Digital gauge. I see your problem.
Well, anything can break & I like the accuracy and despite what many people say I actually find the digital display easier to read at a glance than an analog model, especially given the mounting location I went with. Before installing the gauge I played around with it a bit to see how fast it reacted to temperature changes and it was very responsive, but yeah it is accuracy that isn't really needed in a pyro application on a daily diver work truck. I'm just having fun with this truck and doing some things only for the sake of doing them and learning.
I did contact The Sensor Connection and they sent me this in reply:
Thanks for your purchase. Looks like your system is coming up on about 3 years of age. After use EGT probes will sometimes experience what’s called aging. Aging causes the measurement to shift a bit, 10 degrees is not unheard of. The good news is that this does not mean the probe is going bad. This shift can be taken out in the gauge via the offset function. We’d suggest going into the gauge’s program and adding a -10 offset. Here’s a quick video demonstrating how to program in the offset.
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Nice to know an offset is an option but I'm not going to worry about it. Don't know about the "aging" but at least their suggestion wasn't to just buy another thermocouple and/or gauge.
Nice tidbit of info on Type K thermocouple calibration
https://calibrationawareness.com/thermocouple-wire-calibration-procedure-type-k-thermocouple
CDX was right about them calibrated at 70F, or 73.4F if hairs must be split