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 Post subject: Testing new CHT probe
PostPosted: 12 Mar 2011, 18:22 
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Joined: 01/24/09
Posts: 390
Post Likes: +41
Location: Phoenix, AZ (DVT)
Aircraft: 1961 Bonanza N35
My cylinder head temperature quit working recently and after checking this forum as well as others I did the simple test of shorting the the CHT wire to ground and when I got a full scale reading I knew it was the probe. After more research and checking the I purchased a replacement probe from Spruce, as the original AC probe is no longer available.

I assumed there would some resistance difference between the AC probe and the new probe and I purchased an 82 ohm resister to put is series with the new probe because 82 ohms was the general agreement of series resistance needed to make the new probe work with the origianl 50-year old CHT guage.

This morning I took a hot plate, sauce pan, candy thermometer and a bunch of test wires with aligator clips to my hangar to check to make sure the new probe worked Ok with the guage. I heated a quart of oil in the sauce pan and found 82 ohns to be a little high but found a 50 ohm resister seemed to make the guage register about the same as the candy thermometer.

All I have is the original single probe CHT as well as a simple probe EGT guage. I did a short test flight and found the indicated temperature with the new CHT registers over 400 degrees even at full rich and low power which is higher than with the AC probe.

My question is was I correct to attempt to check the operation of the new probe with hot oil? Is there a difference way I should have tested? The 82 ohm resister would cause the temperature reading to be lower and I'm wondering if i should have simply installed that as opposed to the 50 ohm I installed. The readings are higher than with the old probe and I don't think my #3 cylinder is running too hot. Any feedback will be appreciated.


Bill


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 Post subject: Re: Testing new CHT probe
PostPosted: 12 Mar 2011, 22:01 
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Joined: 12/10/07
Posts: 14256
Post Likes: +932
Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
Username Protected wrote:
My cylinder head temperature quit working recently and after checking this forum as well as others I did the simple test of shorting the the CHT wire to ground and when I got a full scale reading I knew it was the probe. After more research and checking the I purchased a replacement probe from Spruce, as the original AC probe is no longer available.

I assumed there would some resistance difference between the AC probe and the new probe and I purchased an 82 ohm resister to put is series with the new probe because 82 ohms was the general agreement of series resistance needed to make the new probe work with the origianl 50-year old CHT guage.

This morning I took a hot plate, sauce pan, candy thermometer and a bunch of test wires with aligator clips to my hangar to check to make sure the new probe worked Ok with the guage. I heated a quart of oil in the sauce pan and found 82 ohns to be a little high but found a 50 ohm resister seemed to make the guage register about the same as the candy thermometer.

All I have is the original single probe CHT as well as a simple probe EGT guage. I did a short test flight and found the indicated temperature with the new CHT registers over 400 degrees even at full rich and low power which is higher than with the AC probe.

My question is was I correct to attempt to check the operation of the new probe with hot oil? Is there a difference way I should have tested? The 82 ohm resister would cause the temperature reading to be lower and I'm wondering if i should have simply installed that as opposed to the 50 ohm I installed. The readings are higher than with the old probe and I don't think my #3 cylinder is running too hot. Any feedback will be appreciated.


Bill

You're doing a single point calibration and the slope is wrong. With nothing more than the addition of a series resistor you can only get the gauge to read correctly at one temp and the value of the resistor will determine that temp. I suggest you go with the recommended 82 ohms and check the resulting accuracy at the most important temps in the range (i.e. 380-400F). If it's good there the rest doesn't really matter much.

That said, it's also possible you have the "wrong" probe and that there's one which would work better. I vaguely remember a SB (or something in the ABS forum) about this issue and 82 ohm seems familiar as well.

_________________
-lance
Advice in this post may contain errors. Using said advice is totally at the risk of the user.


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 Post subject: Re: Testing new CHT probe
PostPosted: 13 Mar 2011, 02:29 
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Joined: 03/08/10
Posts: 2184
Post Likes: +130
Location: San Jose, CA (KRHV)
Aircraft: 1959 Bonanza K35
My JPI engine monitor went out a few months back and IMO, it was a faulty voltage regulator that I have now have had, replaced.

The new JPI, upon installation was reading about 100degrees high - long story short, it had to do with the configuration of the instrument. However, getting to the point, I called JPI to ask if there was some sort of calibration, or how the heck did I know that this was accurate? His answer was the following: At startup, there should be less than 2 degrees difference between the CHT's of all of your cylinder probes. Also, if the plane has been sitting the reading should be very close to the ambient temp.

I don't really know if my probes are reading correctly - but all 6 do seem to agree and they satisfy the tests JPI tech support gave me. I also, don't know if I would trust a $10 candy thermometer. My A&P uses a laser therm - I would assume they are a heck of a lot more accurate.

_________________
Todd
N6005E "G00SE"
San Jose, CA KRHV


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