Using thermocouples?
Using thermocouples?
(OP)
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.
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.





RE: Using thermocouples?
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Using thermocouples?
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.
RE: Using thermocouples?
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Using thermocouples?
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.
RE: Using thermocouples?
RE: Using thermocouples?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Using thermocouples?
TEMP = TEMP - TEMP / 10
TEMP = TEMP + A6
RE: Using thermocouples?
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/Tempera... 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/parametricsearc... TI has 95 to choose from, and they're going to be easier to implement than a thermocouple.
TTFN
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RE: Using thermocouples?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Using thermocouples?
Thanks very much for all the advice.