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  #1  
Old 03-29-2017, 04:58 PM
Brent 801's Avatar
Brent 801 Brent 801 is offline
 
Join Date: May 2012
Location: Conroe, TX
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Default Pitot Tube wiring past Magnetometer

I'm preparing to run the wires from my Garmin GAP 26 heated pitot inboard to the fuse. Is there any concern about running the pitot heat wiring close to the Garmin Magnetometer which is installed in the left wing where a Dynon ADAHRS would by located if using Dynon?
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Old 03-29-2017, 06:17 PM
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Pitot heaters are heavy current users, this means serious potential for magnetic interference. Best practice would be to run a twisted pair (pwr/grd) from the tube all the way to the switch and ground block and keep the wiring as far as practical away from the magnetometer, then keep your fingers crossed.
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Last edited by Walt : 03-29-2017 at 06:19 PM.
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Old 03-29-2017, 06:57 PM
DerekS DerekS is offline
 
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Location: Ontario, Canada
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Default My Experience

I have mine quite close to the pitot, and am using Dynon's temperature regulating probe which I imagine is noisier than just an on/off spike since it has to regulate.

First time I tested it was close enough to fail the self-test. In my case I was able to reroute the wires so that they were 6 inches further away, at which point I no longer saw any impact when I turned it on or off during the self test.

Distance really does matter a lot, and 3 or 4 inches is too close.

(In my case not a twisted pair install)


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Old 03-29-2017, 07:50 PM
KeithB KeithB is offline
 
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Yes, pitot heat power will interfere with the Garmin magnetometer mounted on the ADAHRS shelf. I ran a power wire in the "bundle" in the wing with a ground to a rib by the pitot - failed miserably. Then I ran a ground wire (not twisted at this point as Walt has recommended) - better, but still failed. Then I rerouted the ground farther from the magnetometer, twisted it around the power wires(s) near the magnetometer and got better results. It still fails Garmin's interference test for about 3 seconds during power up and power down, but I called it a victory.
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Old 03-29-2017, 07:56 PM
rockwoodrv9 rockwoodrv9 is offline
 
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I know there are differences between the 9 and 14, so is there a reason the magnetometer located in the wing rather than the back tail deck area? Im not flying yet so I don't know how mine will work, but there isn't nearly as many wires back there as by the pitot. There must be a reason. Thanks
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Old 09-04-2017, 07:00 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by KeithB View Post
Yes, pitot heat power will interfere with the Garmin magnetometer mounted on the ADAHRS shelf. I ran a power wire in the "bundle" in the wing with a ground to a rib by the pitot - failed miserably. Then I ran a ground wire (not twisted at this point as Walt has recommended) - better, but still failed. Then I rerouted the ground farther from the magnetometer, twisted it around the power wires(s) near the magnetometer and got better results. It still fails Garmin's interference test for about 3 seconds during power up and power down, but I called it a victory.
Is there any reason not to run coax for pitot heat? I'm guessing it would be much better than twisted pair.
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Old 09-04-2017, 07:29 PM
JDA_BTR JDA_BTR is offline
 
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The physics of twisted pair is better than coax for this application. So coax isn't better, and it is bulkier to boot.

In hindsight I wish I had installed the pitot to the right wing; that would have been better overall. But my Gretz mast is firmly installed on the left and won't get moved. The shelf is in the left wing too - also not going to move. One thing that may make a difference for me is I have the Dynon pitot heat and not the Garmin, with the rest of my kit Garmin. Maybe it will act a little different current-wise.

Its interesting because for sure with pitot heat off it will all work fine. Its when I turn it on that it will be wonky - maybe. But as the pitot heats up, the current variations will diminish, and the magnetometer should settle down albeit with some new fixed error. I wonder what that will be.

If the magnetometer in the wing is a total loser I suppose I can at anytime make a shelf in the tail section and put it off to the right side up there.

I think I'd rather have a card-compass on the dash than a second magnetometer. What happens in the G3X system if you don't install a magnetometer? Can that be ignored?
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Old 09-04-2017, 07:32 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by Aluminum View Post
Is there any reason not to run coax for pitot heat? I'm guessing it would be much better than twisted pair.
Other than cost and weight, no - but make sure the center conductor and shield can handle 10 amps dc without significant loss.
We're concerned with a steady state magnetic field here, not RF. A tightly twisted pair should be nearly as good as a coaxial arrangement. But (in either case) this only works if the ground return is not connected to the pitot tube or its mounting harware. You need the return current thru one wire to equal the feed current thru the other.
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Old 09-04-2017, 08:26 PM
SabreFlyr SabreFlyr is offline
 
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Quote:
Originally Posted by BobTurner View Post
But (in either case) this only works if the ground return is not connected to the pitot tube or its mounting harware. You need the return current thru one wire to equal the feed current thru the other.
I'm a few months from needing to worry about this and haven't studied it yet but this confuses me a bit. You DON'T want the ground connected to either the pitot tube or its mount?!?
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Old 09-04-2017, 09:00 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by SabreFlyr View Post
I'm a few months from needing to worry about this and haven't studied it yet but this confuses me a bit. You DON'T want the ground connected to either the pitot tube or its mount?!?
You want the 'return line' (often called the ground wire) from the heater to go back along side of, or better twisted with, the wire which supplies the current to the 'hot' or input side of the heater. Ground it on the firewall, or much better, at a universal grounding point/block. The return wire should not be connected to the metal tube or any metal structure, which would allow the return current to flow thru the metal structure. You want the only possible path to be thru the return wire, so its current is equal (but opposite) to the input wire, so their magnetic fields will cancel out. If your pitot tube has its return line connected internally to the metal structure, you will not be able to easily cancel out the input wire's magnetic field.
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