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mprather(at)spro.net
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 Posted: Wed Dec 06, 2006 7:42 pm Post subject: Pitot Current Draw |
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Here's an idea that may draw snickers, jeers, and possibly gasps of
horror. My sister (like lots of peoples' sisters) has a curling iron
heated by butane.. Why doesn't anyone have a pitot tube heated directly
by the combustion of a hydrocarbon (gasoline/jetA might be problematic
without a pump). It would be tremendously efficient compared to hauling
around a gigantic alternator. It would run for hours on a tiny amount of
fuel. It could be thermostatically controlled, and be amazingly light
weight.
I know, it's not an "electric" idea...
Matt-
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armywrights(at)adelphia.n
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| Posted: Wed Dec 06, 2006 7:55 pm Post subject: Pitot Current Draw |
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For developing your own answer based upon your stated needs, I’ll defer to Bob N. and the other smart aeroelectric types. For an off-the-shelf solution that may also fit your needs, have you looked at http://www.gretzaero.com/ga1000.html?
Rob Wright
RV-10 with Gretz Aero
Fuselage
~snip~
The GA-1000 is Gretz Aero's totally new design of a heated Pitot tube. It is molded from a very high-tech Polymer material that has nearly the same thermal conductivity as
--> -->metal. This new design has electronics embedded in the Pitot tube. It also comes with a small electronic control module that will mount close to the Pitot tube in the wing. This electronic control module controls the heat of the Pitot and provides information to the pilot regarding thermal and heater performance of the Pitot tube. This information is presented by way of a 3/16 inch diameter multi-conductor cable that has a very small circuit board with three LED's on it. This LED circuit board is already attached to the cable. The LED circuit board is designed to be placed behind the panel and allow the LED's to protrude through the panel at a convenient location to the pilot.
The three LED's are: Green = circuits are working and temperature of Pitot tube is well above freezing: Yellow = current is being provided to the heater due to low temperature: Red = temperature is low at the Pitot tube and needs heat. There will be times when one or two LED's will be lit. If no LED's are lit, there is a problem with power getting to the Pitot tube.
The GA-1000 is great for small electrical generating systems in light aircraft. The heater in the Pitot draws 7 Amps, and that is only when the electronic control module calls for heat. The heater will cycle on and off as needed to maintain the proper temperature. During the times when heat is not needed or called for, the electronic control module draws less than .1 Amp.
The Gretz Aero GA-1000 heated Pitot tube comes complete with: GA-1000 heated Pitot tube, electronic control module, LED circuit board and cable, screws for Pitot tube to mounting bracket installation, mounting hardware for mounting the electronic control module, and of course, complete instructions. The complete weight of the entire kit is 397 grams or 14.0 oz.
The GA-1000 Pitot tube has a very sleek and modern shape that is pleasing to the eye. It is very dark gray to black in color. If the builder feels the need, it can be painted with automotive high temperature engine paint. This is available at most auto parts stores. The GA-1000 will fit the standard Gretz Aero Pitot Tube Mounting Bracket Kits.
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Speedy11(at)aol.com
Sent: Wednesday, December 06, 2006 7:57 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Pitot Current Draw
Chuck,
You have just experienced the main thing I dislike about this forum. When you ask for advice or assistance, you will commonly get critique on why you should not do what it is you want to do. It takes several attempts at explaining that you don't care about opinions - you simply want assistance. For example, I've asked how to sample volts and amps in multiple places in my electrical system. I got "you don't need to know that info while you're flying." Regardless, I want to be able to do that so how can I do it? Answer: since I don't think you should do that I'm not going to tell you how to do it.
As you've discovered, getting an answer can be very frustrating.
Still, I've learned a lot about electrical stuff here and I continue to learn. So, I still find the forum of value.
I wish I had the answer for you - I would provide it. But, I don't. Hopefully, you will get an acceptable answer.
