More on CHT probe replacement

[Jim Burns]

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

[[email protected]]

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

[RST Engineering]

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

[George Patterson]

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.

[Denny]

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

[Jim Burns]

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.

[RST Engineering]

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]

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.
>
>

[Dave Butler]

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

[Jim Burns]

> 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

[Denny]

As long as the higher wattage resistor fits physically and is the
appropriate resistance (82 ohms in this case), it will work just
fine... It will be just loafing at a small fraction of a watt and so
should never 'burn out from thermal stress.... Physically larger is
usually more mechanically robust, as Dave surmises...

Resistors start at 1/8 watt at most electronic suppliers and go up into
the hundreds of watts... The larger units will not be available in all
resistances, especially in the megohm range... For what you need it
in the airplane, a wirewound "flameproof" of 2 watts is fine...

denny

[RST Engineering]

Jim ...

The three main resistor characteristics are value (ohms), tolerance (%), and
power handling capability (watts). There are other second-order effects,
but let's not worry about them right now.

You've got the value nailed.

You've got the tolerance pretty well in hand. Tolerance is pretty much
linear with dollars. That is, a 5% resistor might cost a dime. A 0.5%
resistor may cost a dollar. A 0.05% resistor might be $10, and so on. For
your "hammer, file, kick in the edges and weld shut" project 5% is well
within any reasonable design consideration.

Wattage is a little like a wooden deck. A fly lands on it, no big deal.
You drive a motorcycle on it and it may groan, but will probably not break.
Drive a HummV on it and you've got matchsticks. Ohm told us that the
power dissipated in a resistor is equal to the current through the resistor
(squared) times the resistor value (P = I^2 * R).

Let's say, just for grins, that the current through the resistor is 10 mA
(ten milliamperes) or so. This 82 ohm resistor will then be dissipating
about 8 milliwatts. (0.008 watts). A 3 watt resistor won't even start to
get warm.

The general rule of thumb is that you select a resistor with double the
expected wattage next-size-up. That is, if your resistor is actually
dissipating 1 watt, double is 2 watts, next size up is 3 watts. However, to
be dissipating 1 watt, the current through the resistor is going to be about
100 mA, which is WAY THE HELL more than I really think you are dissipating.
That's why I asked you to check the current through the resistor (or the
voltage across it, same thing) so we could see what the resistor was REALLY
dissipating. You'd probably want to make this measurement fairly close to
the top end of the scale where the current is most probably the greatest.

Some posters have postulated that a larger wattage resistor will be more
resistant to vibration and the like. I respectfully disagree. I'd much
rather something about the size and mass of a toothpick than size and mass
of a large ball bearing bouncing around. If we can get a handle on exactly
how much power this sucker is dissipating, we may be able to do a Rat Shack
fix for a buck or so.

Keep me posted.

Jim



"Jim Burns" > wrote in message
...
> Thanks Jim, I appreciate the link.

> 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.

[Jim Burns]

Damn, you're a pretty good teacher! ;) Thanks. Everything you wrote was
basically what I thought, but wasn't sure of. Seems every electrical item
has a sizing procedure, and that's what I didn't know. I can size motors,
fuses, wiring, and larger items, but I had no idea how to figure the
required size of a resistor.

Thanks again for the explanation and your help. I'll hook it up when I get
the parts and let you know the draw.
Jim

"RST Engineering" > wrote in message
...
> Jim ...
>
> The three main resistor characteristics are value (ohms), tolerance (%),
and
> power handling capability (watts). There are other second-order effects,
> but let's not worry about them right now.
>
> You've got the value nailed.
>
> You've got the tolerance pretty well in hand. Tolerance is pretty much
> linear with dollars. That is, a 5% resistor might cost a dime. A 0.5%
> resistor may cost a dollar. A 0.05% resistor might be $10, and so on.
For
> your "hammer, file, kick in the edges and weld shut" project 5% is well
> within any reasonable design consideration.
>
> Wattage is a little like a wooden deck. A fly lands on it, no big deal.
> You drive a motorcycle on it and it may groan, but will probably not
break.
> Drive a HummV on it and you've got matchsticks. Ohm told us that the
> power dissipated in a resistor is equal to the current through the
resistor
> (squared) times the resistor value (P = I^2 * R).
>
> Let's say, just for grins, that the current through the resistor is 10 mA
> (ten milliamperes) or so. This 82 ohm resistor will then be dissipating
> about 8 milliwatts. (0.008 watts). A 3 watt resistor won't even start to
> get warm.
>
> The general rule of thumb is that you select a resistor with double the
> expected wattage next-size-up. That is, if your resistor is actually
> dissipating 1 watt, double is 2 watts, next size up is 3 watts. However,
to
> be dissipating 1 watt, the current through the resistor is going to be
about
> 100 mA, which is WAY THE HELL more than I really think you are
dissipating.
> That's why I asked you to check the current through the resistor (or the
> voltage across it, same thing) so we could see what the resistor was
REALLY
> dissipating. You'd probably want to make this measurement fairly close to
> the top end of the scale where the current is most probably the greatest.
>
> Some posters have postulated that a larger wattage resistor will be more
> resistant to vibration and the like. I respectfully disagree. I'd much
> rather something about the size and mass of a toothpick than size and mass
> of a large ball bearing bouncing around. If we can get a handle on
exactly
> how much power this sucker is dissipating, we may be able to do a Rat
Shack
> fix for a buck or so.
>
> Keep me posted.
>
> Jim
>
>
>
> "Jim Burns" > wrote in message
> ...
> > Thanks Jim, I appreciate the link.
>
> > 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.
>
>