[Dusk Blue Z]

Sam,

Spot radiometers are like digital cameras, the more they cost the higher the resolution and the narrower the field. I think you’re about right with the 4” target size estimate. The reason the instructions state that it is "Not recommended for measuring on shiny surfaces” is because heat is reflective. Try and measure the temperature of the chrome on your bumpers on a sunny day…

As a coincidence guess what the emissivity of black electrical tape is, yup 0.95. That’s the reason Thermographer’s use it as a constant.

On a practical note I don’t think you are trying to hold a set temperatures like some manufacturing process need to do, just trying to make sure the gauge is reading close. Based on your thermometers accuracy I would just cover the top of the thermostat housing with electrical tape to get an even surface coverage. Start the engine and watch the temperature rise. You should be able to see when the thermostat opens and closes. Just compare these readings to what is being seen on the gauge in the car.

On an impractical note I think MOROSO used to make a thermometer that would screw in to one of the pipe threads on the intake manifold. You could verify the temp of that by using boiling water on the stove (keep in mind your elevation, that changes the boiling point of water).

Mike