CHT sensor error

[Whack777]

Does anyone know if a CHT sensor is in error is that error consistent throughout the temperature range. ie if it's overreading at higher temps such as climb, will it be over reading at idle or can the degree of error vary with temp?

 

 

[sain]

Accuracy for a K-type thermocouple (whats normally used for CHTs I believe) does vary with temperature, but the variance isn't large - see here: Thermocouple Accuracy Table . Over the operating temperature range of say a rotax 582 you'll be on the same +-1.5degree error.

 

Therefore I believe that if its in error it should be fairly consistant. call it an educated wild XXXX guess. Easiest way to tell for sure would be to pull the sensor and test it. A 582 operating range is 110-150 degrees, which you

 

could duplicate with a frying pan on a stove. You can probably find the temperature range for your engine in its operators manual.

 

Hope it helps.

 

*edit*

 

Your aircraft is listed as a jabiru, so I'm assuming your running a jab engine. Max CHT for a 2200 is 200C. normal max (cruise) is 180C, so you can probably at least get close to that on a stove. Error may creep up to +-1.6degC at the top of the range

 

 

[crashley]

I believe cht use a J type thermocouple and the exhaust temp uses a k type thermocouple

 

the error may or may not be consistant I have checked a few guages and found some to have a uniform error across the range which is easily fixed and some to have an error which increases as the temp rises if there is any joins in the cable which have not been done correctly variances in the ambient temp will affect the reading also the best way to check is to use a simulator on the guage or as suggested heat the sensor to a known temp and compare,

 

 

[sain]

Heh, learn something new every day. There seems to be a mix of thermocouples, with J-types being most prelevant for CHTs, but with some brands using K-types. The ones I've been playing with are K-types, and I just assumed they all were. And we all know what they say about assuming: :mulie:

 

Thanks Crashley.

 

Oh yeah, its also worth noting that the guage itself may not have a uniform movement. Chrashley's suggestion of a simulator would show that issue up nicely.

 

 

[Whack777]

Thanks for that guys. It always amazes me the knowledge that lies just beyond the keyboard.

 

 

[skydog]

boiling water

 

take the thermocouple off and place into some boiling water.

 

Should be 100 celcius right.

 

:big_grin:

 

 

[jetboy]

a probe placement error will not be transferred throughout the range. A cold junction error will.

 

If you are asking about a standard Jabiru installation you have both errors built in.

 

The tab under a sparkplug reads the plug temperature minus the cooling temperature around the lug. Typically reads under by 20 deg c but if the plug washer is leaking can over read. the proper way to fit the probe is clamped inside a hole in the head or clamped on the surface of the head with insulation under the clamp.

 

If the cold junction is at 50 deg c as specified in the installation manual you get another 50 deg (less ambient) under reading.

 

Some gauges have cold junction compensation, some dont.

 

Proper handling of a cold junction is that the end of the thermocouple wires terminates to the gauge wires or terminals at the same ambient temperature as the gauge, and gauge is a compensated one.

 

Then again, if you are dealing with a Jabiru the factory limits are set with the above errors in mind, so you have to decide what reference you are to use. I understand new engines have a hole in the head (dont they all?) for a probe.

 

To check a gauge, an accurate electrical digital voltmeter can be used, find the readings and corresponding temp. for the corresponding K or J thermocouple from the web.

 

Voltages are only a few millivolts, and you add the ambient temp. to the reading because most DVM dont do thermocouple corrections.

 

Dont sweat over the detail, your setup will be close enough unless exhaust gas or cooling air is getting across the probe.

 

Ralph

 

 

[Dieselten]

It's not just the probe, the display-instrument which the probe feeds also needs to be temperature-compensated - and only the good (i.e. expensive) ones are.

 

 

[Thruster87]

Which is correct A spark plug gasket probe generally results in a CHT reading that's about 40°F hotter than a normal, threaded probe on the same cylinder. or The tab under a sparkplug reads the plug temperature minus the cooling temperature around the lug. Typically reads under by 20 deg c but if the plug washer is leaking can over read. the proper way to fit the probe is clamped inside a hole in the head or clamped on the surface of the head with insulation under the clamp.

 

 

[facthunter]

Boiling water is only 100 c at 1013.2 millibars. atmos . Nev

 

 

[Oscar]

As I participated in a calibration test of the temperature thermocouples on a (J2200) test engine two days ago, this is an interesting thread!. The chts tested are thread-in-head types so I can't comment whether the results apply to the under-plug types, but the entire test installation was tested using a pair of labaratory-quality mercury-in-glass thermometers with the temps averaged between them and tested both on the rise and the fall of temp. in a large brass block, to ensure that any position error between the thermometers and the actual probe positions on the block for both rise and fall of temp was averaged.

 

The end result is that there are minor changes in recording AT THE INSTRUMENT through the range 60C - 200C for each probe, and each probe had slight variances for where divergence occurred - but they were, on the whole, very accurate. This was on a 'test' engine set-up, with the (compensating cold junction) instruments located in a separate room from the engine itself so one can count on the cold junction environment being entirely stable. The tests were conducted on the actual probes and the complete wiring used on the test engine.

 

Suffice it to say that provided the actual cht reporting set-up is reliable from probe to instrument read-out, the accuracy of the temps. reported by decent-quality probes will be better than one can realistically assimilate in real-life flying as a guide to what the engine is doing at any particular moment. Having a recording capability that one can examine later, however, could be very illuminating as to how the engine behaves.

