How EGT Probe / Sensors Work
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 ‘Seebeck 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'. 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.
