More on CHT probe replacement

I've been researching how to replace an old AC Spark Plug CHT probe in our
Aztec. AC doesn't make them anymore and the replacement Rochester brand
probe operates at a different resistance range requiring the gauge to be
removed, shipped out and recalibrated. Not a fun proposition.

Rumor, and a couple parts houses, have told me that the American Bonanza
Society published an article about this problem and a solution is to install
an 82ohm resistor inline with the probe to account for the difference
between the AC and the Rochester gauges. Sounds easy enough.

Is there a group member that belongs to the ABS that could verify the
article and possibly post or email me a copy??

Any other thoughts or comments about this??

Thanks again, again!!

Jim

On Wed, 31 Aug 2005 12:53:46 -0500, "Jim Burns"
wrote:

I've been researching how to replace an old AC Spark Plug CHT probe in our
Aztec. AC doesn't make them anymore and the replacement Rochester brand
probe operates at a different resistance range requiring the gauge to be
removed, shipped out and recalibrated. Not a fun proposition.

Rumor, and a couple parts houses, have told me that the American Bonanza
Society published an article about this problem and a solution is to install
an 82ohm resistor inline with the probe to account for the difference
between the AC and the Rochester gauges. Sounds easy enough.

Is there a group member that belongs to the ABS that could verify the
article and possibly post or email me a copy??

Any other thoughts or comments about this??

IMHO, an accurate CHT indication is one of the things that can tend to
contribute to engine longevity.

Unless you were planning on installing a PMA'd primary replacement
http://www.buy-ei.com/Single%20Primary.htm or
http://www.buy-ei.com/Dual%20Primary.htm
at some point in the near future, I would recommend doing the "right"
thing and getting your probe replaced and your indicator calibrated.

I probably would consider installing a resistor to be a minor
alteration, but would still recommend getting the system
checked/cal'd.

Actually, I would recommend getting both CHT gages cal'd and replacing
both probes. I allegedly used to maintain a bunch of working GA
aircraft (including PA23's), one of the things we ALWAYS did at engine
o-haul was replace the CHT and oil temp probes, and have the gages
cal'd (APoLH). Did the EGT/TIT systems also, but we had the
capabilities to do 'em in-house.

Compared to the cost of engine overhaul/installation, this is cheap
peace-of-mind.

Remember, this advice is worth exactly what you paid for it...

TC

And remember, there is an absolutely wonderful and free calibration point at
212°F available for nothing more than a pan of water and a hotplate.

Jim



wrote in message
...
Remember, this advice is worth exactly what you paid for it...

TC

RST Engineering wrote:
And remember, there is an absolutely wonderful and free calibration point at
212°F available for nothing more than a pan of water and a hotplate.

If you're at sea level. If your elevation is substantially different from that,
you need to look up the boiling point of water.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

I
guess we could also calibrate it with a pan of oil, a hot plate, and a
candy
thermometer.

RIght on! This is the sort of thing that is taught in high school
physics (or was, in my day, right after the earth first cooled)

denny

But I think the width of the needle of our gauge will compensate for that.
It's an old style gauge with only a dot at 200F and the next dot is at 400F.
The best we can do is try to keep the needle below the 400F dot a tad. I
guess we could also calibrate it with a pan of oil, a hot plate, and a candy
thermometer.

Jim

"George Patterson" wrote in message
news:cgvRe.12917$LK.8705@trndny09...
RST Engineering wrote:
And remember, there is an absolutely wonderful and free calibration
point at
212°F available for nothing more than a pan of water and a hotplate.

If you're at sea level. If your elevation is substantially different from
that,
you need to look up the boiling point of water.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

BTW, that 82 ohm power resistor (??) you need is available at www.mouser.com
5 watt cement coated fireproof resistors (280 CR5 series) are 39 cents in
onesies. 3 watt flame retardant resistors (283 series) are also 39 cents in
onesies.

Mouser has no minimum order and a decent shipping policy.