Stan Sutterfield
| Quote: |
Okay, I appreciate everyone holding forth that it's stupid to fly in
ice....I'm not that dumb, I only look it. However, the question stands,
how does one knock down the heat and current draw. Richard mentioned a
'strut switch'....whoever that is. Can someone enlighten us
non-electrical types. What I'm concerned about is being in the clouds,
close to freezing and loose electrical--I'd just as soon not have to
turn off the pitot heat because it's sucking the battery dry in short
order; I'd rather it be in long order.
Chuck Jensen www.aeroelectric.com |
0123456789012345
[quote][b]
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rdunhamtn(at)hotmail.com
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| Posted: Thu Dec 07, 2006 4:15 am Post subject: Pitot Current Draw |
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Grasshopper,
When I was in the Navy, practicing emergency medicine at NAS JAX Hospital, a
corpsman came to me and in the course of conversation brought up the idea of
vapor locking the heart. We had been
having a conversation about how the human body handles gases and he asked me
how much air one could inject into a vein in order to vapor lock the heart
and kill the person. Well, I was training these guys to be ready for
anything in combat so I didn't think anything about the question. I shot out
a WAG number of cc's and the conversation drifted on to other topics.
A few days later he didn't show up for work. One of his buddies informed me
that he had tried to commit suicide by injecting air into his veins! Nobody
else overheard our "vapor lock" conversation
and nobody knew I had advised him on the "proper" technic for vapor locking
the heart. Not even me!
Well, that young and foolish sailor lived through his crisis and I was
relieved. And since that day, I am reluctant to give "answers" just because
inquiring minds want to know. You see, I believe I have some measure of
culpability for the actions of others if I have enabled them in some way to
DO something bad with the information they're gathering. And, I believe the
listers here believe that too!
We listers want everyone here to play safe! Play yes, absolutely! BUT, play
safe! And if you ask questions that raise suspicion that you're not going to
play safe, don't expect your buddies here on the list to simply not notice!
You're asking them for advice precisely because they have expertise greater
than your own! Think about it. Sometimes the teacher has to backtrack the
pupil's training a step or two and illustrate why their [the student's]
thinking is wrong. Don't get your panties in a wad when that happens. The
enlightened student GETS the light bulb phenomenon and realizes that he did,
in fact, "get an acceptable answer."
Rodney in Tennessee
DO NOT ARCHIVE
| Quote: | From: Speedy11(at)aol.com
Reply-To: aeroelectric-list(at)matronics.com
To: aeroelectric-list(at)matronics.com
Subject: Re: Pitot Current Draw
Date: Wed, 6 Dec 2006 20:56:45 EST
Chuck,
You have just experienced the main thing I dislike about this forum. When
you ask for advice or assistance, you will commonly get critique on why you
should not do what it is you want to do. It takes several attempts at
explaining
that you don't care about opinions - you simply want assistance. For
example,
I've asked how to sample volts and amps in multiple places in my electrical
system. I got "you don't need to know that info while you're flying."
Regardless, I want to be able to do that so how can I do it? Answer: since
I don't
think you should do that I'm not going to tell you how to do it.
As you've discovered, getting an answer can be very frustrating.
Still, I've learned a lot about electrical stuff here and I continue to
learn. So, I still find the forum of value.
I wish I had the answer for you - I would provide it. But, I don't.
Hopefully, you will get an acceptable answer.
Stan Sutterfield
Okay, I appreciate everyone holding forth that it's stupid to fly in
ice....I'm not that dumb, I only look it. However, the question stands,
how does one knock down the heat and current draw. Richard mentioned a
'strut switch'....whoever that is. Can someone enlighten us
non-electrical types. What I'm concerned about is being in the clouds,
close to freezing and loose electrical--I'd just as soon not have to
turn off the pitot heat because it's sucking the battery dry in short
order; I'd rather it be in long order.
