Infra red devices are very useful. They measure block body radiation, not temperature directly. All metals have well known emission curves for temperature changes that dont involve a change of state. They are relatively insensitive to surface finish
at the temp ranges we are interested in.
Previously we had to use temperature sensitive paints, max temp indicator strips and instrumented thermocouples. Much less convenient than the hand held devices.
I have calibrated the hand held devices. They are consistent for steady state temperatures which is more important than their absolute calibration.
If you keep a log of temps then you'll have a baseline for troubleshooting. On the engine
for example you are looking for hot spots or unexpected variance on cooling
system or exhaust
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