CHT Gauge

[jcruffle]

Has anybody experienced a faulty CHT gauge? Mine has only recently begun to display irregular and illogical behaviour. Even with a slightly warm engine well after a flight, it indicated 175 degrees today even though I could handle the spark plug with comfort. I disconnected the sender and it still read approx 125 degrees.

 

No. 6 rear plug looks fine. Oil temp and pressures are fine. The CHT readings in climb and cruise at 2850 RPM are way high.

 

After commencing descent at 2600 RPM and 120 knots, the CHT rose.

 

Up to now the temps have been good. I have a J230 factory built approx 90 hours. Am I missing something??

 

John

 

 

[Yenn]

Can you swap the wiring to a different sender to see if it is the gauge or the sender that is at fault.

 

Having 6 cylinders I guess that it is a Jab engine, which I think depends upon 12V power supply. I cannot remember how they work but there may be a poor connection somewhere.

 

 

[Bruce Tuncks]

Hi John, the cht sender is just a tabbed washer with 2 wires crimped to the tab. The washer Jabiru use is a standard electrical one which isn't a very good fit at the spark plug. If the seal is not good, hot combustion gas will leak out and make the sender real hot.

 

To check things out, dunk the sender into boiling water to see if you get 100 or not.

 

The washer under the plug is a nuisance when you change plugs, I reckon it would be possible to put the 2 wires into a shallow drilled hole, retained with a pop rivet as a plug.

 

......Bruce

 

 

[Guest rudix]

Hi John, the cht sender is just a tabbed washer with 2 wires crimped to the tab. The washer Jabiru use is a standard electrical one which isn't a very good fit at the spark plug. If the seal is not good, hot combustion gas will leak out and make the sender real hot.To check things out, dunk the sender into boiling water to see if you get 100 or not.

The washer under the plug is a nuisance when you change plugs, I reckon it would be possible to put the 2 wires into a shallow drilled hole, retained with a pop rivet as a plug.

 

......Bruce

I am afraid it is not quite that simple, the CHT probe is not just a washer, it is a washer with a thermocouple that actually senses the temperature. The thermocouple can go faulty so if it is not a bad connection that would be my next guess.

 

The boiling water trick works well to see if it works and is accurate, up to a point at least.

 

Fly safe,

 

Rudi

 

 

[jcruffle]

Thanks for your input. It will give me things to consider once I can get to my plane without the need for webbed feet. (It's been raining here a bit). I will keep you posted F.Y.I.

 

John

 

 

[Bruce Tuncks]

CHT sender

 

The sender really is just a tabbed washer, I made 4 of them myself to monitor each cylinder. The thermocouple is the 2 dissimilar wires, and the difference in temp between the hot and cold junctions is what drives the thing. The hot junction is the 2 wires joined up in the folded tab of the washer.

 

John, sorry I can't read your message, something about my system won't let it display.

 

cheers, Bruce

 

 

[Guest Martin J400]

I have a J400/3300 and took the opportunity recently to test the CHT sender by placing the washer portion into a container of boiling water. I did it several times to gain a reasonably accurate reading. The CHT gauge read between 245 and 250 degrees fahrenheit which is roughly 120 degrees C.

 

So mine must be over reading by 20 degrees C. I've no idea whether this difference would vary by temperature - perhaps someone could offer a view?

 

 

[Bruce Tuncks]

thermocouples

 

Martin, the output of a thermocouple is not exactly linear, the emf actually peaks at some temperature, eg 285 C for copper-iron and 1050 C for constantan-iron.

 

But for our purposes if it over-reads by 20 at 100 degrees it will over-read by a similar amount at 200.

 

The display gauge is just an ammeter which sees a current equal to the emf divided by the total circuit resistance, but it is marked in temperature . So each setup needs adjusting. In the case of over-reading, a bit more resistance needs to be added in series with the gauge. Or, you could re-mark the gauge.

 

There are different wire types around, as well as different gauges. The one provided in my SK kit worked ok, but it was supplied as a package which VDO had obviously set up.

 

cheers, Bruce

 

 

[Guest Martin J400]

Many thanks for your input Bruce. I'll just accept that it over reads for the time being...

 

 

[hfrensch]

Martin, a thermocouple reads the voltage produced between two dissimilar metals which changes with temperature. This is not linear and the gauge is calibrated accordingly. The J type thermocouple used for the CHT is iron constantine and will read with relative accuracy to 750 degC( well above the expected temp) The point where it will read is where the two wires are joined and the quality of this joint will have a bearing on its accuracy.

 

The problem with the Jab system is the thermocouple wires are crimped to copper wires leading back to the gauge. This creates a cold junction and the temperature of this junction has a bearing on the temp read by the gauge. when I built my first Jab in 1996 I did a lot of testing on this particular gauge and found out that the gauge reads accurate temps if this junction is at 50 deg C. IF the junction is colder the the gauge reads high, if the junction is hotter it reads low and a error of 20 deg is possible. Do not take the error as always being 20 deg high it will vary with temperature.

 

Expensive gauges are calibrated for a cold junction and usually run thermocouple wire up to the gauge. My conclusion was that the jab gauge was calibrated to expect a cold junction temp of 50deg C so I located it in a hot part of the engine bay. Hope that helps clear it up a bit.

 

Cheers Helmut.

 

 

[Bruce Tuncks]

cht

 

Good comment Helmut,

 

the cold junction can obviously vary by 20 degrees depending on the day and just where it is in the engine bay. And this will change the displayed temp by a similar amount.

 

Thanks, Bruce

 

 

[jcruffle]

The sender really is just a tabbed washer, I made 4 of them myself to monitor each cylinder.

Bruce, I'm told that these washers are available commercially. Are you aware where? The one supplied with the VDO instrument is 14mm (the spark plug is 12mm and therefore not a good fit). I would be good to access a 12mm or even a 13mm washer.

 

John

 

 

[Bruce Tuncks]

washers

 

John,

 

the washers I used (12mm I think.....anyway, the size down from 14mm) were from an electrical trade-supply place, where they also have the 14mm ones which the factory uses. This size does not quite fit on the plug, and it needs to be opened out with a tapered reamer till it fits.

 

This is a fiddly job, and you can't blame the factory for using the 14mm one.

 

This works, but I don't like the washer business much anyway, the problem being that when you tighten the plug, the washer wants to rotate a bit and this strains the thermocouple wires. It also happened with the 14mm washer.

 

If the hot junction were inserted about 3mm into a suitable spot of the cylinder head, and fixed in with say a 3/32 pop rivet, then it would make plug changes much easier.

 

Maybe somebody has already done this, I'd like to hear about it.

 

cheers, Bruce