Chuck Jensen
|
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raymondj(at)frontiernet.n
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| Posted: Thu Dec 07, 2006 4:50 am Post subject: Pitot Current Draw |
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Rodney (O Wise Master),
Pretty arrogant of you to think that you have control of or responsibility
for another's actions and to think that you have any right to try to impose
your values on another by attempting to control their actions.
Raymond Julian
Kettle River, MN.
--
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nuckollsr(at)cox.net
Guest
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| Posted: Thu Dec 07, 2006 5:22 am Post subject: Pitot Current Draw |
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At 09:14 PM 12/6/2006 -0500, you wrote:
| Quote: | Ernest,
Please explain what you mean by a cockpit referenced backup altimeter. I
don't think I've ever heard of any thing like that. And how would it help
keep you from hitting the ground?
Maybe you're referring to alternate static air. But the question wasn't
on heating the static port - it was on losing pitot ram air to ice.
|
"Cockpit referenced" is a specific term for "vented to
cockpit atmosphere" . . . which is what the vast majority
of alternate static sources do.
The piper Pacer and Tri-Pacer didn't have static systems.
The altimeter was simply mounted to the panel and vented
to cockpit as was any VSI and/or altitude encoders added
at later dates.
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
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cjensen(at)dts9000.com
Guest
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| Posted: Thu Dec 07, 2006 5:27 am Post subject: Pitot Current Draw |
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Mike,
If I was able to draw the sarcasm out of you, I'd had to see what a pro could do! ( ). Actually, I've done the calculations and am satisfied of the results, even if not confirmed by empirical data. The current pitot draws about 15 amps. By comparison, my older, but very adequate (as in--never iced up) pitot drew 7 amps. While not on a first name basis with any electrons, I assume that an amp is an amp and is generally going to produce a certain amount of heat, which is apparently adequate, based on empirical experience. So, if 7 amps was good enough in the old pitot, I was thinkin' that 7 amps was probably good enough for this pitot. I could hire a thermodynamist to confirm all this, but I'll go out on a limb and employ common sense instead--it's cheaper and often, just as accurate.
As to your suggestion of simply using a bigger battery, I didn't notice any calculations showing how big and how many batteries I should have. However, if by limiting heat output in the pitot, I cut my amperage in half, I can calculate that the draw from my current battery will be 1/2 as much and last twice as long (but not for the full load, since the pitot is only a fraction of the total load; unfortunately, a high fraction). As to the suggestion to just turn it off.....duh, the reason we have a heated pitot is--never mind, it makes my head hurt.
Sorry for my poor response and sarcasm, but I think you brought it out of me.
All the forgoing is just kidding. Not many people intentionly flying their GA craft into icing and not many lose electrical power generation, but when the two are combined, even though the odds are it happening are small, the odds of ending up dead are quite large. So, if there was a reasonably simple mechanical/electrical means of reducing the current draw of the pitot, I wanted to do it. Kind of like flying an airplane--not many of us have real good reasons for doing it, but we just want to do it.
Chuck Jensen
Mike wrote....
The only way I can answer this question is to tell a story. First, flying in the ice is something many pilot do every year and sometimes often. If you know what you’re doing and are prepared then the risks are minimal. I have been flying in ice with airplanes that are able to handle it for better then 20 years. It’s not like flying on a sunny day. But once you get use to it and prepare for it, it’s not that big of a deal. A few words of advice, no two icing days are the same. Only experience will help you here. My personal feeling on ice in piston powered planes, you don’t have many out when the weather gets bad. When in ice greater then light find a place to land ASAP.
As for you electric question; First the easy answer is to put a bigger battery in the airplane. Second, why would you think that using less power for pitot heat is going to be any better then turning the pitot heat off all together? Unless you run a thermal equation on the speed, temperature, and heat (layman’s terms) required to guarantee the anti-ice properties needed, then everything your doing is just a big guess anyway. And I guessing that since I have to go to the book to do said equation thoroughly, I would guess the electrical formula by comparison would be a piece of cake. I know this is not what you want to hear but the facts are the facts. The reason that the pitot uses so much power (in general) is that is what it takes to keep ice off the probe in 100% humidity at temperatures < 10dec C.
