DAQ for current & Thermocouple

[loghmani]

Is it possible to read thermocouple signal with NI 9207 or it is necessary to use NI 9219?

I know that thermocouple generate mv versus temperature rising. So we could read it with 9207 and transfer it to temperature by lookup table? Is this voltage linear? Is it possible ?

[lvrat]

NI 9219 has a build in thermistor for CJC calculation. With NI 9207 there are voltage channels but selecting the DAQ will also depend on what is the O/P voltage you want, resolution, number of channels, sensor current rating etc.

[loghmani]

The question is this:

Can I use a DAQ which supports only voltage(mv , v) for reading thermocouple 's data? and how can I do this?

[TimmTheEnchanter]

Hi loghmani,

Theoretically, you can use any analog input voltage device to read a thermocouple input, including the 9207 DAQ device.  However, in practice, it is a bad idea.  There are a few reasons for this, including that the 9207 does not have any CJC compensation onboard, and that the 9207 is designed to look at a broader range of voltages outside of the thermocouple range.  As such, the 9207 will have less resolution for the thermocouple.  Referring to p. 16 and 17 of the 9207 manual, it looks like you can only set +/- 10v, which means your resolution will be spread over that entire range.  Considering thermocouple temperature changes are specified in microvolts per degree, your resolution and accuracy will suffer using the 9207.

You might also find this DevZone helpful:

Signal Conditioning necessary for Temperature Measurements

http://zone.ni.com/devzone/cda/tut/p/id/11892

Furthermore, because the 9207 has a delta-sigma converter, it is more designed to measure transients in a signal, not precise, slower measurements, as a thermocouple would provide.  I would highly recommend looking into another device, such as the 9213, which is designed for measureming thermocouples.  The manual (see Resources>>Manuals>>User Guide and Specifications) has detailed information about accuracy for various thermocouple types.

Hope this helps,

Andrew

[TimmTheEnchanter]

Hi loghmani,

I just realized my earlier post wasn't incredibly clear.  While both the 9207 and 9213 feature a delta-sigma ADC, the 9213 is more designed for these kinds of temperature measurements (slower, not changing as quickly).

Regards,

Andrew