WebDAQ 316 temperature data jumps unexpectedly

[DAQman]

Posted on behalf of a customer

We are trying to measure the surface temperature on a water-cooled stainless-steel shell that is heated by a DC current from 0 to 8 vdc. Our test lasts for 4 seconds with the temperature peaking at 150 C°. The instrument we chose is the WebDAQ 316 thermocouple data logger because of its 75 Hz maximum sample rate.

When heating the shell, the data results give us an unwanted jump in the temperatures. When the DC current source is turned off, the temperatures seem behave correctly. We have tried both unshielded and shielded thermocouples and the results are the same.

We use K-type thermocouples that have the weld exposed and we attach it to locations on the shell in the attached picture.  The WebDAQ 316 input channels are specified to be differential so we believe 0.0 to 8.0 vdc should not be a problem. Could you provide guidance as to how to correct this behavior?

 

 

[DAQman]

Hello,

The WebDAQ 316 channels are not electrically isolated, and each has a reference resistor connected to ground (COM). The design is such that an accurate temperature measurement is not possible if the grounded thermocouple tip exceeds ±1.2 vdc. This is the common mode specification for its ±78vdc input range.

Because the temperature is not extremely high, you could investigate using temperature conducting epoxy to attach the thermocouples. If used correctly, the epoxy can also isolate the tip from the metal surface. Another possible solution is Kapton© high temperature tape as an insulator.

John Rys

Measurement Computing