Sorry for the poor and somewhat sarcastic answer, but I think Chuck brought it out in me.
Regards,
Mike Larkin
-----Original Message-----
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Speedy11(at)aol.com
Sent: Wednesday, December 06, 2006 6:57 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Pitot Current Draw
Chuck,
You have just experienced the main thing I dislike about this forum. When you ask for advice or assistance, you will commonly get critique on why you should not do what it is you want to do. It takes several attempts at explaining that you don't care about opinions - you simply want assistance. For example, I've asked how to sample volts and amps in multiple places in my electrical system. I got "you don't need to know that info while you're flying." Regardless, I want to be able to do that so how can I do it? Answer: since I don't think you should do that I'm not going to tell you how to do it.
As you've discovered, getting an answer can be very frustrating.
Still, I've learned a lot about electrical stuff here and I continue to learn. So, I still find the forum of value.
I wish I had the answer for you - I would provide it. But, I don't. Hopefully, you will get an acceptable answer.
Stan Sutterfield
| Quote: |
Okay, I appreciate everyone holding forth that it's stupid to fly in
ice....I'm not that dumb, I only look it. However, the question stands,
how does one knock down the heat and current draw. Richard mentioned a
'strut switch'....whoever that is. Can someone enlighten us
non-electrical types. What I'm concerned about is being in the clouds,
close to freezing and loose electrical--I'd just as soon not have to
turn off the pitot heat because it's sucking the battery dry in short
order; I'd rather it be in long order.
Chuck Jensen -- Please Support Your Lists This Month -- (And Get Some AWESOME FREE Gifts!) November is the Annual List Fund Raiser. Click on the Contribution link below to find out more about this year's Terrific Free Incentive Gifts provided |
0123456789012345678901
--
11/22/2006
--
11/22/2006
2 [quote][b]
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cjensen(at)dts9000.com
Guest
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| Posted: Thu Dec 07, 2006 5:36 am Post subject: Pitot Current Draw |
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Wow, this thread is about everything except how to reduce the pitot
current draw, though I'll follow up on the Gretz pitot heat control
idea.
I don't think Rodney was arrogant in thinking he had control or
responsibility for another's actions...though he might be mistaken.
Information and knowledge is a tool, how it is used is generally not
under the control and responsibility of the teacher. If that were the
case, Driver Eds teachers would be a horribly conflicted, guilt-ridden
bunch.
Chuck Jensen
Do Not Archive
--
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bob(at)bob-white.com
Guest
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| Posted: Thu Dec 07, 2006 7:24 am Post subject: Pitot Current Draw |
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On Thu, 7 Dec 2006 08:27:22 -0500
"Chuck Jensen" <cjensen(at)dts9000.com> wrote:
| Quote: | <snip> The current
pitot draws about 15 amps. By comparison, my older, but very adequate
(as in--never iced up) pitot drew 7 amps. While not on a first name
basis with any electrons, I assume that an amp is an amp and is
generally going to produce a certain amount of heat, which is apparently
adequate, based on empirical experience. So, if 7 amps was good enough
in the old pitot, I was thinkin' that 7 amps was probably good enough
for this pitot. I could hire a thermodynamist to confirm all this, but
I'll go out on a limb and employ common sense instead--it's cheaper and
often, just as accurate.
<snip>
|
Not quite true Chuck. Your old pitot tube was about 2 ohms. You were
drawing 7 amps from around 14 volts. Power is I^2 * R = 98 W. Now
you have a pitot that's about 1 ohm drawing 15 amps from 14 volts (0.933
ohms but who's counting). Drop the current in this pitot to 7 amps and
power is only 49 W. It was suggested by someone that the dropping
resistor could be mounted on the pitot also which would help, but still
not be equivalent.
