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JDA_BTR
Joined: 05 Sep 2015
Posts: 3
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 Posted: Tue Sep 08, 2015 1:55 pm Post subject: Pitot Heat Ground |
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I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on.
I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery.
So I'm struggling with the idea that my ground wire will return the current to the battery in a sufficient way to stifle interference of the ADAHRS magnetometer. I figure a lot of current will take the controller box path and a lot will take the wire path, but since I don't own two ammeters I won't be knowing for sure.
So if the controller box is going to ground, seems like taking the ground wire to a lug near the box might as well be done, because I won't get an interference benefit running the ground wire back to the battery. Lots of people ground the controller locally without trouble despite the reccs by Dynon to run a wire all the way back.
Anyone have an expert opinion?
I'm about at the point of blowing off the long ground return and just getting a remote magnetometer if it makes a difference.
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ceengland7(at)gmail.com
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| Posted: Tue Sep 08, 2015 2:26 pm Post subject: Pitot Heat Ground |
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On Tue, Sep 8, 2015 at 4:55 PM, JDA_BTR <dudley(at)jdatkinson.net (dudley(at)jdatkinson.net)> wrote:
| Quote: | --> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net (dudley(at)jdatkinson.net)>
I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on.
I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery.
So I'm struggling with the idea that my ground wire will return the current to the battery in a sufficient way to stifle interference of the ADAHRS magnetometer. I figure a lot of current will take the controller box path and a lot will take the wire path, but since I don't own two ammeters I won't be knowing for sure.
So if the controller box is going to ground, seems like taking the ground wire to a lug near the box might as well be done, because I won't get an interference benefit running the ground wire back to the battery. Lots of people ground the controller locally without trouble despite the reccs by Dynon to run a wire all the way back.
Anyone have an expert opinion?
I'm about at the point of blowing off the long ground return and just getting a remote magnetometer if it makes a difference.
|
 My 1st inexpert reaction is, why is the magnetometer 'built in' the same wing as the heated pitot? And 2nd reaction is, why can't it be built into (moved to) the other wing? Never built a -14 or installed a Dynon, but I'll bet their install instructions for the magnetometer say to keep it as far away from high current wires and RF as possible.
[quote][b]
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JDA_BTR
Joined: 05 Sep 2015
Posts: 3
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| Posted: Tue Sep 08, 2015 2:37 pm Post subject: Re: Pitot Heat Ground |
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It's a good thought. The ADAHRS location in the wing is very convenient to the pitot for AOA. The magnetometer can be had as a remote unit and put elsewhere.
Nonetheless I would like to know about the situation as presented, because I'm curious - why would the controller box be grounded if it is so important to carry a ground wire all the way back to the battery; and when the faqs here say the ground can be local and simple. Would running that wire reduce magnetometer interference even if I ran it?
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ceengland7(at)gmail.com
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| Posted: Tue Sep 08, 2015 3:14 pm Post subject: Pitot Heat Ground |
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Again, inexpert opinion...But with a high-current device like a pitot tube heater, they are probably trying to guarantee a solid ground path for the heater. With all the variations in building techniques, and hundreds of 'joints' between the pitot & the battery negative, it's conceivable that one could have several ohms of resistance in the ground path when using the airframe. Could be worse with a tube/fabric airframe, through the steel. Another thought is that the instructions are intended to be 'universal', so no matter the airframe material, a good ground is provided.
But I'd hope someone at Dynon could give you the real answer.
If the magnetometer is built into the ADAHRS, you could just prepare for having to remote it (a few extra wires), and just try it before purchasing the extra magnetometer.
On Tue, Sep 8, 2015 at 5:37 PM, JDA_BTR <dudley(at)jdatkinson.net (dudley(at)jdatkinson.net)> wrote:
[quote]--> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net (dudley(at)jdatkinson.net)>
It's a good thought. The ADAHRS location in the wing is very convenient to the pitot for AOA. The magnetometer can be had as a remote unit and put elsewhere.
