Non contact thermometers

[jimbo]

Well, this is really a cooking question....but here goes!


We had the idea of using an infrared non contact thermometer....like for example the HVAC guys use, to check the temp of water heating in a pan, or oil heating in a fryer. I have a small, inexpensive one, and when I try to do water ( haven't tried it on oil yet) it seems to read right through the liquid and pick up the temperature of the steel pan. I put a pan on, the water was still cold, and almost immediately, the bottom of the pan was hot enough to burn your finger and the thermometer was reading 128

Is it possible to measure the temp of cooking oil with a non contact thermometer?

Help me out here!!!

[Joe Steam]

Most non-contacts us IR radiation to measure temperature, and rely on the emissivity of an object to emit these IR waves. So that a black object will show a higher temperature than a shiny object. So when you point your device into a pan filled with water or oil, it tries to read the temp of the bottom of the pan, but the water/oil block the IR waves. I think it would be very difficult to get a good reading using an inexpensive non-contact that does not have multiple wavelengths or emissivity adjustments for your application.

[Thatguy]

I would not have expected some of these values.
http://www.infrared-thermography.com/material-1.htm

[Dana]

Water is highly emissive, but not fully transparent to infra-red. The bottom of the pan and lower film of water can be quite hot, while the surface is at a much lower temperature. 128F isn't hot enough to burn you finger immediately, but it will over several seconds. Reflected IR off the sides of the pan can also interfere with an accurate measurement.

Stainless steel, carbon steel (not rusted), chromium, aluminum, and copper are all realtivly low-emissivity, and will give a false-low reading with an IR thermometer.

The color in the visible spectrum isn't a very good indicator of a material's IR emissivity. It's easy to think that black as most-emissive and white as least emissive, but there are myriad counterexamples of both. "Cool roof" finishes tend to appear white in the visible spectrum (for better solar reflectivity), but has quite a different character in the infra-red to be able to shed heat via radiation. Bare aluminum or chromiuma have high reflectivity across the spectrum, but will get hotter than most other materials in direct sun due to it's inability radiate away even the smaller fraction of heat that it absorbs, and has to rely on convection cooling to the air.

The IR emissivity of cooking oil is also quite high- higher on average than mineral oil, according to this table.

[jimbo]

I appreciate the feedback. To summarize, in English!, it sounds like if I want to know the temp of water of oil, I just need to dip a probe thermometer in there!

[jadnashua]

I appreciate the feedback. To summarize, in English!, it sounds like if I want to know the temp of water of oil, I just need to dip a probe thermometer in there!

And, the better IR thermometers also have the ability to use a probe.

[hj]

The better multimeters also use a thermocouple to read temperature.

[jimbo]

I have lots of thermometers, including probes for my DMM. Wifey was attracted by the seeming speed of measurement, and not getting a probe "dirty". This was an "as seen on TV" dealybop, but I think I will pass on it

[jadnashua]

The better IR thermometers allow you to adjust them to the emissivity index, and thus get a decent temp reading from most anything (assuming you can find the proper valve to dial in!).