Bob W. (Not a thermodynamist but I learned ohms law at an early age.)
--
N93BD - Rotary Powered BD-4 - http://www.bob-white.com
First Flight: 11/23/2006 7:50AM - 0.7 Hours Total Time
Cables for your rotary installation - http://www.roblinphoto.com/shop/
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cjensen(at)dts9000.com
Guest
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| Posted: Thu Dec 07, 2006 7:55 am Post subject: Pitot Current Draw |
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Bob,
Thanks for the (correct) information. As a child, I lead a sheltered
existance and wasn't allow to play with Ohms, Amps and other foreigners,
so this is new to me. There's a whole new, big, world out there to
learn about. P.S., I still want to reduce my current draw!
Chuck Jensen
--
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echristley(at)nc.rr.com
Guest
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| Posted: Thu Dec 07, 2006 9:40 am Post subject: Pitot Current Draw |
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raymondj wrote:
| Quote: |
Rodney (O Wise Master),
Pretty arrogant of you to think that you have control of or responsibility
for another's actions and to think that you have any right to try to impose
your values on another by attempting to control their actions.
Oh, get over yourself already. Until you start forking over some cash,
|
you have no right to demand that anyone answer your questions at all,
let alone in any particular fashion. Many of us sign in here because it
is a good place to mull over and exchange ideas. That includes not only
how to do things, but why to do things. This list, like most, is a
conversation, not just an answering service. I get to choose how I
share what I know. That's MY right. It's pretty arrogant of you to
think that you have control of how another person shares his own
knowledge, and to think that you have any right to try to impose your
values on another by attempting to control their actions.
That said, the original poster was provided with several avenues to
pursue: a power cycler, a series resistor, and Eric's thermal gradient
resistors (sorry, Eric, can't remember the proper name). The merits and
drawbacks of each were discussed. So you he had to spend a few days
watching the ideas bounce around a bit. How much more did that cost vs.
paying a consultant? Was anything learned in the interim that is
probably useless right now, but was free to you none the less, and
possibly useful in the future? I reached deep and gave what little
knowledge I had. Others did the same, teaching me some stuff which I
consider more than adequate payment for my efforts. I controlled no
one, and no one controlled me.
--
,|"|"|, Ernest Christley |
----===<{{(oQo)}}>===---- Dyke Delta Builder |
o| d |o http://ernest.isa-geek.org |
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Mark Phillips in TN
Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN
|
| Posted: Thu Dec 07, 2006 9:44 am Post subject: Pitot Current Draw |
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|
In a message dated 12/07/2006 7:31:48 AM Central Standard Time, cjensen(at)dts9000.com writes:
| Quote: | | So, if there was a reasonably simple mechanical/electrical means of reducing the current draw of the pitot, I wanted to do it. |
>>>
Not sure if this would work for you- just a possible solution:
http://mpja.com/productview.asp?product=4057+MD
It is a pulse-width modulation speed control for DC motors. I used one for dimming my panel LEDs. It is rated for 10 amps, so if the pitot heater drew more, you "might" have to add an appropriate resistor between the board output and the pitot, but not sure about this. You could use a DPDT switch on the panel for Pitot Heat with "Full" in one position, wired direct to the the heater, and the other side "Adj" (or something appropriate) that would supply power to the speed controller. This would also require adding the control pot to the panel to adjust the controller, or you could just set it to an acceptable partial setting and do without the adjustability.
DISCLAIMER: lest somebody take this suggestion and go out and kill themselves, don't blame me! No sarcasm at all here, and please remember this is a venue for advancing the art of experimental aviation electrical systems, not commentary on opinions of personal philosophy- let's stay on mission, folks...
From The PosumWorks in TN
Mark Phillips
[quote][b]
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mprather(at)spro.net
Guest
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| Posted: Thu Dec 07, 2006 9:55 am Post subject: Pitot Current Draw |
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Hey Chuck,
Putting a resistor in series does reduce your current draw.. Are you
happy with how much so? Has your question been answered? Your original
question didn't quantify your goal. As has been mentioned, there are
solutions to this problem which can deliver more power to the pitot tube
for less total current expended, but they require more circuitry/figetry
to accomplish.
