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Paul Eckenroth
Joined: 29 Jun 2010
Posts: 14
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 Posted: Mon Jul 04, 2022 1:58 pm Post subject: Thermocouples: grounded or ungrounded |
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I would like advice on which thermocouples to use on my Revmaster engine, grounded or ungrounded. The thermocouples I will buy have a flag surrounding the thermocouple end with a hole for a bolt . The end will be bolted to the stator using one of the attachment bolts so the outer surround will be grounded. The thermocouple reader has continuity between the thermocouple ground and the power ground. So the concern with using a grounded thermocouple is the potential for a ground loop. The question is will any resultant ground loop be sufficient to cause a audio problem. The ungrounded probe has a sheath of brass while the grounded uses stainless which I think would be more robust. The grounded also has quicker response to temperature change. So, help me avoid a coin toss here. Thanks.
Paul
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nuckolls.bob(at)aeroelect
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| Posted: Mon Jul 04, 2022 9:09 pm Post subject: Thermocouples: grounded or ungrounded |
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At 04:58 PM 7/4/2022, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Paul Eckenroth" <jeckenroth(at)nbn.net>
I would like advice on which thermocouples to use on my Revmaster engine, grounded or ungrounded. The thermocouples I will buy have a flag surrounding the thermocouple end with a hole for a bolt . The end will be bolted to the stator using one of the attachment bolts so the outer surround will be grounded. The thermocouple reader has continuity between the thermocouple ground and the power ground. So the concern with using a grounded thermocouple is the potential for a ground loop. The question is will any resultant ground loop be sufficient to cause a audio problem. The ungrounded probe has a sheath of brass while the grounded uses stainless which I think would be more robust. The grounded also has quicker response to temperature change. So, help me avoid a coin toss here. |
What's your readout instrument? Does
it's installation manual suggest one
type over the other?
Insulated thermocouples are never 'bad'
but depending on the signal conditioning
circuitry of the display, the common mode
noises endemic to grounded thermocouples
may affect measurement quality.
Speed of response would not be an issue
here. The ground loop would not be a potential
problem with other avionics, just the
temperature display.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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Paul Eckenroth
Joined: 29 Jun 2010
Posts: 14
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| Posted: Tue Jul 05, 2022 8:13 am Post subject: Re: Thermocouples: grounded or ungrounded |
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Thanks for the reply Bob. Unfortunately my GRT EFIS cannot read Thermocouples except as used for EGT and CHT. Therefore I am using small inexpensive readers off Amazon. I seriously doubt if they have any specialized circuitry other than that require to give a basic reading of the thermocouple. Does the ground loop have the potential of rendering the reading totally unusable or is it more like a 5% loss of accuracy. The ungrounded probes are around twice as expensive as the grounded probes. I would certainly be willing to consider a better reader if it can be had in a small format. The Onex is a single seat very small plane with limited panel space. Thanks for your help.
Paul
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N509RV(at)eckenroth.com
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| Posted: Tue Jul 05, 2022 3:00 pm Post subject: Thermocouples: grounded or ungrounded |
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Bob, here are the links:
Probes: https://cdn.automationdirect.com/static/specs/psthermoprobebors.pdf
Reader: https://www.amazon.com/PEMENOL-Fahrenheit-Temperature-76F-999F-Thermocouple/dp/B07X37WH5Z/ref=asc_df_B07X37WH5Z/?tag=&linkCode=df0&hvadid=386775205645&hvpos=&hvnetw=g&hvrand=7435587142193397440&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9006641&hvtargid=pla-841393864259&ref=&adgrpid=77847887454&th=1
Thanks
Paul
On Tue, Jul 5, 2022 at 1:55 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 11:13 AM 7/5/2022, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Paul Eckenroth" <jeckenroth(at)nbn.net (jeckenroth(at)nbn.net)>
Thanks for the reply Bob. Unfortunately my GRT EFIS cannot read Thermocouples except as used for EGT and CHT. Therefore I am using small inexpensive readers off Amazon. I seriously doubt if they have any specialized circuitry other than that require to give a basic reading of the thermocouple. Does the ground loop have the potential of rendering the reading totally unusable or is it more like a 5% loss of accuracy. The ungrounded probes are around twice as expensive as the grounded probes. I would certainly be willing to consider a better reader if it can be had in a small format. The Onex is a single seat very small plane with limited panel space. Thanks for your help. |
Okay, best bet is go grounded thermocouples PLUS
take your power ground wire for the instrument(s)
out to some handy spot on engine.
Got a link for these probes?
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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N509RV(at)eckenroth.com
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| Posted: Wed Jul 06, 2022 2:57 pm Post subject: Thermocouples: grounded or ungrounded |
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Plan to attach two thermocouples to the stator with the stator attachment bolts. There is also a thermocouple imbedded in one of the generator coils by Revmaster, but the lead wires are solid and extremely thin so I'm not anticipating a long life. I will also attach a thermocouple to the voltage regulator. The most important temperature readings will be of the stator. Unfortunately since I can't run the readings through the EFIS, I will not have an alarm.I will look into a rotary switch. The grounds on my plane are all connected to 2 forest of tabs fore and aft on the firewall with a #4 wire to the engine, so I think that connecting the reader ground to this grounding point would be the same as connecting directly to the engine. Thanks for the comments.
Paul
On Tue, Jul 5, 2022 at 8:20 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 03:53 PM 7/5/2022, you wrote:
Okay, how do you plan to attach the probes to the
stator . . . bolts? We used to weasel small t/c
wires under the stator windings of a motor.
How many total points to you plan to monitor?
Have you considered a single instrument and rotary
selector switch to manually scan through the
array of thermocouples?
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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N509RV(at)eckenroth.com
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| Posted: Fri Jul 08, 2022 1:05 pm Post subject: Thermocouples: grounded or ungrounded |
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[img]cid:ii_l5cxf3460[/img]
Bob
I agree with what you are saying concerning the ideal method of monitoring the coils. However I need to work with what I have and I don't think I can monitor 12 individual coils with the tools at my disposal. I do plan on using your idea of the rotary switch and I appreciate the schematic. I thought all might be interested in a picture of the stator after removal. The one coil suffered catastrophic damage with the phenolic spacer completely destroyed by the heat. I have machined the case to allow more ventilation and will add two blast tubes directly at the coils. And hope for the best.
Paul
I did check with GRT and their EIS/ EFIS cannot read thermocouples except through the 4 or 6 EGT and CHT inputs.
On Thu, Jul 7, 2022 at 3:22 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | | Quote: | | The most important temperature readings will be of the stator. |
yeaaahhh . . . sort of. Temperature numbers of the stator
stack adjacent to mounting bolts will be greatly
influenced by heat transfer to the crankcase.
We know that the stator failures are generally limited
to one pole. How do the wires look on remaining poles?
To precipitate this 'isolated' failure suggests a thermal
runaway event where the resistance of copper rises with
temperature which in turn, increases energy dissipated in
the wires. I.e. a positive feedback for temperature
excursion.
To catch a pending failure suggest a need to monitor
each winding . . . unless field experience shows that
failures are concentrated.
In any case, other temperatures around the system's
components may be interesting data but I'm skeptical
as to their ability to flag impending failure any
one coil.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
|
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