A Thermocoupler consists of two wire legs made from different metals. These legs are welded at one end to create a junction and this is where the temperature is measured. If heat is applied to the junction, the metal expands generating a tiny electrical pulse called ‘See beck voltage’. The voltage being generated is directly correlated to the heat being applied and the type of metals being used in the thermocoupler. Conversely, if we know the volts being generated, we can figure out the temperature.

1. An analog EGT (Exhaust Gas Temperature) probe is essentially a temperature detector
that that works without electricity because it relies on thermocouplers.
A Thermocoupler consists of two wire legs made from different metals. These legs are
welded at one end to create a junction and this is where the temperature is measured. If
heat is applied to the junction, the metal expands generating a tiny electrical pulse
called ‘See beck voltage’. The voltage being generated is directly correlated to the heat
being applied and the type of metals being used in the thermocoupler. Conversely, if we
know the volts being generated, we can figure out the temperature.
Of course, one does not need to sit with a calculator
or for that matter, do any calculation to figure out
the temperature in the aircraft cylinder. Each GT
gauge is carefully calibrated to indicate the correct
temperature based on the voltage received.
However, it is important to know that since the
voltage being generated is very tiny (measured in
milli volts), it is vital to ensure that the wires leading
to the gauge are not near any electrical source or
anything that might even generate static electricity.
Ideally, the wires should be adequately shielded to
prevent contamination from any external electrical
source otherwise; the Digital Gauges might not indicate the correct reading.
There should be one EGT Gauges per cylinder so you can compare readings from both.
Each analog EGT probe has two wires; one to be connected to the #1 connector of the
EGT gauge in the cockpit and the other (black) should be connected to #3 connector of
the EGT gauge.
Be sure to get the polarity right and also ensure there is good continuity all the way to
the gauge. Also, do not cut the Fuel Gauges leads. If you need to lengthen the wire, use
standard 18-gauge wire to a maximum of 15'.

2. Tip: Before you actually install an EGT, check probe and gauge using a heat source (e.g.
propane torch).
The Visual (spark plug) test for EGT probes.
This is a rough visual check to ensure the EGT probes are working well. This test works
on the basis of the fact that spark plugs change colour depending on the heat being
generated within the cylinder head. At cooler temperatures the spark plugs are dark
coloured but as the temperature within the aircraft cylinder rises, the spark plugs
slowly change colour to tan and at even higher temperatures; to grey / white.
Here’s how the test is done:
After fitting the EGT probe(s), run the aircraft engine and do a visual check of the EGT
gauge(s) and the spark plugs. Initially, the spark plugs would be dark coloured but
slowly, as the EGT gauge indicates 1100 to 1200 degrees F, the spark plugs will have
become tan coloured and at very high temperatures, the spark plugs will become almost
white. You now know that your EGT gauge and probe is working reasonably well.