Matt-
[quote]
<cjensen(at)dts9000.com>
Bob,
Thanks for the (correct) information. As a child, I lead a sheltered
existance and wasn't allow to play with Ohms, Amps and other foreigners,
so this is new to me. There's a whole new, big, world out there to
learn about. P.S., I still want to reduce my current draw!
Chuck Jensen
--
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pilot4profit(at)sbcglobal
Guest
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| Posted: Thu Dec 07, 2006 3:02 pm Post subject: Pitot Current Draw |
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Hello people, new to list here, and just thinking about the RV-10. I have a bit of experience in electronics and things electrical. While I don’t have a circuit right now to show you for the desired purpose of regulating current to the pitot (I’ll need a bit of time to look up a good one) things to keep in mind are:
Just a power resister to drop the current will waste ½ the power in the limiting resister.
Use the PWM with caution, as it may produce noise in the electrical system (depending on the frequency of the oscillations) that may affect your other systems, and be a cause for great frustration in locating the interference. Give me a few days to do some looking, and maybe I can find a better solution. Lower frequency is better, go for something that cycles in seconds, not milliseconds.
Craig Smith.
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Fiveonepw(at)aol.com
Sent: Thursday, December 07, 2006 12:42 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Re: Pitot Current Draw
In a message dated 12/07/2006 7:31:48 AM Central Standard Time, cjensen(at)dts9000.com writes:
| Quote: |
So, if there was a reasonably simple mechanical/electrical means of reducing the current draw of the pitot, I wanted to do it. |
Not sure if this would work for you- just a possible solution:
http://mpja.com/productview.asp?product=4057+MD
It is a pulse-width modulation speed control for DC motors. I used one for dimming my panel LEDs. It is rated for 10 amps, so if the pitot heater drew more, you "might" have to add an appropriate resistor between the board output and the pitot, but not sure about this. You could use a DPDT switch on the panel for Pitot Heat with "Full" in one position, wired direct to the the heater, and the other side "Adj" (or something appropriate) that would supply power to the speed controller. This would also require adding the control pot to the panel to adjust the controller, or you could just set it to an acceptable partial setting and do without the adjustability.
DISCLAIMER: lest somebody take this suggestion and go out and kill themselves, don't blame me! No sarcasm at all here, and please remember this is a venue for advancing the art of experimental aviation electrical systems, not commentary on opinions of personal philosophy- let's stay on mission, folks...
From The PosumWorks in TN
Mark Phillips
| Quote: | | www.aeroelectric.comwww.buildersbooks.com |
012345678901234
[quote][b]
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mprather(at)spro.net
Guest
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| Posted: Thu Dec 07, 2006 4:32 pm Post subject: Pitot Current Draw |
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|
Mmm sort of.. I've seen the details of this topic danced around a bit so
far..
First, let's assume that it's not practically possible to mount the
dropping resistor in such a way that it dissipates it's heat into the
pitot tube. Best case scenario mounts the resistor to the same fasteners
that attach the heater to the wing. Many inches from the place that needs
to get hot.
Point: Let's assume the old, adequate heater used 100Watts when wired to
the bus. Let's further assume the new one uses 200Watts when wired to the
bus. That the old heater was adequate says that 100Watts is enough power
(the right amount) for what Chuck wants to do. The challenge is to get
the new heater to only use 100Watts. The way pick a dropping resistor
which will do that is slightly complex.
First, find the equivalent resistance of the new heater. That can be
found by noting R=V^2/P = 14V*14V/200W = 0.98ohm. Great.
