You could use an analogue meter like in a classic car, red-lining around 120 Deg C
I have a thermocouple probe that clips on to my multimeter that is calibrated and quite accurate too, but the access to the thermal imager is a tool I can use because I can. I also had a few other jobs around the house to use it on too.
Ambient temperature by the way was 12.7 degrees at the time...I did record that which I suppose was a good average temperature of our annual climate. A good Temperature for mid March.
I am trying to get a max reading around the expansion tank, not in it, as the sensor I want to use is a contactless level sensor that I plan to use as a coolant level alarm.
About the same price as a level switch and tank replacement unit, but one that will not come in contact with the liquid. It will jump across a smallish gap and will just clip on to my existing tank. I will not even need to drain the coolant.
It has a maximum working temperature of 85 deg but the manufacturer tells me it can be exceeded.
I just wanted to get a good idea of if its possible or not.
Its another option that may be easier to fit and have more advantages, like self checking, will not clog up with oil or debris and is failsafe in operation.