I see alot of threads about pyros and egt temp ranges,
I can't help but wonder what the lifespan of a pyro probe is.
When I used to work aircraft turbines we would check the calibration of the cockpit guage
and the engine probes at certain intervals. We would check the guage with a calibrated device
and check the probes for proper resistance values.
I wonder if automotive pyro probes lose their calibration over time, should they be changed
after a certain # of miles or years to maintain accuracy?

 

AFAIK, the probe is a thermocouple like is in your gas furnace or WH. A thermocouple is simply 2 dissimilar metals such as iron and copper bonded together at the tip. When it is heated it produces a voltage which is dependent on the metals used and the temperature.

I don't believe the voltage would change over time but the joint could break or you could lose continuity elsewhere which would cause a zero reading. But I could be wrong since it's just a SWAG.

Edwin

 

Alot depends on what shielding your probe has and how hot you get it. I had a standard Isspro prode and it lasted about 4 years or about 600K miles.

 

The way a thermocouple works is that it produces a voltage based on temperature DIFFERENCES. Two dissimilar wires (the materials used are based on what temperature range you want to measure) are simply twisted together at the "hot" point. Then a volt meter measures the voltage at the "cold" point. The voltage is generated over the LENGTH of the wire. So it's based on temp. differences. So...the probe is simply the point where the wires are twisted together then covered with some sort of suitable heat-resistant shield...in our case, usually stainless steel I believe. I pyrometer gauge is simply a fancy/accurate volt meter. Different voltage readings can by mapped/correspond to the temperature difference between the hot end and the cold end.

If the voltage were actually generated in the tip, then you could produce a perpetually motion machine. Imagine a closed system, an oven maybe. You have pyrometer probe, which generates voltage with heat, connected to a small resister. The resister heats up from the voltage produced by the pyrometer probe which generates more voltage which generates more heat in the resistor which generates more voltage in the probe... You get the picture. So you see why the voltage generation is dependant on the differences in temperature.

So you could also think about compensating for in-cab temeruture. The pyrometer gauge reading could vary by as much as 100* I imagine dependant on the temperature of the cold end of the wires, or your in-cab temperature.

 

http://www.omega.com/prodinfo/thermocouples.html

A really good reference on how thermocouples work.

K.