Using thermocouples? 1

[ziploc]

I'm building a relatively simple circuit based on an Arduino to serve as a console for my powered paraglider. I want it to monitor RPM, cylinder head temperature, altitude, etc. At this point I have some questions about interfacing with the thermocouple used for monitoring cylnder head temperature.

I've currently got a J-type thermocouple plugged directly into what I'm pretty sure is a completely passive gauge (i.e. no powered). Works great. But I want to read that same thermocouple with my Arduino for this project. From what I've learned, there are two problems with reading this sensor directly with an Arduino analog input.

The thermocouple really only reports relative temperature (the hot junction relative to the cold junction. I want the actual temperature of the cylinder head.

Also, it only puts out 55 µV/°C - so I'll need an amp.

Thus - I found this IC that provides both the cold junction compensator and the necessary amplification:

http://cds.linear.com/docs/Datasheet/LTK001fa.pdf

Unfortunately, I'm significantly too dumb to understand from its data sheet how to use it.

Among my questions:
J-type thermocouple connects to pins 2 & 3?
10 mV/deg-C output goes to Arduino analog input? Is this pin 6?
What is "Vos Trim" (pins 1 & 8)?
What is V+ and V- (pins 7 & 4)? I'm assuming that for use with my J-type thermocouple this is just Vcc and Gnd (0 and 5V).
What is "over comp" (pin 5)?
I don't understand this business in the description:
"direct thermocouple outputs of
60.9µV/°C (E), 51.7µV/°C (J), 40.6µV/°C (K, T), and
5.95µV/°C (R, S). It also has a 10mV/°C output which can
be scaled to match any arbitrary thermocouple."

How do I tell it which type of thermocouple I'm using?

Admittedly, I'm even more lost than usual on this one. Thanks for any tips.

 

[Skogsgurra]

I can understand that you feel a bit lost. You have chosen one of the more complex and complete solutions. I do not think that you need all these things.

First, this device works with tvo supply voltages and you are going to use one, +5V.

Second, you will probably not need cold junction compensation. It is much easier, and usually adequate, to adjust for that in the Arduino. Or not adjust at all and accept the error.

Third, using a simple amplifier with an opamp with low offset and temperature drift is a lot easier. It will also work well with a single +5V supply.

Google on until you find something that you find more understandable and go from there. LT stuff are for demanding applications and are demanding in themself.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ziploc]

Thanks for the response Gunnar. I wondered about using something simpler, and there is in fact a Maxim IC that provides temperature compensation and A2D conversion. I would have to read that as I2C instead of analog (which I think is OK), the only down-side is that it's only available in a surface mount package (SO) - so it's less convenient for me to prototype with.

That being said, I'd still like to understand some of the stuff on the LT chip - if only for the sake of learning.

You say: "this device works with tvo supply voltages and you are going to use one, +5V."

I assume you mean "two" supply voltages - right? I think that's right. It's output can go negative or positive, but in my application will always be positive. If I understand the spec correctly I should be able to supply it with a single 5V source. I assume that means I'd connect -V to Gnd and +V to 5V. Is that right?

Regarding the adjustment for cold junction compensation in the Arduino, I assume you're meaning that I can just add the ambient temperature to the temp measured with the thermocouple - is that right? That would be an easy solution since I am getting temperature and pressure from a BMP085. When you suggest just accepting the error - wouldn't that be a very significant error if I compare flying at 80 deg-F vs. flying at say 35 deg-F? It seems that would change my cylinder head temperature reading by 45 degrees. Is that right?

But in the end, I think I like your idea of using a simple op-amp with low offset and temperature drift. Do you have one you'd suggest?

Thanks again.

 

[Skogsgurra]

Yes, two, of course.
Yes, you can run it off a single +5V supply. Didn't read the data sheet.
If you have ambient temperature available, that would be fine to compensate for cold junction with.
Re opamp, I have used the lowly 324 quad opamp and calibrated away the offset. The offset drift isn't so terrible and if you need better opamps, there are really good ones available at LT and other linear houses.
Yes, 45 degrees error it is. If that is a significant error or not, I can't say. I thought that an engine was running quite hot and that you were interested in seeing significant and sudden changes. But if you want to know the real temperature, a 45 degree error can't be ignored.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ziploc]

Great. Thanks again. I've got some instrumentation amps from a previous projects, but they're very small surface mount components - so I'll pick up some DIP op amps.

This is a 2-stroke engine that's running at relatively high performance, so it tends to run near enough to the edge that a 45 degree error would make me a little nervous. But this is also about me learning.

Thanks for your help.

 

[ziploc]

How do you feel about this op-amp:

http://www.datasheetcatalog.org/datasheets/166/49945_DS.pdf

 

[Skogsgurra]

The typical input offset drift is 7 microvolts/C, which is an order of magnitude less than the thermocouple output. So, yes, I think it will do a decent job. With the Arduino, you can use the other opamp to measure offset drift and compensate for it. They are usually identical between the two opamps on a chip, so with this technique, you can get excellent results.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[OperaHouse]

I built a fridge temperature controller with the UNO and I used five diodes in series for the temperature sensor fed from +5V through a 2K resistor. Easy to calibrate using a glass of ice water and just anbient temp and inserting those numbers into the MAP function. One problem I found was the numbers I used powering it from the USB cable didn't match those when it was powered by the on board regulator. I fixed this by cutting into the USB cable and disconnecting the +5v line. I "averaged" the numbers using these two lines of code. Divisor is the multiplier and number of samples.
TEMP = TEMP - TEMP / 10
TEMP = TEMP + A6

 

[IRstuff]

Seems like the temperature monitoring is absurdly complicated for a DIY project. I mean, if you want to do this much complexity, knock yourself out, but just know that you are reinventing a whole bunch of wheels. Consider instead,

http://www.ti.com/lit/ds/symlink/lm92.pdf

which is a SINGLE COMPONENT, DIGITAL OUT, ACCURATE, temperature sensor. It's accurate to better than 2°C from -25° to +150° OUT OF THE BOX. It would take a couple of days to calibrate and verify a bare thermocouple analog sensor module to that level of accuracy for a DIY.

OR, even better, I wrote all that stuff before I went to SparkFun, which I should have done first, there's

https://www.sparkfun.com/products/9418

http://www.sparkfun.com/datasheets/Sensors/Temperature/tmp102.pdf

which is 0.5° accurate from -25°C to +85°, ready to be integrated with the Arduino, and comes with sample code.

For other temperature sensor choices, see:

http://www.ti.com/paramsearch/docs/...log&familyId=611&uiTemplateId=NODE_STRY_PGE_T

TI has 95 to choose from, and they're going to be easier to implement than a thermocouple.

TTFN
faq731-376

 

[Skogsgurra]

A cylinder top is probably hotter than 150 degrees - and definitely hotter than 85.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ziploc]

Yes, my CHT runs in the 200-240 deg-C range. I think I'm really liking the idea of putting the thermocouple through a simple op-amp (already ordered), and compensating for the cold junction using the temperature I'm already getting from the BMP085 (which I nominally use for altitude and vertical speed). If I didn't have that temp. available I might well use the 5 diodes in series. Very clever.

Thanks very much for all the advice.