Nonetheless I would like to know about the situation as presented, because I'm curious - why would the controller box be grounded if it is so important to carry a ground wire all the way back to the battery; and when the faqs here say the ground can be local and simple. Would running that wire reduce magnetometer interference even if I ran it?
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=446901#446901
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enginerdy(at)gmail.com
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| Posted: Tue Sep 08, 2015 3:26 pm Post subject: Pitot Heat Ground |
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There are a couple of potential answers to this. One simple one is that the ground connection between the chassis of the box and the power ground inside the box is not low-impedance. In designing circuits, you do what you can to minimize radiated and conducted noise, and one of these ways is to segregate grounds internally. The connection between the two may be a giant bonding braid, a screw through the board, or even a 1 MOhm resistor.
Another reason that the installation manual may encourage a wire pair instead of a single conductor and chassis ground is that high current devices create larger magnetic fields. Magnetic fields are proportional to several things, two of which are current and current loop area. By taking two wires back to the battery, you minimize loop area. When using the chassis ground method, you have no real control over the loop area.
There may be another line of reasoning, but that is only known to the engineer who wrote the installation manual unfortunately.
I am not that guy, so I have no idea, but that should give you some possibilities to consider.
—Daniel
| Quote: | On Sep 8, 2015, at 5:37 PM, JDA_BTR <dudley(at)jdatkinson.net> wrote:
It's a good thought. The ADAHRS location in the wing is very convenient to the pitot for AOA. The magnetometer can be had as a remote unit and put elsewhere.
Nonetheless I would like to know about the situation as presented, because I'm curious - why would the controller box be grounded if it is so important to carry a ground wire all the way back to the battery; and when the faqs here say the ground can be local and simple. Would running that wire reduce magnetometer interference even if I ran it?
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=446901#446901
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JDA_BTR
Joined: 05 Sep 2015
Posts: 3
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| Posted: Wed Sep 09, 2015 3:31 am Post subject: Re: Pitot Heat Ground |
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I spoke with a knowledgeable person at Dynon and was told that the MOSFET power transistor in the controller uses the case for a heat sink and thus is grounded to same; so the ground to the chassis is by design and necessary. It will also be low impedance in this configuration. In a metal airplane even if a wire is carried back to the battery most if not all of the current will pass to the mounting points of the controller box; so grounding the box to the airframe locally will make as much sense as carrying a wire back to the battery.
There won't be a way to make a balanced wire run for the pitot heat controller.
I could mount the controller in the fuselage and run the actual pitot wires balanced out to the heater. Will have to consider that; but I'm thinking now to just wire it up and if the magnetometer is affected (likely) then I'll remote it to another location, or relocate the ADAHRS/magnetometer.
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Kellym
Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ
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| Posted: Wed Sep 09, 2015 6:11 am Post subject: Pitot Heat Ground |
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IIRC the general guidance is that high current devices like pitot heat
and landing lights, nav lights do not need separate ground wire back to
grounding block, only sensitive low current electronic devices like
radios, intercoms, etc need central grounding. I haven't flown yet, but
have the Dynon heated pitot with the controller mounted within 6" of the
pitot mast. If you can't mount the combined ADAHRS in the fuselage
clear of magnetic influences. they also offer a separate, remote
magnetometer that you could mount out near a wing tip.
On 9/9/2015 4:32 AM, JDA_BTR wrote:
| Quote: |
I spoke with a knowledgeable person at Dynon and was told that the MOSFET power transistor in the controller uses the case for a heat sink and thus is grounded to same; so the ground to the chassis is by design and necessary. It will also be low impedance in this configuration. In a metal airplane even if a wire is carried back to the battery most if not all of the current will pass to the mounting points of the controller box; so grounding the box to the airframe locally will make as much sense as carrying a wire back to the battery.
There won't be a way to make a balanced wire run for the pitot heat controller.
