Thermocouple changing setting

I have three SNAP-PAC-AITM8 cards in my system along with various other cards (analog, digital, etc.). The system is running a combustion system for testing. Lately, something is malfunctioning in the system and some or all of the t/c’s will change from type K to +/- 75 mv. Usually the other problem that corresponds with this is the three blowers that are controlled via VFD’s with a serial card will also start operating erratically. The speed will change say from 40% output to 0% and back to 40% output randomly even with the operation in manual mode instead of in PID mode). When the problem is detected, I shutdown the system and I have to use PAC Manager to change the t/c’s back to type k’s. This will happen randomly. Sometimes multiple times a day or sometimes it won’t happen for over 2 weeks.

Two things, firstly, check your rack voltage. This sounds a LOT like you are getting some low voltages on the rack.
It could be either a bad PSU or it could be noise on the mains and the PSU is not able to filter it and is causing the rack to glitch. (This possibility is going to tough to track down, but I have had this exact issue with some racks at the hospital a few years back - we installed a small double conversion (this bit is important) UPS to test if the mains were the problem, they were).

You need to measure between 5.0 and 5.1 volts between the input terminal (-ve) and the far end of D2 (+ve) on the SNAP rack.

After doing this, and making note of it over a few hours, if it looks rock solid, give support an email and get them to start working the issue.

Thank you for your help. I checked the voltage and sure enough the voltage was around 4.99 and 5.0 V at the rack at the location you pointed out. We will be increasing the voltage and hopefully we won’t see the problem again.
We have a UPS system, but I am not sure when it was last checked so we will check that and replace if needed.

That 0.1 volts makes a difference to the modules at the end of the rack.
For sure you should turn the voltage up a bit. You can go as high as 5.2, but no higher.

Some docs to help;

Longish post about UPS’s and rack power supplies in the Forums; Opto 22 SNAP PAC power supply and UPS recommendations

We had this same problem – Type K getting reset to default (+/- 75 mv) on any of the SNAP-AITM modules. The way we fixed it was to make sure you Store the configuration to flash memory from PAC Manager, any time you make a change to the I/O config on the rack. We use EB1 brains, but I believe this is true for any rack-mounted brain/controller. Whenever there is a power bump, if you do not have the config saved to flash memory, the thermocouple I/O point resets itself to the default, which is always +/- 75 mv.

So, saving config to flash and having the rack on a UPS are two layers of protection for your failure mode. Yes, you should check your UPS. Good luck!