Thermocouples used in big jets.... AS coursework
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Thermocouples used in big jets.... AS coursework
Hi there, I'm just doing my AS level coursework and am using a thermocouple, testing specs, accurace and all the rest of it. I was just wondering if anyone had any interesting uses or thermocouples on modern jet planes. The anti-ice system would be obvious I'd have thought, or in the engines- can anyone spcifically tell me how they work in particular situations and why they are used?
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Engine Exhaust Gas probs typically 7-11 probes all round connected in parallel to a junction and then forward with compensated leads. Probs gen alumel chromel junction. Typical fault is EGT under reading or spikes in the EGT trend.
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yes chromel and alumel are the most common,thinks its in the k-type congif,but copper and constantan may be used for measuring ''cylinder gas head'' temps as i read in mod15
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so is an average taken from several different probes (I would assume so). Also, approximatly what temperature will these probes measuring engine exhaust be measuring?
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old ones used JPT too (Jet pipe Temp)
There is also ITT on some (Inter Turbine Temp) - it just depends where the T/couples are mounted (without meting)
You actually want the temp AT the turbine but it can be inferred from cooler positions.
There is also ITT on some (Inter Turbine Temp) - it just depends where the T/couples are mounted (without meting)
You actually want the temp AT the turbine but it can be inferred from cooler positions.
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Thermistors don't tend to handle temps over about 300*C due to materials.
Thermocouples can be chosen to handle all sorts of tmp ranges due to the choice of material types.
Thermocouples can be chosen to handle all sorts of tmp ranges due to the choice of material types.
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Thermocouples generate their own voltage too so no need to run current carrying power cables into a hostile environment (even if they didn't melt)
Oh and Thermocouples are cheap and easily produced.
Oh and Thermocouples are cheap and easily produced.
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Apologies for minor thread drift - I've been working on engines for 20 years and am still amazed at how the EGT system for the RR engines and other marques fail in the most unpredictable manner. Does anybody else see this - or am the only unlucky powerplant wallah?
PS - TET - Turbine Entry Temperature
PPS - Remembering that the wires from the T/C have to also be alumel chromel....
Brgd's
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PS - TET - Turbine Entry Temperature
PPS - Remembering that the wires from the T/C have to also be alumel chromel....
Brgd's
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N1,
Must say that TGT failures in my experience are not that common.
The RB21 has 17 thermocouples that all route through a ballast resisitor.
Serveral themocouples can be u/s (Open circuit) but not have a dramatic effect on readings.
I have know up to half a dozen u/s with no marked indication problem.
There is usually a few degrees drop in indicated TGT
Of course there is also indicator failure but generally these are obvious faults
Sticking or FSD.
Must say that TGT failures in my experience are not that common.
The RB21 has 17 thermocouples that all route through a ballast resisitor.
Serveral themocouples can be u/s (Open circuit) but not have a dramatic effect on readings.
I have know up to half a dozen u/s with no marked indication problem.
There is usually a few degrees drop in indicated TGT
Of course there is also indicator failure but generally these are obvious faults
Sticking or FSD.
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RR Allison T56/501 engines have 18 thermocouples (3 per combustion liner). Any 1 thermocouple failing can cause a loss of up 24°C TIT at take off power. Not a lot you say - probably true, but the TD system will then increase fuel flow, i.e. increase TIT, until TIT signal from thermocouples matches input from the coordinator: result turbine overtemp, and eventual burn-out of turbine.
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GTT-shirt,
wasn't suggesting RB system is unreliable - in-fact I agree that I've only seen perhaps 1 headscratching defect a year and the local airline has 500 engines on-wing. But, the point I wanted to make was the peculiarity of some of the final fixes. Especially now with causes including FADEC technology, C-duct heatsheld material interference and even one event on a CFM where the Arinc 429 data wire in the pylon actually caused an EGT signal fluctuation.....
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wasn't suggesting RB system is unreliable - in-fact I agree that I've only seen perhaps 1 headscratching defect a year and the local airline has 500 engines on-wing. But, the point I wanted to make was the peculiarity of some of the final fixes. Especially now with causes including FADEC technology, C-duct heatsheld material interference and even one event on a CFM where the Arinc 429 data wire in the pylon actually caused an EGT signal fluctuation.....
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