I could mount the controller in the fuselage and run the actual pitot wires balanced out to the heater. Will have to consider that; but I'm thinking now to just wire it up and if the magnetometer is affected (likely) then I'll remote it to another location, or relocate the ADAHRS/magnetometer.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=446911#446911
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nuckolls.bob(at)aeroelect
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| Posted: Wed Sep 09, 2015 9:07 am Post subject: Pitot Heat Ground |
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At 04:55 PM 9/8/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net>
I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on.
I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery. |
NOBODY should ever need to run any applicance
wire(s) ground or otherwise, directly to the
battery. Can you point us to a copy of the
system wiring diagram? Do you KNOW how much
power the pitot heater needs?
Bob . . . [quote][b]
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lyleapgmc
Joined: 19 Feb 2014
Posts: 57
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| Posted: Wed Sep 09, 2015 10:16 am Post subject: Pitot Heat Ground |
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I was going to say what Bob says here. The battery posts are for one, and only one, purpose. That is the connect for the main supply cables for the vehicle. No other wire should be attached at that point. Terminal strips are for that purpose.
Wiring connected directly to the battery posts will corrode. Even large battery cables will corrode internally. I have a ten inch cable that ran from the battery to the starter solenoid that was green under the insulation for its entire length. This reduces the useable conductor in the cable and restricts its ability to carry the required current. With smaller gauge wires that reduction is tantamount to cutting the wire.
On 9/9/2015 12:06 PM, Robert L. Nuckolls, III wrote:
[quote] At 04:55 PM 9/8/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net> (dudley(at)jdatkinson.net)
I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on.
I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery. |
NOBODY should ever need to run any applicance
wire(s) ground or otherwise, directly to the
battery. Can you point us to a copy of the
system wiring diagram? Do you KNOW how much
power the pitot heater needs?
Bob . . . [b]
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ribrdb(at)gmail.com
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| Posted: Wed Sep 09, 2015 10:52 am Post subject: Pitot Heat Ground |
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I don't see anything in the dynon installation manual about connecting directly to the battery.[url=http://dynonavionics.com/downloads/Install_Guides/Heated_Unheated_AOA_Pitot_Probe_Installation_Guide_-_Rev_C.pdf`]http://dynonavionics.com/downloads/Install_Guides/Heated_Unheated_AOA_Pitot_Probe_Installation_Guide_-_Rev_C.pdf`[/url]
It says:
Do not connect the Black (Ground) wire to the airframe as a Ground connection. Doing so will introduce high currents (10A) into the airframe. This can introduce a significant voltage drop, and potentially cause engine instrumentation, avionics, and audio system electrical problems.
Also it says the ground wire should be:
Constant connection to ground (not routed through a switch or fuse / circuit breaker). Must be sized to conduct 10A with minimal voltage drop. Ground connection must be constant FOR PITOT HEAT STATUS LINE (White wire) to operate when the controller is powered Off or not functioning.
On Wed, Sep 9, 2015 at 11:21 AM Lyle Peterson <lyleap(at)centurylink.net (lyleap(at)centurylink.net)> wrote:
[quote] I was going to say what Bob says here. The battery posts are for one, and only one, purpose. That is the connect for the main supply cables for the vehicle. No other wire should be attached at that point. Terminal strips are for that purpose.Â
Wiring connected directly to the battery posts will corrode. Even large battery cables will corrode internally. I have a ten inch cable that ran from the battery to the starter solenoid that was green under the insulation for its entire length. This reduces the useable conductor in the cable and restricts its ability to carry the required current. With smaller gauge wires that reduction is tantamount to cutting the wire.Â
On 9/9/2015 12:06 PM, Robert L. Nuckolls, III wrote:
| Quote: | At 04:55 PM 9/8/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net> (dudley(at)jdatkinson.net)
I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on.