Now we need to look at how much current needs to be shoved through the
0.98ohm resistor to get 100W. That can be found by using I = sqrt(P/R) =
sqrt(100/0.9 = 10.101A. Need to pass 10A through a 1ohm resistor to
burn 100Watts. Good, passes the bs check.
Now, let's look at what the total series resistance needs to be in order
to get 10.1A through the circuit. That is R= V/I = 14V/10.1A = 1.386ohm.
So, the total loop resistance needs to be 1.386ohm. 0.98ohm of that is
coming from the pitot tube, and the other 0.41ohm comes from the dropping
resistor.
The power lost in the resistor can be found a couple of ways; I'll show
one... P= (I^2)*R = 41Watts.
The current without the dropping resistor is I = P/V = 200W/14V = 14.3A.
Summary:
To use a dropping resistor to get the equivalent amount of heat from the
pitot tube (goal) reduces the current from 14.3A to 10.1A, and wastes
41Watts on the resistor.
As has been mentioned, picking a resistor value equal to the equivalent
resistance of the heater divides the current consumed by the circuit by
two, but the power delivered to the tube itself by four. Maybe that's not
an adequate amount of power for the tube to function. Certainly it
wouldn't perform as well as the old pitot tube.
Regards,
Matt-
snip
| Quote: |
Just a power resister to drop the current will waste ½ the power in the
limiting resister.
|
snip
| Quote: | Craig Smith.
_____
From: owner-aeroelectric-list-server(at)matronics.com
[mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of
Fiveonepw(at)aol.com
Sent: Thursday, December 07, 2006 12:42 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Re: Pitot Current Draw
In a message dated 12/07/2006 7:31:48 AM Central Standard Time,
cjensen(at)dts9000.com writes:
So, if there was a reasonably simple mechanical/electrical means of
reducing
the current draw of the pitot, I wanted to do it.
>>>
Not sure if this would work for you- just a possible solution:
http://mpja.com/productview.asp?product=4057+MD
It is a pulse-width modulation speed control for DC motors. I used one
for
dimming my panel LEDs. It is rated for 10 amps, so if the pitot heater
drew
more, you "might" have to add an appropriate resistor between the board
output and the pitot, but not sure about this. You could use a DPDT
switch
on the panel for Pitot Heat with "Full" in one position, wired direct to
the
the heater, and the other side "Adj" (or something appropriate) that would
supply power to the speed controller. This would also require adding the
control pot to the panel to adjust the controller, or you could just set
it
to an acceptable partial setting and do without the adjustability.
DISCLAIMER: lest somebody take this suggestion and go out and kill
themselves, don't blame me! No sarcasm at all here, and please remember
this is a venue for advancing the art of experimental aviation electrical
systems, not commentary on opinions of personal philosophy- let's stay on
mission, folks...
>From The PosumWorks in TN
Mark Phillips
|
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nuckollsr(at)cox.net
Guest
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| Posted: Thu Dec 07, 2006 5:49 pm Post subject: Pitot Current Draw |
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|
At 08:22 AM 12/7/2006 -0700, you wrote:
| Quote: |
On Thu, 7 Dec 2006 08:27:22 -0500
"Chuck Jensen" <cjensen(at)dts9000.com> wrote:
><snip> The current
> pitot draws about 15 amps. By comparison, my older, but very adequate
> (as in--never iced up) pitot drew 7 amps. While not on a first name
> basis with any electrons, I assume that an amp is an amp and is
> generally going to produce a certain amount of heat, which is apparently
> adequate, based on empirical experience. So, if 7 amps was good enough
> in the old pitot, I was thinkin' that 7 amps was probably good enough
> for this pitot. I could hire a thermodynamist to confirm all this, but
> I'll go out on a limb and employ common sense instead--it's cheaper and
> often, just as accurate.
>
<snip>
Not quite true Chuck. Your old pitot tube was about 2 ohms. You were
drawing 7 amps from around 14 volts. Power is I^2 * R = 98 W. Now
you have a pitot that's about 1 ohm drawing 15 amps from 14 volts (0.933
ohms but who's counting). Drop the current in this pitot to 7 amps and
power is only 49 W. It was suggested by someone that the dropping
resistor could be mounted on the pitot also which would help, but still
not be equivalent.
