John,

The sensitivity of the accuracy of IR measurements to the emmissivity
depends both on the absolute temperatures involved and the ambient
temperature (if the emmisivity is 0.5 you pick up 50% less energy
from the sample, but you also pick up the 50% of the ambient energy
that is reflected by the sample). The emmisivity adjustment on simple
temperature probes assume the ambient is enough colder than the sample
for ambient reflections not to be significant. If this is not true
then even if you know the real emmisivity you wont get an accurate
reading.

Having said that I would have expected a rubber roller to have a fairly
high emmisivity & if your ambient temp is fairly stable then you should
get reasonable accuracy by calibrating the sensor using a thermocouple
fixed to a stationary roller.

David Whalley
Loughborough University

>We have recently been measuring the exit temperatures of our layers 
>and panels using a IR Temperature Probe.  These temps were in line 
>with current specifications.  We verified the exit temperatures with the 
>built-in thermocoupler and a temperature probe. However, one of our 
>local sources allowed us to use their IR Temp Probe that took into 
>consideration the Emissivity of the material measured, in this case, the 
>rubber lam rollers.  The exit temp was then 25~30 degrees higher than 
>our initial testing.  
>
>How critical is the emissivity and can it be a factor here?  Does 
>anyone else rely on emissivity to read their exit temps?  The 
>recommended emissivity is .86 (20~30 degrees out of spec) but when 
>using 1.0, the required temp was achieved.
>
>Please advise.
>
>John Gulley
>Compuroute Inc.

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