I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery. |
  NOBODY should ever need to run any applicance
  wire(s) ground or otherwise, directly to the
  battery. Can you point us to a copy of the
  system wiring diagram? Do you KNOW how much
  power the pitot heater needs?
 Bob . . .
ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
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[b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Sep 10, 2015 4:44 am Post subject: Pitot Heat Ground |
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| Quote: |
But I'd hope someone at Dynon could give you the real answer. |
Agreed. I've had some conversation with Dynon
in years past . . . conversations that gave
the impression of solid technical competence.
Given their successes in this market, your
expectations for the quality of their support
is not unwarranted.
| Quote: | If the magnetometer is built into the ADAHRS, you could just prepare for having to remote it (a few extra wires), and just try it before purchasing the extra magnetometer.
On Tue, Sep 8, 2015 at 5:37 PM, JDA_BTR <dudley(at)jdatkinson.net (dudley(at)jdatkinson.net)> wrote:
--> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net (dudley(at)jdatkinson.net) >
It's a good thought. The ADAHRS location in the wing is very convenient to the pitot for AOA. The magnetometer can be had as a remote unit and put elsewhere.
Nonetheless I would like to know about the situation as presented, because I'm curious - why would the controller box be grounded if it is so important to carry a ground wire all the way back to the battery; and when the faqs here say the ground can be local and simple. Would running that wire reduce magnetometer interference even if I ran it? |
Exactly, that "ground wire directly to the
battery" phrase has popped up over the
years in several OBAM aviation installation
manuals . . . were anyone at King Radio, Collins,
Narco or ARC to make such an assertion, the
writer should be tasked with a "lunchtime
learning session" for colleagues and instructed
to "bring us up to date on the physics of
your assertion." I would LOVE to be in
the audience for THAT one!
I'm not clear as to where the all the
components are located on the airplane
with respect to each other . . . pitot
tubes are generally on the wing, the AHRS
is out there too? Help me understand . . .
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Sep 10, 2015 5:05 am Post subject: Pitot Heat Ground |
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At 01:50 PM 9/9/2015, you wrote:
| Quote: | I don't see anything in the dynon installation manual about connecting directly to the battery.
[url=http://dynonavionics.com/downloads/Install_Guides/Heated_Unheated_AOA_Pitot_Probe_Installation_Guide_-_Rev_C.pdf`] http://dynonavionics.com/downloads/Install_Guides/Heated_Unheated_AOA_Pitot_Probe_Installation_Guide_-_Rev_C.pdf`[/url]
It says:
Do not connect the Black (Ground) wire to the airframe as a Ground connection. Doing so will introduce high currents (10A) into the airframe. This can introduce a significant voltage drop, and potentially cause engine instrumentation, avionics, and audio system electrical problems. |
Good find . . . the author of these words demonstrates
a poor understanding of grounding for the purposes
of achieving (1) reliable power to the device and (2)
minimizing risk of noise propagation to other
systems.
| Quote: | Also it says the ground wire should be:
Constant connection to ground (not routed through a switch or fuse / circuit breaker). Must be sized to conduct 10A with minimal voltage drop. Ground connection must be constant FOR PITOT HEAT STATUS LINE (White wire) to operate when the controller is powered Off or not functioning. |
Just why anyone would run a ground wire through a
switch is a mystery . . .
It's sad to discover these kinds of muddy thought
in the manuals for what should be a no-brainer
installation. Those kinds of statement inject what I
have dubbed a "divide by zero" factor into
the document.
[img]cid:.0[/img]
Whether you're a politician, engineer, philosopher, CEO
. . . what ever. Validity of many hours/pages of ideas become
suspect when you include the "divide by zero" assertion
into your 'formula for success'.
Such assertions will at best make the rest of your
ideas suspect . . . if not nullify them entirely.
It's sort like spending an hour on the phone with tech
support before somebody notices that the thing isn't
even plugged in . . .
