MCC USB-2408-2AO thermocouple noisy signal

[Joe S]

A USB 2408-2AO is configured for seven Type J thermocouples as diffwerential signals on channels 0 to 6 and a 0-5V analog single ended input on Ch7.  With thermocouples connected to the instrument on panel mounted jacks and dangling in air the values displayed are within a degree and steady over time.  The thermouples are stainless steel armored TCs with ungrounded junctions. The TCs tips are sgnetecured to a small aluminum block which is within a horseshoe magnet.  When the magnetic TC assembly is attached to the surface of a heated steel platen on a plastics molding machine all TC values swing wildly and stabilze once again when removed.  We have a strong grounded pwer supply to the instrument.  Is their an isolation/suppression DIN rail module that can restore signal integrity?  What else should we pursue?

[Fausto]

Hello @Joe S.

In InstaCal, set the data rate of the thermocouple channels to 60 Hz.  

 

You mentioned a total of 7 input signals (6 thermocouples wired in differential mode and a signal wired in single-ended mode).  If there is no additional signal, wire the last signal in differential mode with a 100k Ohm resistor from the CH7L terminal to the AGND terminal and retest your application.

If the thermocouple measurements are not stable, then retest your measurements with the plastics molding machine not powered.  

If the thermocouple measurements continue to be unstable, with the plastics molding machine powered off, then disconnect all the thermocouples from the horseshoe magnet and retest your measurements.

If that is not a viable option, then use a thermocouple signal conditioner with a good analog low-pass filter (i.e. 8B47K-13, 8B47K-14 from Dataforth). The mV output signal would connect to the analog input channel on the USB-2408-2AO which should be configured in InstaCal as voltage channel types.  The voltage signal would then need to be scaled to temperature data using a custom written application.

Regards,

Fausto