[ccsmith51]

I have what appears to be a Westach dual CHT/EGT gauge, although there are no markings on the gauge.

It has a ring sensor for CHT mounted under the #3 bottom plug. I'm considering replacing the ring sensor with a bayonet for better accuracy.

I see J and K sensors listed on ACS. How can I tell which I should use with this gauge?

 

[az_gila]

Color coded

I have what appears to be a Westach dual CGT/EHT gauge, although there are no markings on the gauge.

It has a ring sensor for CHT mounted under the #3 bottom plug. I'm considering replacing the ring sensor with a bayonet for better accuracy.

I see J and K sensors listed on ACS. How can I tell which I should use with this gauge?

Look at your existing ring sensor leads.

White/Red is a J-type
Yellow/Red is a K-type

 

[ccsmith51]

Excellent, Gil, thanks!!!

 

[ccsmith51]

Look at your existing ring sensor leads.

White/Red is a J-type
Yellow/Red is a K-type

Looked today. There is heat shrink at the cable termination at the ring, so I can't see the colors. The two wires that come out of the braid where they connect to the gauge are black and white....

Would that be a J type?

 

[az_gila]

Looked today. There is heat shrink at the cable termination at the ring, so I can't see the colors. The two wires that come out of the braid where they connect to the gauge are black and white....

Would that be a J type?

Black/White is an alternative International color code for aa Type-J.

However, just to confuse us all --

Black translates to White
White translates to Red

Note the +/- signs

 

[ccsmith51]

Got it, thanks again.

 

[az_gila]

Got it, thanks again.

Another twist....

If it is an actual Westach CHT ring thermocouple, their catalog talks about a type J device, but then also talks about a "black sleeve" on the red wire of the J-type red/white leads.

Before you hook up the new CHT thermocouple cut the shrink tubing off both ends of the old thermocouple and check for wire colors...

 

[PaulT]

An easy way to tell if your sensor is J type (Iron/Constantan), is to test with a magnet. One of the conductors is Iron.
However the total lead and probe resistance is an important consideration with analogue (steam) thermocouple gauges, as are the extra joins you may have to introduce to achieve the length required.
The errors that these can cause may offset any advantage in using a better quality probe.
I would probably stick with the original.
Paul.

 

[rocketbob]

Westach uses J-type probes for their cylinder head instruments. Its more common to see K-type on both CHT and EGT probes. Some of their CHT instruments will work with both. I have a neighbor that had to replace some CHT probes with a Westach gauge, and I gave him some K-type probes. He called the factory and asked if they could be used and they said yes but wire them to a different terminal on the gauge.

 

[az_gila]

Westach uses J-type probes for their cylinder head instruments. Its more common to see K-type on both CHT and EGT probes. Some of their CHT instruments will work with both. I have a neighbor that had to replace some CHT probes with a Westach gauge, and I gave him some K-type probes. He called the factory and asked if they could be used and they said yes but wire them to a different terminal on the gauge.

In some instruments, but they are not interchangeable. Given a choice, the J-type are better for CHT as they generate about more voltage than the K-type. Every bit extra helps when the measurement needed is of milliVolts.

They do not have as high an upper operating temperature limit, so type-K are needed for the hight EGT temperatures.

 

[ccsmith51]

Yikes, this is getting complicated!

The reason for considering the change is last weekend I checked my CHT reading by placing the ring probe in boiling water. It read about 190?, about 20? low. I was thinking that if I was going to replace the probe, hoping to get a more accurate reading, that a bayonet would be the way to go.

Maybe I'll just leave it alone and add 20? to whatever I read, hoping that the reading is off by 20? over the operating range.

That makes me think of something else. Would one or the other probe give a "truer" CHT reading? I'm assuming that they are reading temps in a different location...

 

[rocketbob]

Thermocouples are very non-linear in their voltage output depending on the temperature of whats being measured and the cold-junction temperature. I have done quite a bit of work integrating microprocessors with thermocouples and when I saw the insides of a Westach gauge I thought there's no way that thing could be accurate. You would be better off buying an electronic instrument that does the math to properly display an accurate temperature.

 

[az_gila]

Thermocouples are very non-linear in their voltage output depending on the temperature of whats being measured and the cold-junction temperature. I have done quite a bit of work integrating microprocessors with thermocouples and when I saw the insides of a Westach gauge I thought there's no way that thing could be accurate. You would be better off buying an electronic instrument that does the math to properly display an accurate temperature.

Yes... even the Westach literature says that they assume that the reference temperature (cold-junction in Bob's post above) inside the instrument is calibrated to is 70 F.

If you take off on a hot AZ day after your plane has be tied down outside and increase in the instrument temperature over 70 F will be an error in the reading.

Instrument temp = 90 F - then - CHT reading of 400 F is actually 420 F

Not a super accurate instrument...