Infrared

" Infra-Red Thermometers - How to use them accurately!

The special thing about IR Thermometers is that they do not contact the surface they're measuring the temperature of. 
What they do is measure the heat emission and then calculate the temperature. This picture shows how it works.

The important point is that not every surface sends the same amount of IR  radiation. It does not only depend on the temperature, but on the material and structure as well. Dark, rough surfaces emit more heat than bright, shiny ones.

The value that describes this characteristic is called the Emission Value. It is a number between 0 and 1. Per definition, a "black object" has the value 1, so an emission value of 0.5 means that this object emits half as much IR radiation. The emission, absorption and reflexion are connected. Emission is equal absorption, while the sum of emission and reflexio are 1. However, the physics and math are not that important for the average modeler   ;-)
Every IR Thermometer uses the emission value to calculate the temperature. To get accurate results, you need to know the emission value of the surface. On some IR thermometers, you can adjust the emission value. On many, you can't. Most of those have the value set to ~0.95. That value is all right for most surfaces, but not for bright or shiny ones. To get more accurate results, paint the surface black or put tape over it.

I am working on exact measurements for different cooling heads. My goal is to determine their emission values, so that owners of an IR thermometer can measure the temperature more accurately. I'm working with purple, black and silver cooling heads. If you want the values for green (CMB), orange (R&B) or another colour, you can send me such a cooling head or a complete engine (preferably a new R&B WSII :-D )

Here are the first results. They're from a purple cooling head for the HPI .15FE. What I did first is calibrate my contact thermometer to 0°C with ice water. Then, I calibrated it to ~40°C using warm water and a fever thermometer.

For the first measurements, I heated the cylinder head to >160°C using a hot air gun. While the aluminum cooled down, I noted down the readings from the contact and the IR thermometer. To have good heat flow between the aluminum and contact sensor, I drilled a 3mm hole and used castor oil. With the IR thermometer, I measured at the top of the cooling head and at the plug. On the right is the data from the comparison. (Click for full scale)

The picture on the left proves that even an expensive IR Thermometer is very inaccurate when used incorrectly! (Click for full scale)

With the knowledge of how big the error is, you can correct the reading... but it's even better to measure correctly. My next task was to find out the emission value of that purple anodized cooling head. To do so, I heated the aluminum to 100°C and adjusted the IR thermometer to various emission values. On the right is the table with the results (Click for full scale)

For the purple HPI cooling head you see on the pictures, I measured an emission value of 0.85

Measurements for black and silver cooling heads will follow.

Greetings, Thomas

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