the video and some things are not clear to me yet. Hopefully someone can help with clarification.

1. The nist tables work on assumed reference junction temp of 0, if so how do you compensate for ambient temp. Substract temp from final reading or do you take the diff in mv from ambient and measured?

2. If you use for example type J cable which I understand is Iron/Constantan to a point and from there you use a copper connector and copper wires to a temperature controller it will mess up the actual readings because of the 3rd different metal you introduce?

 Yes, it will mess the reading up. Every transition from TC wires to copper wires forms 'new' junctions that add or subtract from the actual measurement. That is why TCs require TC lead wiring (including special screw terminals made from the same metal material as the TC wires use) from the TC sensor tip to the first TC to copper transition which is where the TC compensation sensor measurement must be taken.

 I worked for 30 years with TCs at an oil refinery, we had thousands of them. There main advantage is the very large temperature range that an TC can handle without 'burning open'. They are not particularly accurate outside of lab conditions using expensive 'certified' TC material.

 We started converting over to 3 or 4 wire platinum 100 ohm RTD sensors the last couple of decades for new installations, as they had come down in price over the years relative to TCs and are much more accurate in real word installations. However they could only be used up to about 500 F without using extra rugged and expensive ones that could push up to about 1,000 F. RTD is a much better choice these days in my opinion expect if you have to go up beyond 1,000 F.