Thermocouple k type fluctuation reading

[Jacouille]

Hello everybody,

I have a problem with the value of thermocouple K with the device USB 6225 - fluctuating value & non sense value.

My question is :

Thermocouple K the ideal thermocouple with USB 6225 ?

Would not it be better to have another thermocouple like PT100 or other thermocouple ?

Many thanks

[aputman]

Briefly looking at the device page, the USB6225 is a multifunction DAQ board and is not specific to measuring temperature.  I have not had any luck trying to read thermocouples with multifunction devices.  You'll need one with cold junction compensation (CJC).  Otherwise you will get fluctuating and inaccurate readings, as you have discovered.

[MHawk]

We use a similar DAQ (USB-6229) for measuring type K thermocouples.  We measure the cold junction temperature by adding a precision resistor and a thermistor in series (voltage divider) to an Analog Output channel.  We set the Analog Out to a constant voltage, then use one of the analog input channels to measure the voltage drop across the thermistor. Usings Ohm's law to calculate the resistance, it is just a matter of looking the temperature up in a table built from the data sheet.  The whole thing is calibrated annually to ±1 C or better.  YMMV, since it depends on the accuracy of the cold junction circuit and the thermocouples used.