I used to have a contact in Dynon management . . . haven't
talked in years. I'll see if he's even still there . . .
Bob . . .
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gerry.vandyk(at)eastlink.
Guest
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| Posted: Thu Sep 10, 2015 5:59 am Post subject: Pitot Heat Ground |
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Bob, perhaps I'm misunderstanding this thread. You said: | Quote: | Just why anyone would run a ground wire through a switch is a mystery . . . It's sad to discover these kinds of muddy thoughts in the manuals for what should be a no-brainer installation.
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The quote from the installation manual is:
| Quote: | | Quote: | | Constant connection to ground (NOT routed through a switch or fuse / circuit breaker). |
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Isn't the manual telling the installer to bring the ground wire back to the main ground block, presumably the forest-of-tabs on the firewall? Why would this manual be cause for concern?
Gerry
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Sep 10, 2015 7:37 am Post subject: Pitot Heat Ground |
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| Quote: | | Quote: | | Quote: | | Constant connection to ground (NOT routed through a switch or fuse / circuit breaker). |
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Isn't the manual telling the installer to bring the ground wire back to the main ground block, presumably the forest-of-tabs on the firewall? Why would this manual be cause for concern? |
Why would such a statement even be necessary?
Are there cases in the wild where someone's
on-purpose design did take ground through a fuse
or switch? The mere mention this would probably
bring smiles to the system integrators at
Textron but it plant's seeds of an exceedingly
poor idea in the minds of their target
market . . . the neophyte builder/installer.
"Gee, are there instances where ground paths
SHOULD go through switches or breakers?"
This is why I have always encouraged folks
to supply lucid wiring schematics with their
products. No pictograms. Schematics are
to electronics as numbers and operator
symbols to mathematicians. The universal
language understanding of how best to assemble
something is the annotated schematic . . .
like those you see in Appendix Z of the
book, diagrams at the back of a Cesnna
Service Manual, or the EXCELLENT diagrams
published for KIA cars. Those diagrams offer
a path for understanding between designers and
users. I once spent a couple weeks in CA
giving Japanese technicians an overview of
a television time base corrector that my
company built . . . and I wrote the manual
for. Their English was poor, my Japanese nil,
but they understood television, waveforms
and SCHEMATICS.
If an idea can be easily depicted on the
wiring diagram, then do it. This completely
negates the need for words. If words ARE
offered, they should re-enforce the schematic
with very low risk for doubt.
Words are 'icing' on a an underlying
'cake' that should stand alone on the
language of universal symbology and
mathematical expression.
I would hope the author of the ground wire
statement was simply trying to compensate
for the lack of lucid illustrations. As
we've seen in this thread, vagaries in
the use and understanding of language have
generated a whole lot of conversation trying
to figure out what COULD have been described
in a few lines on a page. Unfortunately,
other words in the document do not suggest
that the author has a good handle on how
the various appliances on an airplane can
be encouraged to happily co-exist.
This may well be an example of a condition
common to many tech manufacturers. Marketing
hands down goals, engineering designs to
the goals, production experts package and
manufacture the thing and 'tech writers'
put out the books. It's the Silo-Syndrome
where good stuff can get blown to the winds
as a task is pitched from one silo to the next.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Sep 10, 2015 7:44 am Post subject: Pitot Heat Ground |
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At 04:55 PM 9/8/2015, you wrote:
| Quote: | Subject: AeroElectric-List: Pitot Heat Ground
From: "JDA_BTR" <dudley(at)jdatkinson.net>
To: aeroelectric-list(at)matronics.com
--> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net>
I read the faqs, and know the recommendation to just take the pitot ground to the chassis. |
Let's see if we can zero in on the physics. Is
The pitot heat controller in conductive enclosure?
Does its ground wire make electrical connection
to the enclosure?
If the above are both 'yes' then run the power
ground to a point on the airframe separate from
but in the vicinity of where the controller
lives in the airplane.
