Non-Contact (Optical) Pyrometer & Exhuast Gas Temperature

mrmain@gmail.com · science forum beginner Joined: 27 Jun 2006 Posts: 2

I'm trying to determine if an optical pyrometer can be used to measure
gas temperature in an application such as sensing exhaust gas
temperature of an engine. Normally, this is accomplished with a
thermocouple.

It seems that optical pyrometers operate by detecting energy (normally,
infrared) emitted by an object. If the emisivity of the object is
known, then the temperature of the object can be calculated. It is not
clear if this technology can be used to measure the temperature of a
gas, without supplying a "target" (such as a metal vane) in the gas
flow.

Is it possible to focus the sensor to look at a "spot" in space, and if
so, does the temperature of objects outside of the focal plane (such as
the opposite side of the exhaust tube) greatly affect readings?

My guess is that the IR sensor will be affected by anything in its
line-of-sight, but I'm unsure if it is possible to create optics that
would provide the equivalent of a tight "depth-of-field" (as in
cameras). If it is possible, then sensitivity to emission outside of
the focal plane would rapidly diminish.

Informed persons, please comment...
thanks,
R. Main.

mrmain@gmail.com · science forum beginner Joined: 27 Jun 2006 Posts: 2

I've researched this enough to have decided on a solution, which I post
here for posterity.

The following resources were instrumental:
http://www.omega.com/pdf/temperature/Z/pdf/z063-066.pdf
http://www.omega.com/pdf/temperature/Z/pdf/z067-069.pdf
http://www.flirthermography.com/ media/2004-061-Vollmer_FINAL.pdf

The upshot: Hot gases have low emissivities. Exhaust gases of an engine
will have particulates that should increase the overall emissivity, but
other factors will still make direct optical measurement difficult and
impractical (preventing fouling of the optical window into the exhaust
system, visibility of the opposite wall, etc).

This site:
www.exergen.com/industrl/ irtc/technotes/technote_058.htm
suggests use of an "immersion thermowell" (a closed-end tube inserted
into the gas stream), which will support use of an optical pyrometer.
They claim better accuracy than thermocouples. The only downside I can
see is the potential for a "blow-out" if the thermowell is not
maintained, eventually corrodes through, which allows hot gases to fry
the sensor. Maintenance will always be an issue, and replacing a tube
should be cheaper than replacing thermocouples.

I hope this proves usefull to others...
Cheers,
R. Main.

mrmain@gmail.com wrote:

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