measuring millivolts cDAQ NI 9205

[IshanG]

Hi guys

I am trying to measure a voltage (in millivolts from the ends of a thermocouple) using NI9205. However when i tried to enter the minimum and maximum value for the voltage in the virtual channel to measure voltage i am unable to feed in voltage in the range of 1 mV.

Can anybody help me.

Ishan 

[Mark_SAI]

Hello Ishan,

From what I can understand, you are trying to set the minimum/maximum value of the voltage range to 1mV range.

The minimum allowed range value is -0.2V and the maximum is 0.2V. This will give you a resolution of around 0.006mV per bit.

If I have misunderstood you, please clarify!

I hope this helps!

[IshanG]

hi macaba

yeah you got my question right.. i was trying to set the minimum and maximum for the virtual channel.. and when i used an ice bath as a cold junction in a conventional experiment (using constant temp bath) my readings were varying from 1.6 to 3.5 mV for temp range from 50 C to 110 C... i had posted a doubt previously where i wanted to know how i could measure such voltages in a 9205 since this is the only module i have at my disposal... can you please provide me with some insight.

ishan

[IshanG · Accepted solution]

i was thinking of using an amplifier circuit with a provision multiplexer so that we can have multiple inputs.. please advice

[Mark_SAI]

Hello,

Have you tried using the DAQ Assistant?:

I see no reason why this would not work with your 9205 module, you just don't have any Cold Junction Compensation (CJC) built in.

Hope this helps!

[IshanG]

hi mark

thanks for taking all the pain of explaining me. I tried to peform the experiment using 9205.  In the setting panel, there is an option for calibration. I clicked on it and entered the sampling rate and sample number (as multiples of 60 and 100 resply). Next in asks about multiple channel calibration and three columns title "reference", "uncalibrated" and "difference". what do these mean. I suppose it is asking for a reference temperature for which it takes the corresponding voltage (to be used for cjc). what does 'difference' mean? am i right in assuming these facts.. if possible can you explain me the steps please.

thanking you once again.

Ishan 

[Mark_SAI]

Hello Ishan,

I notice in your other thread that your voltage scaling question has been answered by Sara.

The calibration option in DAQ Assistant is there if you are using a thermocouple module and you have 2 known reference temperatures, this will improve the accuracy even more. However if you are using the 9205, this will not help a great deal.

For future knowledge:

Reference is the temperature of your external source

Uncalibrated is the temperature read from the thermocouple module

Difference is (reference - uncalibrated)

If you are still wanting to measure the mV values, then I suggest the following DAQ Assistant:

With these settings:

This will then give readings like:

Where the oscillation you see is 50hz mains noise. You could average this out.

I hope this helps!