CHT sensor validation

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ahmebane
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CHT sensor validation

Post by ahmebane »

With all the discussion on CHT temps, how do we know for a given aircraft what the accuracy is of the sensor system? Does anyone know of a calibration procedure? I'm asking because of a user that recently replaced their VM1000 with a JPI & saw 20-40 degree differences in CHT readings afterwards.

I constantly struggle to keep my CHTs down, even on a not-so-warm day (mid 80s), mainly on climb-out. If I knew that my CHT readings were off, that would help. I also have the VM1000.

TIA.
Last edited by ahmebane on Wed Nov 30, 2022 3:29 pm, edited 1 time in total.
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MarkA
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Re: CHT sensor validation

Post by MarkA »

One test is to look at the CHTs right after the master is turned on before engine start with a cold engine. They all should be close to the ambient air temp. On my G1000 plane, the CHT and EGT probs all report within 1 degree of each other at this point.
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Re: CHT sensor validation

Post by ahmebane »

I considered that, but if the sensors are off by a percentage (rather than a fixed amount), I don't think it would tell me much. For example, if the ambient is 70 degrees & the readings are off by 5%, they would read 67, which is close. But at 400 they would be off by 20 degrees. Its a first step for sure. I talked to the shop about it & they said maybe they could use an IR gun to look at the CHT with the motor running & the cowling off. Not sure how accurate that would be either.

Thanks, I'll try that today just to get a data point. It does seem like the oil temp & CHT temp before starting should be at ambient. Now that I'm thinking about it, does the CHT sensor even go down to ambient?

With all the fixation on readings that are just 20 degrees LOP or whatever, it seems like being off just a small amount matters, so it would be good to know.
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Re: CHT sensor validation

Post by Steve »

You could remove one of the CHT sensors from the cylinder head (the easiest one would probably be #1) and immerse it in boiling water with the Master ON. Should read 212 degrees F...
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Rick
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Re: CHT sensor validation

Post by Rick »

ahmebane wrote: Wed Nov 30, 2022 3:33 pmWith all the fixation on readings that are just 20 degrees LOP or whatever, it seems like being off just a small amount matters, so it would be good to know.
For LOP operation, you are looking at the change in EGT instead of the absolute CHT - and the actual EGT values are pretty irrelevant anyway. So even if the EGT sensors were off by 10% or even 20%, the relative difference between the peak value and 20 degrees off the peak would still be plenty accurate enough to use. On the other hand, if you are trying to keep your CHTs under 400, for example, knowing that your CHT sensors are accurate is more important!

I also try to scan the values at startup as a check. I find that if the ambient temperature has been constant for a few hours, all my sensors (OAT, EGTs, CHTs, and oil temp) will read within a degree of each other. But if the OAT is changing, they "catch up" at very different rates, with the CHTs lagging behind everything else, of course.
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Re: CHT sensor validation

Post by ahmebane »

Rick wrote: Wed Nov 30, 2022 4:05 pm
ahmebane wrote: Wed Nov 30, 2022 3:33 pmWith all the fixation on readings that are just 20 degrees LOP or whatever, it seems like being off just a small amount matters, so it would be good to know.
For LOP operation, you are looking at the change in EGT instead of the absolute CHT - and the actual EGT values are pretty irrelevant anyway. So even if the EGT sensors were off by 10% or even 20%, the relative difference between the peak value and 20 degrees off the peak would still be plenty accurate enough to use. On the other hand, if you are trying to keep your CHTs under 400, for example, knowing that your CHT sensors are accurate is more important!

I also try to scan the values at startup as a check. I find that if the ambient temperature has been constant for a few hours, all my sensors (OAT, EGTs, CHTs, and oil temp) will read within a degree of each other. But if the OAT is changing, they "catch up" at very different rates, with the CHTs lagging behind everything else, of course.
Uh yeah, I had a brain fart on the LOP comment! That's EGT, not CHT. But yes, I am trying to keep the CHTs below 400 & whether they're 380 or 420 makes a big difference. Thanks .
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Re: CHT sensor validation

Post by ahmebane »

I checked the cht and oil temperature readings today before starting the plane. I also checked the outside ambient temperature with a calibrated thermometer. Cloudy day, so I don't think much heating of the airframe from the sun.

Ambient temperature was 68. Oil temperature read 60. Cht read 77.

Oil temp appears to read low, cht appears high. All cylinders read the same 77.

Thoughts?
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Re: CHT sensor validation

Post by Rich »

This is a parallel set of data, as I still have the VM1000 (to be replaced at some point). Today I did a temperature comparison in the hangar:

- All 4 EGTs and CHTs showed 32 degF
- Oil temp showed 27 degF. (In this situation it's really upper accessory case temperature.)
- Davtron showed 36 degF (appx. +2 degC)
- Aspen showed +1 degC (appx. 34 degF)

Given this was about 10:30 AM with the sun shining on the left wall of the hangar and the overnight low had been about 22 degF this all seems legit, considering the known minor discrepancies between the Davtron probe and the Aspen RSM.
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Re: CHT sensor validation

Post by ahmebane »

And a set of data from early-ish this morning:

All CHT and EGTs: 50
Oil: 45
Davtron: 62

Calibrated thermometer: 61

Overnight low, as reported by airport METER was 47, 2 hours before I took the readings off the plane.

To me these also seem fairly reasonable. Maybe I should just believe what they're telling me.
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