thermocouple reading not stable.

[Angelantoni]

Hai

I am reading the data from the thermocouple in 3 differnt ways.

Case 1: creating a task with thermocouple as input, with temperature range -200 to 200deg C

in this case if i am reading the temperature by stating the CJC temp as internal, the temperature read is very stable.

Case 2:created a task with channels as analog input. voltage range to read -0.1V to 0.1v

The voltage is not stable at all, every sample gives a completely different value, but the CJC voltage is very stable.

Case3:Only channels which have thermocouple connected to it are included and a task is created. voltage is read with range as -0.1v to 0.1v

In this case only one channel is very stable, the other 2 channels are giving different voltages for ev
ery samples.

Can anybody tell me what is wrong and how do i rectify it?

Thanx
Arun

[JaredA]

Hello Arun,

I'm a little confused by why you are even trying to measure a thermocouple measurement without directly specifying that it is a thermocouple task. One of the advantages you get from specifying a thermocouple task is that you get to line-by-line configure your ranges and thermocouple type directly in the DAQ driver software. Also, it allows you to specify the CJC source with each channel. As you are probably aware, having an available and accurate CJC is extremely important when making any thermocouple measurement.

There are two things which I believe to be contributing to the variations in measurement you are seeing. 1) Thermocouples produce a very, very small voltage and the accuracy of your device, or the visible range you are analyzing is not s
ufficient enough to capture a proper measurement. 2) There is no associated CJC value with you AI Voltage task, thus you are looking at the raw voltage across the cold-junctions of the thermocouple.

If you can explain to me why you are experimenting with the different methods, I may be able to give you some suggestions on improving them. Otherwise, I would direct you to use the thermocouple task configurations.

Best of luck.

Jared A

[JaredA]

One more thing that I wanted to include in my post was a link to the thermocouple measurement tutorial available on ni.com. It may be found here.

Jared A

[Angelantoni]

Hai Jared

Thanks for the reply, Am trying to use voltage task to acquire the thermocouple reading because i need to perform caliberation on the voltage received. Also i need to acquire CJC temperature sepreately to supply it to the interface. I think it is better to use my own code to acquire the data (from the scratch) so that any bug can be solved. Am not sure if this is a stupid reason...if it is please let me know. Is it preferable to use MAX to configure channels? since this is very transperent to the user am little uncomfortable.

Thanx for the reply

Arun

[JaredA]

Hello Arun,

By no means is this a stupid idea. There's a reason for everything, it's just a matter of understanding it. Just keep in mind that a temperature measurement is not all that complex and that the NI-DAQmx driver has been created and tested exhaustively to guarantee that the Thermocouple instance of the DAQmx Read.vi appropriatly compensates with the CJC specified and converts the voltage signal to appropriate temperature units.

If you are going to read the voltage from the thermocouple and the CJC on different channels, then I recommend using a VI called Convert Thermocouple Reading.vi. This VI may be found on the Function Palette >> NI Measurements >> Traditional NI-DAQ >> Signal Conditioning. This VI will allow you to gi
ve it a voltage waveform, specify the thermocouple type, CJC source and value, basically everything that the DAQmx task set to thermocouple would allow you to specify as well. You can also dig down into this VI to see the actual mathematical calculations that take place if you are unsure of it.

I'm not sure if by MAX being transperent to the user is what you want or not, but I'm going to also recommend that you do use MAX to configure your DAQmx channels and tasks. Once you have them created, you can actually test your settings in MAX such that you can determine whether or not the data you are getting is correct or not before you ever touch LabVIEW. If you have any questions on how to utilize your channels or tasks in LabVIEW, please let me know and I can assist.

Jared A

[Angelantoni]

Hai Jared

Thanx for the reply. As you said am already using the "Convert thermocouple reading VI" to read the voltage and convert it to temperature. But if i directly configure my channels as temperature channels, the reading am receiving is perfect. If i configure them as voltage with range of measurement as +/- 0.1 V and then use the "convert thermocouple reading VI" the converted values are errorneous. Is the range creating the problem? but that cant be as one of the channels gives correct reading while other channels does not, inspite of the fact that all channels are configured identically.

thanx for the answer again.

Arun

[JaredA]

Hello Arun,

Let's try and figure out what's going wrong here. I need you to supply me a few pieces of information and I'll then try and find the appropriate solution for you. As I understand it, you are trying to acquire several voltages off of your thermocouples while also acquiring a voltage from a CJC sensor. You are then converting the raw voltages to temperature using the built-in Conver Thermocouple Reading.vi.

The questions that I have are:

1. What DAQ device are you using?

2. Are you acquiring continuously (what sample rate?), finite (what sample rate?), or on demand?

3. Are all three thermocouples the same type? What kind are they?

4. How are you connecting them to your DAQ device? Signal condi
tioning or a terminal block? What's the model?

5. What is the source of your CJC voltage? Is it stable?

If you can help me out with these, I should be able to setup a quick test here and get something working for you, if not figure out what might possibly be going wrong.

Best,

Jared A

[Angelantoni]

hai Jared

Thanx for the reply.
I found the problem to be actually setting of the range of the voltage, the voltage range set for the thermocouple were very high. on decresing the range the reading were very stable.

thanx for the support

Bi

[JaredA]

Ah, the all too common range issue. That's what I thought it may have been. I'm glad you were able to fix it.

Best of luck.