Somebody needs to measure the current that these things are sucking. In
order to dissipate 1 watt in an 82 ohm resistor, you need a bit more than
100 mA (0.1 amps) to flow. My gut says that this is not happening.

Jim



"Jim Burns" wrote in message
...
But I think the width of the needle of our gauge will compensate for that.
It's an old style gauge with only a dot at 200F and the next dot is at
400F.
The best we can do is try to keep the needle below the 400F dot a tad. I
guess we could also calibrate it with a pan of oil, a hot plate, and a
candy
thermometer.

Thanks Jim, I appreciate the link.
I also found some 82ohm 2W 5% flameproof resistors on ebay.
Some more goggling found that this Rochester probe w/82ohm resistor "work
around" has been used by owners of Pipers, Bonanzas and Commanders, all with
the AC Spark Plug CHT gauge. The 82ohms is the one constant in everything
I've read, the W varied between 2 and 3Watts and the tolerance was either 2
or 5%.

Not having a background in electronics, what ultimate difference would a
resistor of higher wattage rating have on the circuit, the gauge, or the
probe? If the circuit is only drawing mA's, 1) are there "smaller" mA rated
82ohm resistors available or 2) with a mA rated resistor would the required
ohm rating change? Forgive my ignorance regarding resistors, I basically
understand what they do, but do not know the implications of the
sizes/capacities. I'm just looking for a way to get out of several hours of
tedious PITA work with my upper torso stuffed up underneath our panel.

Instrument Tech of Dallas TX seems to have quite a bit of experience with
the combo and emailed me the American Bonanza Society newsletter that
contains the article. The original question was published June 1990, and
the explanation and work around was published some time shortly after
however I do not have that date.

Jim


"RST Engineering" wrote in message
...
BTW, that 82 ohm power resistor (??) you need is available at
www.mouser.com
5 watt cement coated fireproof resistors (280 CR5 series) are 39 cents in
onesies. 3 watt flame retardant resistors (283 series) are also 39 cents
in
onesies.

Mouser has no minimum order and a decent shipping policy.

Somebody needs to measure the current that these things are sucking. In
order to dissipate 1 watt in an 82 ohm resistor, you need a bit more than
100 mA (0.1 amps) to flow. My gut says that this is not happening.

Jim



"Jim Burns" wrote in message
...
But I think the width of the needle of our gauge will compensate for
that.
It's an old style gauge with only a dot at 200F and the next dot is at
400F.
The best we can do is try to keep the needle below the 400F dot a tad.
I
guess we could also calibrate it with a pan of oil, a hot plate, and a
candy
thermometer.

Jim Burns wrote:

Not having a background in electronics, what ultimate difference would a
resistor of higher wattage rating have on the circuit, the gauge, or the
probe? If the circuit is only drawing mA's, 1) are there "smaller" mA rated
82ohm resistors available or 2) with a mA rated resistor would the required
ohm rating change? Forgive my ignorance regarding resistors, I basically
understand what they do, but do not know the implications of the
sizes/capacities.

I don't have a background in electronics, either, but I'd think 3 watts is way
overkill from an electrical point of view, *but* a higher power rated resistor
is also more robust from a mechanical point of view, and may stand up better to
the vibration environment. Just an idea from an ignorant observer.

Dave

I don't have a background in electronics, either, but I'd think 3 watts is
way
overkill from an electrical point of view, *but* a higher power rated
resistor
is also more robust from a mechanical point of view, and may stand up
better to
the vibration environment. Just an idea from an ignorant observer.

Dave

You summed up my thinking pretty close. I'm thinking if 3W is overkill,
what are the downsides of it? Shorter probe life? shorter gauge life?
shorter resistor life? If it doesn't detrimentally effect the operation of
either the probe or the gauge and the life of the components isn't
compromised, then the only downside I see is about $0.10. But like I said,
I don't have much knowledge in this area but I'm willing to learn so I defer
to the pros.

Thanks
Jim