[Stoves] burning rice husk / IR measurements

[ajheggie at gmail.com]

On Wednesday 12 October 2011 21:31:26 Frank Shields wrote:

> From what I have read from people on this list;
> It seems the IR does not actually measure temperature (like expanding
> mercury) but interprets UV waves(?) coming off an object.

IR doesn't measure anything, it's just one part of the electromagnetic 
spectrum. An IR thermometer will attempt to collect all IR int receives 
and interpret that into a readout, this will be an approximation of the 
surface temperature of the object radiating in the IR range.



> Different 
> colors and texture can change those waves so the instrument must be
> adjusted to estimate the real temperature of the object / surface the
> waves are coming off.

Sort of, all objects will emit a spectrum of wavelengths depending on 
their temperature above absolute zero. Planck's law defines this spectrum 
but you will see that it is a spread of frequencies, so the IR 
thermometer has to collate this spread and use this to interpret the 
temperature. 

On top of that not all bodies at the same temperature are as efficient at 
emitting radiation, a shining reflection surface not only reflects 
incident radiation ( which is why you can see your face in a mirror) but 
they won't emit much either, so will always look cooler than their real 
surface temperature to an IR thermometer. That is why perfectly radiating 
bodies are referred to as black bodies, they both absorb all radiation 
that strikes them and radiate all the wavelengths that Planck's law 
predicts at a power rate determined by the fourth power of their absolute 
temperature. All bodies are also receiving radiation from the hot bodies 
all around them.
>
> In a box of constant temperature everything in that box is at that
> temperature. If the readings differ depending on color-texture we
> should be able to adjust the IR detector to the right temperature. Then
> next time we have the IR adjustment set to that surface and we can
> measure the temperature without the accurate bi-metal thermocouple in
> place. Is that how you look at it?

Yes, one thing that helps us is that most surfaces discolour and lose 
their shine with age and temperature, so most household object settle to 
emitting a percentage of the radiation which they would emit as a perfect 
black body and this is often around 95% of black body radiation at lower 
( <500 Kelvin). When balancing radiators in a hydronic heating system it 
is normal to stick a strip of thin dark, matt tape to a surface to make 
comparisons.
>
> I purchased the IR detector you suggested. I like that it comes with a
> probe reading also.
>
> But adjusting the IR reading with emissivity dial to match the probe is
> difficult. Perhaps because when the door is open one must be fast!

And flames are notoriously difficult to measure, you are normally 
measuring  a surface they are heating.