Circuit to output difference in temperature between two thermistors

Big boys use auto-zeroing loops ;-)

...Jim Thompson

John's probably thinking of the crop of inexpensive chopper amplifiers that is (are?) available these days. Eg. OPA333 which has 10uV max Vos (2uV typical) and maximum 50nV/K dVos/dT (20 typical). Noisy, but for temperature measurement in most situations we probably don't care.

Of course the output of a inverse series pair of thermocouples is not exactly proportional to the difference in temperature. Eg. for type K (Chromel-Alumel):

-50'C~-40'C : 36.2uV/K

So there's something like a +2%/-11% variation in "gain" over -50'C to

Best regards, Spehro Pefhany

As I said, use the same spool of wire for both thermocouples. And something like an AD8628 is typ 1 uV + 2 nV/K offset, and you won't find one that's worse than about 2.5 uV... still plenty good enough for 0.1K net offset. Microvolts used to be intimidating, but need not be nowadays.

As far as offset verification goes, just put the two tc junctions together for a while and note the output. Simple.

There are compensating advantages to the tc's low output voltage, namely their low output impedance, zero self-heating, and inherent accuracy. We routinely get 0.1 to 0.2K accuracy from tc's that are in theory (ie, according to ANSI accuracy classes) not supposed to be that good. I suspect the metallurgy has gotten very good over the decades.

John

Well, somewhere up there I did say "Thermocouples are nicely suited to delta-T measurements reasonably near room temp."

Every temp sensor is nonlinear, even the ICs. With the tc's, the "zero offset" of delta-t measurements will stay very low over a wide range of temps, especially if both tc's are from the same batch of wire.

John

Chopper stabilized anyone?

On a sunny day (Sat, 10 Jan 2009 11:30:19 -0500) it happened Phil Hobbs wrote in :

Phill I tried, and will try again, but I have no access to a.b.s.e so I use:

Same difference... both rely on establishing a zero volt "pedestal".

...Jim Thompson

Not easy to do when there are cables involved. If you just want to measure the temperature of the circuit board, great. There are problems with thermal offsets between two different copper wires--it can be in the ballpark of 100 nV/K. If your thermocouple has a 40 uV/K response, you can easily be 2 degrees off without knowing it. There are a lot of fat 'n happy temperature controllers out there that have this problem.

IC temperature sensors measure the temperature of their leads rather than of the air, because copper's alpha is 300-400 W/m/K as opposed to

Cheers,

Phil Hobbs

I put it on my web site:

Cheers

Phil Hobbs

Przactly.

On a sunny day (Sat, 10 Jan 2009 13:24:45 -0500) it happened Phil Hobbs wrote in :

OK, got it, thank you. Had to look at that a long time .... I agree thermistors are more accurate. But I think perhaps initial accuracy, or tolerance, is bigger then ICs? That is important in case you have no calibration source, or for other reasons calibration is not practical. I like the LM135 because it has analog out, and you can trim it if you want, and it has 1°C initial accuracy. So, with calibration you can set it as close as needed, and it is nice and linear, something you can directly feed into an AD (PIC?) and then, for the OP, subtract there. No lookup tables, no special math.

On a sunny day (Sat, 10 Jan 2009 13:10:53 -0500) it happened Phil Hobbs wrote in :

But there are ICs with digital output, making any cable length irrelevant. Anyways, a PIC next to a LM135 does the same, can send the temp via RS232, or any other protocol, if must be. It all depends on the application. The IC wires may well be in the same area that needs to be measured.

If your application is squishy enough, they can work fine. For precision temperature stabilization--even at a single temperature--they're junk.

Cheers,

Phil Hobbs

On a sunny day (Sat, 10 Jan 2009 14:59:00 -0500) it happened Phil Hobbs wrote in :

I see you are a person of extremes :-) I just wonder about the difference between 'stabilization' and 'stabilisation'. My spell checker insists it is 'stabilisation'.

I do not like thermistors, those are not linear. I like linear for most purposes.

Of course. ;) Millikelvin temperature stabilization is often needed in order to keep diode lasers from mode hopping, especially when there is some practically-unavoidable level of optical feedback.

Your spell checker is an obvious Brit. I'm glad it's got over its New Year's hangover. I'm Canadian, which is an intermediate energy state...we use 'u' like the Brits but 'ize' like the Yanks. This obvious indecisiveness is why the old country is in such a mess. :(

Linear transducer curves are certainly convenient, and (speaking in generalities) things that are linear are also stable, so my prejudices agree with yours. For wide range, lower stability things, I like to use platinum RTDs, as John L. suggested.

Because their R vs T curve is concave-upwards, thermistors can be linearized to some level by adding a resistor in parallel, but since their nonlinearity is so large it works only over a smallish range.

RTDs have much less nonlinearity, but their R(T) curves are concave downwards, so to fix it you need a negative shunt resistance. A buffer with a small amount of positive feedback can do an amazing job of linearizing--you want a noninverting stage with a net negative input resistance (including the bias resistor) of about -40 times the RTD's nominal resistance. Of course it frequently isn't worth the trouble unless you're doing the control in analogue.

Cheers,

Phil Hobbs

There are circumstances where this isn't true. Thermistors don't offer particularly good thermal contact to the substrate whose temperature they are measuring, and if you really want to push down the Johnson noise from your resistance sensor, a big platinum resistance sensor in good thermal contact with its substrate can dissipate enough extra heat to overcome the 10:1 difference in sensitivity.

I spelled this out in a comment I published back in 1978

Sloman, A.W. "On microdegree thermostats", Journal of Physics E: Scientific Instruments, 11, 967-968 (1978).

It's not of much practical relevance - the comment has never been cited by anybody else - unless you really wanted to push the envelope.

I agree that thin-film RTDs on alumina are good for fast response, but very small glass-bead devices like the YSI matched ones are good too--provided that you don't pot them in epoxy or do something silly like that. It's certainly easier to get a good fast thermal interface with a flat piece of alumina, e.g. by indium bonding. I usually use drill holes with hemispherical bottoms and glass bead thermistors, attached with alumina-loaded epoxy and held down by springy leads during cure.

The other problem is the bandwidth. Like any other feedback loop, temperature controllers live and die by their loop gain, which depends on the sensor speed, which in turn tends to slow down quadratically with size and distance from the source. So there's a huge premium on small sensors and actuators, placed very close to the cold plate.

I like to use the monitor photodiode inside the laser can for temperature control--you can get sub-second response times, which means that you can maintain decent control over a wider bandwidth. My best one was for a pocket-sized 3D scanner (sort of a cross between a coordinate measuring machine and a digital camera)...it had a TO5-can diode laser sitting on two big (2520 size) surface mount resistors for heating, the monitor PD for a temperature sensor--it had about a 1.5 Hz control bandwidth, which is pretty good for a temperature controller. (Laser cooling systems can achieve kilohertz bandwidths on very small systems like molecules or micron-sized particles.)

Another cute feature is that its collimation was electrically adjustable--there was a pre-squashed Nitinol ring between the collimator and the laser can, so you just cranked up the heat until the Nitinol expanded just the right amount. That was intended for one-time use during manufacturing--there wasn't enough of a preload to squash the Nitinol down again.

Cheers,

Phil Hobbs

Dang, I thought *I* invented that.

I did once manage a 3-wire RTD connection with zero offset, gain trim, and linearization with one dual opamp.

John