Adding a 'ground wire' off to some other
portion of the airplane is of no value. It
will simply be a conductor of much higher
resistance than the airframe and carry a
exceedingly small proportion of pitot heater
current.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Sep 10, 2015 1:04 pm Post subject: Pitot Heat Ground |
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Another 'point of physics' that goes to your
question. I just went to the workbench, laid
my handy military surplus compass on the bench
then held a wire over the needle at the optimal
angle to antagonize the readings.
With 3A flowing in the wire, I had to lower
it to within about 5 inches to detect any
movement of the needle. 10A would probably
produce noticed motion at 14 inches or so.
The interference I noted was 1 degree or
less. The most rigorous certification of
hardware under DO160 for magnetic effects
is 'category Z' which may produce 1 degree
of deflection while located 0.3 meters away
from the compass . . . or about 12 inches.
My 'shade tree' magnetic effects experiment
suggests that 12' of separation for a 10A
conductor will produce no observable deflections
from real magnetic heading.
Bob . . . [quote][b]
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gerald.champagne(at)gmail
Guest
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| Posted: Thu Sep 10, 2015 3:52 pm Post subject: Pitot Heat Ground |
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On Thu, Sep 10, 2015 at 4:02 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: |
  With 3A flowing in the wire, I had to lower
  it to within about 5 inches to detect any
  movement of the needle. 10A would probably
  produce noticed motion at 14 inches or so.
|
That doesn't sound right. The strength of a magnetic field should increase linearly with current, but should fall off as a function of distance cubed. A 3A current at 5 inches should show the same deflection as a 10A current at about 7.5 inches, not 14 inches. Did I miss something?
GeraldÂ
[quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Fri Sep 11, 2015 10:31 am Post subject: Pitot Heat Ground |
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| Quote: | | That doesn't sound right. The strength of a magnetic field should increase linearly with current, but should fall off as a function of distance cubed. A 3A current at 5 inches should show the same deflection as a 10A current at about 7.5 inches, not 14 inches. Did I miss something? |
Nope . . . looks like I punched the 1/sqrt button
instead of 1/3rdrt . . . thank's for the heads-up!
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Wed Sep 16, 2015 10:31 am Post subject: Pitot Heat Ground |
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At 04:55 PM 9/8/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net>
I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on. |
| Quote: | I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery.
So I'm struggling with the idea that my ground wire will return the current to the battery in a sufficient way to stifle interference of the ADAHRS magnetometer. I figure a lot of current will take the controller box path and a lot will take the wire path, but since I don't own two ammeters I won't be knowing for sure. |
Do I understand that the power ground for pitot
heat connects to chassis ground of the controller?
In other words, if there were NO external ground wire
attached, the pitot heater would continue to function?
| Quote: | | So if the controller box is going to ground, seems like taking the ground wire to a lug near the box might as well be done, because I won't get an interference benefit running the ground wire back to the battery. Lots of people ground the controller locally without trouble despite the reccs by Dynon to run a wire all the way back. |
Your reasoning seems sound. The LOWEST resistance
ground path will be through the box mounting and
the longer the ground wire, the lower will be its
share of the load.
| Quote: | Anyone have an expert opinion?
I'm about at the point of blowing off the long ground return and just getting a remote magnetometer if it makes a difference. |
You're conducting the grand experiment. I think
your interference effects will be minimal. If
the ground currents were being carried on a linear
hunk of structure adjacent to the magnetometer,
the potential for interference is higher than
for the fields associated with sheet current spread
out over the area of the skin. Could you put the
pitot tube on the other wing?
But your gut feeling is correct . . . separate ground
all the way to battery does not speak well of
Dynon's grasp of the art and sciences involved.
Were I designing such a beast, all power wiring
between tube and box . . . and from box to ship's power
would be on twisted pairs with the box providing
EMC grounding only . . . power would float (this
is the way the BIG guys do it).
