thermocouple types

[Swanie]

Hi all i'll be brief,
I sit systems tomorrow and can't seem to find what the differences are between a;
'rapid-response' thermocouple (as used in turbo-props?)
and a
'stagnation type' thermocouple (turbo jet/fans)

do these actually exist and why are they different for each application?
cheers

[Ned Coates]

Aircraft Instruments by Pallet page 282 figure 11.18(c) and (d)
Application depends on Velocity of engine exhaust gases.. High in jet so stagnation used where entry and exit holes are not in line, not equal in size, so gas is slowed and stagnated/sampledtyp response 1 to 2 seconds
Rapid responce in turbo prop with equal sized entry/exit holes diametrically opposite so gas flows straight over the hot junction so it responds rapidly.typ respnse time is 0.5 to 1 second

[Blacksheep]

You didn't mention the matter of Power Turbine Inlet Temperature (PTIT) as opposed to Jet Pipe Temperature (JPT) or Exhaust Gas Temperature (EGT). Gas velocity isn't as simple as merely looking at the power plant type - it also depends what temperature you're measuring and where. The gas entering the power turbine of a turbo-prop or helicopter power plant may be moving at a much greater velocity and at a much higher temperature than that in the jet-pipe of a jet engine. It may for all practical purposes be indistinguishable from combustion chamber outlet temperature. The Allison engines used in most of the world's C-130 fleet certainly use stagnation type thermocouples to measure PTIT.

[Swanie]

thanks for your help guys,
86% first try
last one flt planning

[Ned Coates]

Swanie

Well done and good luck for the future

[barit1]

Every thermocouple type has some lag in response - and the faster-response types may erode faster and thus need replacement more often.

Some engines use thermocouples as part of a control loop to help prevent engine overtemp. operation. In this case, faster response is necessary to stabilize the control (prevent hunting).