 

 

[Jaba-who]

"the proper way to fit the probe is clamped inside a hole in the head or clamped on the surface of the head with insulation under the clamp."

 

Well, not quite.

 

That is now an option but Is not without problems.

 

I have done a series of trials that show you can't do that and then simply use the jabiru figures ( 180 C for cruise and 200 C for <5 mins) as ideal numbers to shoot for.

 

Those figures were derived for and can only be used for under plug ring placement ( I guess also for clamped on or in head placed probes - if you test and prove to yourself that the head placed probe indicates the same temps as the ring placed probe.

 

I did a series of trials with head placed probes because I wanted to get the ring out from the spark plug where they get in the way etc. Comparing the two positions on the same cylinders on my engine the temps are the same only till about 80 C and then the head placed probes diverge at varying rates till they are anywhere up to 20 C lower than the ring placed probes.

 

I spoke with Stiffy about this to see if they had a new calibrated temp limits for these head positioned probe. He was not aware that the temps changed so much and jab have still not since released any modified numbers despite the advice to position the probes on the head. Obviously you can't use the standard jab temps as your limits if the "real temp"at the plug ring is 20 C higher especially if that temp is actually above the 180 C or 200C.

 

 

[Thruster87]

I wonder how accurate a distance reading infrared transmitter /sensor looking at the cht would be,compared to other types of direct probe readings. cheers

 

 

[Old Koreelah]

Three of my four CHTs probes are still working, after I hacked a set of under-plug sensors and screwed them to the between-plug holes. After removing each one and clamping it to a good temp probe and testing with a heat gun the readings are way out. No real problem; I repeatedly heated and cooled each till I could mark the max cruise temp on the guage.

 

 

[Jaba-who]

OK said "Three of my four CHTs probes are still working, after I hacked a set of under-plug sensors and screwed them to the between-plug holes. After removing each one and clamping it to a good temp probe and testing with a heat gun the readings are way out. No real problem; I repeatedly heated and cooled each till I could mark the max cruise temp on the guage."

 

??? I don't follow, can you re-explain?

 

 

[Guest Andys@coffs]

I wonder how accurate a distance reading infrared transmitter /sensor looking at the cht would be,compared to other types of direct probe readings. cheers

So more expensive non contact instruments have an "emisability" constant that needs to be set as a function of knowing the material that you are measuring. This website http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html has the constants for a bunch of materials. Cast Iron, dull stainless etc range around 0.7 to 0.9 and polished aluminium from memory is around 0.03........obviously if you don't have an ability to set the constant on the device then the final measured temp error between measured and actual is likely to be very different between a machined aluminium head and a dull stainless exhaust manifold for example......

 

I think it would be brave to use a cheapy without the emission constant adjustment on a Jab engine for example.....

 

Andy

 

 

[Old Koreelah]

I have four CHT probes. Instead of installing them under the plugs as designed, I cut each one down and drilled a 4mm hole. After tapping threads into the holes between the plugs, I screwed a probe to each one.

 

Last week I removed them for testing. I clamped each probe to an electronic temperature gauge, then applied a heat gun.

 

The temperatures shown on the two gauges -cockpit CHT and test probe- differed markedly. This means I can ignore the numbers on the cockpit gauge. (I had earlier calibrated the test gauge in boiling water, so I trust it.) After several cycles of heating and cooling, I was able to mark on the CHT the point where actual probe temp is 200 degrees C.

 

 

[Dafydd Llewellyn]

For what it may be worth, here are the results of four CHT calibrations, for K-type thermocouples supplied by Ian Bent, as read by MGL Infinity E-3 gauges, at the end of approximately 3 metre K-type extension leads. The thermocouples had simple crimp-type electrical (plated copper) eyelets which were fastened to the calibration block by 8-32 steel screws. The calibration block was earthed to the system ground, as were the "ground" leads of the E-3 gauges.

 

The E - 3 gauges have cold-junction compensation; the cold junctions were located close to the gauges, i.e. at essentially the same temperature. As you can see, the accuracy was quite good. The graphs are an amalgam of rising and falling temperature data.

 

 

 

 

 

 

[Thruster87]

At the end of the day with all the different ways people go about monitoring their cht's . I'm still wondering what would be the quickest/most accurate/cheapest way of verifying/calibrating the installations on our aircraft in the field cheers

 

 

[Old Koreelah]

At the end of the day with all the different ways people go about monitoring their cht's . I'm still wondering what would be the quickest/most accurate/cheapest way of verifying/calibrating the installations on our aircraft in the field cheers

Mine.

 

 

[Dafydd Llewellyn]

You have to have a reference thermometer, and a block of high thermal conductivity material to which you can attach both the active end of the thermocouple and the reference sensor.

 

The least expensive form of reference thermometer is mercury-in-glass; see attached. However these are fragile and awkward to use. They need a large block, bored sufficiently deeply to allow the thermometer to be entered to its specified immersion depth, and the use of a "coupling fluid" to help transfer heat to the thermometer; and because of the slow heat transfer to the thermometer, the heating rate needs to be low. The results I posted were obtained this way, with a heating rate of around 2 degree C per minute.

 

A more convenient form would be a hand-held meter; however it needs to be a type that uses either a platinum resistance thermometer or has cold junction compensation, if it uses a thermocouple. Either of these features make such a reference thermometer much more expensive; FLUKE make a good one - it's around $aus 1700. http://www.testequipmentdepot.com/fluke-calibration/thermometer-readouts/1523.htm