The Dynon manual speaks to about 3' of harness
between box and tube? Does then cause you mount
the control box close to the magnetometer?
Bob . . . [quote][b]
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Kellym
Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ
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| Posted: Wed Sep 16, 2015 2:21 pm Post subject: Pitot Heat Ground |
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|
I just went through the Dynon Pitot Install manual as a refresher. I agree that whoever wrote the caution about not grounding the negative power lead to the wing, for a metal wing aircraft, makes zero sense,
The peak current in question is 10 amps. Wings on multi-engine aircraft routinely serve as ground path for engine ground and starter currents. I would be surprised to see any magnetometer interference if the current were carried in the wing skin. The only issue would be the changing current as the controller maintained the set temp with more or less current.
The compass calibration is now done in flight, so it could be done without pitot heat, then repeated with pitot heat to see if it degraded at all.
I will have to look at my own install, but since the distance from wing mounting to forrest of tabs was about 12 ft. I probably just used a red/black 14 gauge twisted pair to my central grounding point.
I do not understand mounting the Skyview ADAHRS in the wing if there is any reasonable place for it in the fuselage. If magnetic interference is the issue, Dynon sells a remote magnetometer that can be mounted near the outer end of either wing. Performance of the ADAHRS will be better in the fuselage than within the wing, even is staying within the recommended 6 ft of centerline, in a wing.
On 9/9/2015 11:20 AM, Robert L. Nuckolls, III wrote:
[quote] At 04:55 PM 9/8/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "JDA_BTR" <dudley(at)jdatkinson.net> (dudley(at)jdatkinson.net)
I read the faqs, and know the recommendation to just take the pitot ground to the chassis.
But I have a specific situation in mind, and would like some thoughts on it. I'm building an RV14 and the pitot heat controller will be about 4 feet outboard of an ADAHRS in the wing with a built in magnetometer. I figured if I ran the 14V/Gnd pair together as far from the magnetometer as possible I stood a chance of not interfering with it when the heater is on. |
| Quote: | I've gotten a Dynon heated pitot tube with controller. In this controller the aluminum controller box is connected to ground as is the ground lead leaving the box. Dynon says to a) not use the box for ground, and b) run a ground wire all the way to the battery.
So I'm struggling with the idea that my ground wire will return the current to the battery in a sufficient way to stifle interference of the ADAHRS magnetometer. I figure a lot of current will take the controller box path and a lot will take the wire path, but since I don't own two ammeters I won't be knowing for sure. |
Do I understand that the power ground for pitot
heat connects to chassis ground of the controller?
In other words, if there were NO external ground wire
attached, the pitot heater would continue to function?
| Quote: | | So if the controller box is going to ground, seems like taking the ground wire to a lug near the box might as well be done, because I won't get an interference benefit running the ground wire back to the battery. Lots of people ground the controller locally without trouble despite the reccs by Dynon to run a wire all the way back. |
Your reasoning seems sound. The LOWEST resistance
ground path will be through the box mounting and
the longer the ground wire, the lower will be its
share of the load.
| Quote: | Anyone have an expert opinion?
I'm about at the point of blowing off the long ground return and just getting a remote magnetometer if it makes a difference. |
You're conducting the grand experiment. I think
your interference effects will be minimal. If
the ground currents were being carried on a linear
hunk of structure adjacent to the magnetometer,
the potential for interference is higher than
for the fields associated with sheet current spread
out over the area of the skin. Could you put the
pitot tube on the other wing?
But your gut feeling is correct . . . separate ground
all the way to battery does not speak well of
Dynon's grasp of the art and sciences involved.
Were I designing such a beast, all power wiring
between tube and box . . . and from box to ship's power
would be on twisted pairs with the box providing
EMC grounding only . . . power would float (this
is the way the BIG guys do it).
The Dynon manual speaks to about 3' of harness
between box and tube? Does then cause you mount
the control box close to the magnetometer?
Bob . . . [b]
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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
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