Temperature Measurement

[cuatr0]

I've been working with a 9215 module on  Compact Rio. I know this is not the right instrument for this but is the one available for me. I need to measure temperature with a thermocouple but I'm having issues with data adquisition. First I made the document called "Matriz nueva" which is a simulation. Then I tried to do the same for FPGA on the others files I've attached but I all I read is 0. I also tried using the LabVIEW example for measuring voltage with the 9215, but when I connected the thermocouple I got a sinousoidal reading reaching from -10 V to 0, which is obviously wrong. I really need help with the data adquisition, I think the convertion program for voltage to temperature might work, but I can't get the data right.

[DonhJoe]

Hi,

I first have to mention that the NI 9215 module is a very good simultaneous analog input module that can read from thermocouples however it is not one of NI's modules designed specifically for thermocouple readings (It does not include features like open-thermocouple detection, CJC for high-accuracy measurements, etc). Pay attention to its12-bit of resolution and make sure that it enables the detection level you want.

I would suggest taking the "Analog Input-Getting Started-Scan Mode.LabVIEWproj" as a starting point for your application because the thermocouple readings example projects that you can find in the Example Finder implement some Channel or Module properties not available for the NI 9215 as mentioned above (look at the picture for reference on how to find it).

This project uses the cRIO Scan Mode which lets you take measurements without having to program the FPGA. The acquisition rates you can do in that mode are most of the times enough for temperature measurements.

For this example project, you can simply set the IP address of your controller to the one in the project, but it comes with the NI 9201 module by default so you will need to discover your actual module and update the VI's block diagram with the new I/O variables in order to start reading.

Best!

[MarkCG]

Putting a TC straight into the module is not going to work. A thermocouple is a millivolt signal- 0-80mv. So you would be using less than 1/125th of the module's range. The module is 16 bit so 2^16/125 = 524 discrete levels approximately. 1000C / 524 is approximately 2 degrees per level. Add in noise and you will be lucky to see accuracy < 10 C .

Get a thermocouple signal conditioner that scales the TC to a 0-10V analog voltage.

[cbutcher]

If you're going to end up buying new hardware anyway, and this isn't for industrial scale or range temperature measurements, I'd suggest just avoiding thermocouples... Probably an RTD or thermistor will be more accurate and easier to set up than what you have, and cost less than a signal conditioner. YMMV.

[DonhJoe]

The resolution does not apply only to the module's full range but for the range configured for the specific acquisition... If you set the range to 0-1V you will get 16-bit of resolution withing that range.

[cuatr0]

Thanks to all of you for your replies, I'll consider them all and do some research. I understand I'm not using the best equipment, unfortunately I have to use the instruments available for me at the university, so buying new measurement equipment or another module is not an option for me. Again, thanks for your help, I'll take into account the other comments.