<VV> re: CHT readings

[Stan East]

On Sun, 24 Oct 2004, tom zimmermann wrote:

>           I have a Raytek Rayriger ST2 I want to use to check actual head
temp.<snip>


to which Frank Parker replied:

You need to be careful using an IR meter for measuring temps of alum or
similar shiny surfaces. They are designed for a "black body" surface or an
emission of about 0.9- e.g. typically a black surface. Some meters have a
knob to adjust for measuring the temps of surfaces of different emission.

I had my meter at last weekends C5R race at Laguna and was measuring the
temps of the radiator, oil cooler, thermostat housing ( yes, we use one to
avoid having to tape radiator openings). When I measured the temp of the
dark blue hose and the alum tubing connected to the hose, the alum tubing
measured 60 deg cooler than the hose even tho they are about the same
temp. The meter was fooled by the shiny surface of the aluminum tubing.
Spraying a little flat black paint would solve that problem but I suspect
you do not wish to do that with your heads, tho you could mask a small
area, run your tests, and scrape it off.


Thanks Frank for writing up you experiences, they match mine. I use both a
portable thermocouple instrument (pyrometer) and the Infra Red Rayteck at
work. To your excellent comment I would add be careful near other heat
sources that can radiate in front of the surface you are trying to measure,
the emissivity error can throw off your reading. (e.g. if there is a lot of
heat radiating from the exhaust system the temperature of the cylinder head
behind that radiated heat will not be read by the Raytek IR meter)

At work I have over 50 plastic injection molding machines running 7/24 with
at least 10 thermocouples per machine/mold combination. These control the
temperature of the plastic resin as we melt and mold it. We use the Raytek
IR to check to see if things are in the correct range and then monitor with
a real t/c if things seem out of order. On "stack" molds where a heated
steel spruebar carries the plastic past the first molding face into the
center section of the mold we are always fooled by the heat radiation
emissions from the spruebar and can not get true temperatures of the cores
and cavities on that molding face, therefore in this application the IR
meter is useless.  Raytek and other manufacturers have Tables of Emissivity
to adjust for different materials than black bodies however they often do
not account for different finishes of the material.

On another of the thermocouple issues here on VV, a small correction to
previous statements by others on the list:

The electrical potential called SEEBACK Voltage (Thomas Seeback 1821) is
between the two dissimilar metals of the thermocouple. This voltage (in
millivolts) changes with temperature. This voltage is present without any
compensating junction. In other words in any wires A and B where A and B are
different metals the junction between metal A and metal B with create a
voltage difference (and therefore a voltage current) which can be measured
and equated to a temperature. The reference junctions or compensating
circuits within the meter compensate for the errors created by the addition
of other dissimilar metals within the meter and also give the equivalent of
a reference junction for the temperature measurement.

Practical consideration from this are; make sure all junctions between the
actual thermocouple measurement point and the meter are at the same
temperature on both sides of any junctions (male and female); make sure that
both male and both female junctions to the circuit (metals A and B) are at
the same temperature or you will introduce an error into the temperature
reading. Use the same type of thermocouple wire all the way to the meter (or
gauge) or the materials supplied/recommended by the gauge manufacturer. A
bad connection would be to have an additional junction on metal A to metal A
within the lower engine shrouds and the junction on metal B to metal B
outside the lower engine shrouds in the air stream below the Corvair. The
actual temperature at A to A and B to B is unimportant but the temperature
of A-A and B-B must be the same (the equivalent of an isothermal block).

If anyone would like to learn way more than they need to know about
thermocouples you can order a copy (free) of the Omega Engineering
Temperature Handbook at:

www.omega.com

There is way more detail about t/c, thermisters, RTD and IR temperature
measurement that 99% of us need to know.

Stan East