Using the Measurement Computing DAQ (USB-TC) to measure C-type thermocouples

[Lwin D]

Hello,

I am interested in using a C-type thermocouple for high temperature measurements. Our thermocouple DAQ, Measurement Computing's USB-TC, cannot directly interface with C-type thermocouples (it is not of one of the included settings), but I wanted to try and back-door it with the following method:

1. Connect the C-type thermocouple to one of USB-TC's thermocouple channels (for example, channel 0)
2. Set Channel 0's thermocouple type to a thermocouple type with a similar EMF/voltage range as C-types, such as K or J-types (see below figure of thermocouple output voltage to temperature)
3. Set Channel 0 to read out not as a temperature, but as a voltage reading (I can accomplish this with Measurement Computing's python package, mcculw)
4. Convert Channel 0's read out voltage to a temperature using NIST's chart for thermocouple voltage to temperature for C-types (link: https://sea.omega.com/temperature/Z/pdf/z239-240.pdf)

Currently, my main concern is how to account for the reference junction temperature in the conversion of Channel 0's stated voltage and the final C-type thermocouple temperature, assuming the reference junction is at 20 C. From the USB-TC's manual, I understand that the DAQ automatically corrects for the cold junction compensation by subtracting it from the analog input's thermocouple voltage measurement. However, is this correction performed when I set Channel 0 to read out voltages rather than temperatures? If not, how do I calculate the cold junction compensation voltage myself?

Finally, are there other inaccuracies/obstacles that I should consider that might make this C-type measurement impossible/not useful? Thank you in advance for any and all advice.

[Nick Wright]

I think the voltage reading function in mcculw only applies to the USB-TC-AI and not the USB-TC.  I do not know of a way to get a voltage reading from the USB-TC directly.  You may be able to get it in a roundabout fashion in this manner:

1. Configure the USB-TC to a similar voltage range TC type such as J.
2. Read the thermocouple temperature T1 and the appropriate CJC sensor temperature Tcjc
3. Calculate the corresponding type J CJC voltage using the J thermocouple data (Vcjc)
4. Calculate the thermocouple voltage using the J thermocouple data (V1)
5. Calculate the raw thermocouple reading voltage Vraw = V1 - Vcjc

You can then calculate the type C thermocouple temperature:

1. Calculate the type C CJC voltage using Tcjc and the C thermocouple data (Vcjc2)
2. Calculate the type C thermocouple voltage V2 = Vraw + Vcjc2
3. Calculate the thermocouple temperature T2 using V2 and the C thermocouple data

[Lwin D]

Hi @Nick Wright, thank you for your response. One quick question, how do I calculate the CJC voltage given the thermocouple type and an ambient temperature of ~20 C? Do I just find the corresponding thermocouple voltage for 20 C for J and C type thermocouples?

[Nick Wright]

1 minute ago, Lwin D said:

Hi @Nick Wright, thank you for your response. One quick question, how do I calculate the CJC voltage given the thermocouple type and an ambient temperature of ~20 C? Do I just find the corresponding thermocouple voltage for 20 C for J and C type thermocouples?

Yes, that is correct.  For best accuracy read the CJC temperature reported by the USB-TC because it could be different from ambient.  

[Henry H]

Hello @Nick Wright,

If using a DAQ-2408-OA to measure C type thermocouples:

1. Would the same procedure apply?
2. How can the CJC output be read/recorded using the DAQ-2408?

Thanks in advance for any guidance you can provide

[Nick Wright]

On 7/31/2024 at 12:15 PM, Henry H said:

Hello @Nick Wright,

If using a DAQ-2408-OA to measure C type thermocouples:

1. Would the same procedure apply?
2. How can the CJC output be read/recorded using the DAQ-2408?

Thanks in advance for any guidance you can provide

The USB-2408 and variants can read voltage directly, so you do not need to convert from temperature to voltage.  However, I do not know of a method to read the CJC temperature directly from those devices.