Exhaust gas temp measurement

[Rudi M. Machilek]

The hot side junction is the only part of the thermocouple assembly that is
involved in temperature measurement.  The conduction up the sheath is a
small loss compared to the mass flow of exhaust gas heating the tip.  See:

http://www.omega.com/temperature/Z/zsection.asp

start with Practical Guidelines for Temperature Measurement.

Basically you drill a hole and stick in a thermocouple.  Usually one uses a
compression fitting to hold the element in place.  There is variation in gas
temperature across the cross section of the exhaust, but roughly 1/3 of the
distance across from the top center is a good place to be (or with the tip
in the geometric center of the region with highest velocity).  Once things
turn red, radiation takes over as the major heating source and measuring gas
temperature becomes a black art.  You are also looking for run-to-run
repeatability rather than accuracy.   Keep all of the cold side junctions in
the same place and at the same temperature and your cylinder to cylinder
numbers will be closer.

The size thermocouple element is a trade-off between mechanical stability
and response time.  The Omega tech. ref. section gives response times for
various sheath diameters and junction designs.  I have used 304 SS 3/16"
Type K grounded junction with good success. Inconnel stands up to
temperature better, but moves more.  Clamp the end sticking out of the
exhaust manifold to something or it will snap-off from vibration in a short
time.

There is lots more, but this is a start.

/Rudi

> Can someone please explain how the thermocouple to measure exhaust
> gas temp is placed. I guess you want the sensor in the exhaust gas as
> close as possible to the exhaust port; so I have this concept in
> my head of drilling a hole in the header close to the exhaust port
> and inserting the thermocouple. But how do you insulate it against
> the metal of the header? If you don't, isn't there going to be a
> problem?

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