The CDT probe which measure the air temperature flowing over it.
The Compressor Discharge Temperature (CDT) probe (not to be confused with Conductivity, Temperature, Depth probe used in ships and also called CDT probe), is installed in the engine compartment just after the inter-cooler. The probe is installed with a #40 stainless steel clamp. A large clamp is supplied to fit around the air-port leaving the inter-cooler.
A typical CDT probe kit from J.P Instruments includes the thermocouple type ‘K’ CDT Probe, a stainless steel clamp thimble, one stainless steel exhaust seal washer and one stainless steel screw type clamp.
Working of the CDT in an aircraft: The CDT probe senses compressor discharge temperature of the aircraft compressor and for this purpose, the probe is mounted external to the flow path that receives high pressure air discharged from the compressor. This air flows past at least one thermocouple which in turn measures the high-pressure air temperature and the high-pressure air is then returned to the flow path.
Basically, air from the compressor is discharged through a hole bored into the engine case. This air is then channelled into a housing containing the CDT probe which measure the air temperature flowing over it. The CDT probe in turn sends an electrical signal to the connected Aircraft Engine Monitor or electronic control unit which in turn indicates the compressor discharge temperature to the pilot. The air is then vented to a sink pressure and can be used as cooling or purge flow.
The purpose of wanting to know the comp discharge temp is to prevent chances of detonation that can happen due to excessive heat. The intercooler cools the air so much that the limit temp is never reached, or even approached, so the original equipment CDT gauge becomes irrelevant. Fundamentally you want to know the inlet temp limits for the engine and the temp of the air at this point. If the CDT is below this temp, you are fine. If CDT is above this, it depends on the conditions and the efficiency of the intercooler (or aftercooler as is technical called ).
As we are aware, CDT can be critical under certain circumstances e.g. staging a dual annular combustor for a high bypass turbofan commercial jet engine where fuel-to-air ration can be critical.
JPI CDT probes are superior because of their response speed. Ungrounded (insulated) probes sold by rival manufacturers are too slow to respond to all temperature changes. It is also worth noting that insulated probes, starting at the same point as grounded probes, never achieved true temperatures. The ungrounded, insulated probes are generally 3/16" in diameter whereas, JPI's grounded probes are only 1/16" in diameter. The Fuel Flow indicators manufactured by the rivals have more surface are and therefore lose energy a rate of T'/ This is another reason JPI uses thin probes and not sluggish probes.
Also, JPI's grounded probes are manufactured using a space-age material, Hastaloy-X, that can withstand the harsh sulphur atmosphere of temperature exhaust gas and uses them in all its gages. You can select and buy CDT Probes here: https://www.jpinstruments.com/product-category/probes-sensors/