Bob W. (Not a thermodynamist but I learned ohms law at an early age.)
|
We touched on this in an earlier post. Recall that while you're reducing
the current through the heater by 1/2, this drops the voltage across it
by 1/2 also for a new dissipation value of 1/4 the original which comes
to 29.5 watts for the tube and 29.5 watts dropped across the companion
resistor for a system total of 49 watts.
If one is applying a rule of thumb that "7 amps was adequate" then the
goal is to drop the 15A tube (14 x 15 = 210 watts) down to what the
original tube ran (7 x 14 = 98 watts). Obviously this is not a drop
to 1/4 power but 1/2 power. So to get 98 watts dissipated in
the new tube at 1 ohm resistance then I(squared) x 1 = 98.
The new value for current drops NOT to 7A but to 10 amps. Given
14 volts applied and 10 volts needed for the new pitot heater then
we need to drop 4 volts at 10 amps in the power reduction resistor.
This calculates out to 4/10 or 0.4 ohms. The resistor will dissipate
4v x 10A or 40 watts.
So, to keep the same heat delivered to the pitot tube we can only
drop the current from 15A down to 10A. Total system draw will be
140 watts instead of the original 210 watts with 100 watts in the
tube and 40 watts in the resistor.
The foregoing assumption of equivalency holds only if the two
tubes distribute their energy over the same number of square inches
of tube surface. Equivalency for ice-melting abilities is measured
in watts per square inch of power density.
Bob . . .
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nuckollsr(at)cox.net
Guest
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| Posted: Thu Dec 07, 2006 5:53 pm Post subject: Pitot Current Draw |
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|
At 05:32 PM 12/7/2006 -0700, you wrote:
| Quote: |
Mmm sort of.. I've seen the details of this topic danced around a bit so
far..
|
<snip>
Good job Matt. You picked it right up and beat me to it!
Bob . . .
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mlas(at)cox.net
Guest
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| Posted: Thu Dec 07, 2006 7:51 pm Post subject: Pitot Current Draw |
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Chuck,
I like you! Few people are able to understand my sarcasm and read between the line when I hammer on the keys…. I response to what you just wrote, if in fact 7amps will do and your sure of that. Then what you are saying is the current (no pun) version of pitot tube is over kill (also no pun intended). All these assumptions being true then all you would have to do is reduce the voltage until the current is at 7.
Have fun,
Mike
--
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bob(at)bob-white.com
Guest
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| Posted: Thu Dec 07, 2006 7:52 pm Post subject: Pitot Current Draw |
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On Thu, 07 Dec 2006 19:48:32 -0600
"Robert L. Nuckolls, III" <nuckollsr(at)cox.net> wrote:
| Quote: |
At 08:22 AM 12/7/2006 -0700, you wrote:
>
>
>On Thu, 7 Dec 2006 08:27:22 -0500
>"Chuck Jensen" <cjensen(at)dts9000.com> wrote:
>
> ><snip> The current
> > pitot draws about 15 amps. By comparison, my older, but very adequate
> > (as in--never iced up) pitot drew 7 amps. While not on a first name
> > basis with any electrons, I assume that an amp is an amp and is
> > generally going to produce a certain amount of heat, which is apparently
> > adequate, based on empirical experience. So, if 7 amps was good enough
> > in the old pitot, I was thinkin' that 7 amps was probably good enough
> > for this pitot. I could hire a thermodynamist to confirm all this, but
> > I'll go out on a limb and employ common sense instead--it's cheaper and
> > often, just as accurate.
> >
><snip>
>
>Not quite true Chuck. Your old pitot tube was about 2 ohms. You were
>drawing 7 amps from around 14 volts. Power is I^2 * R = 98 W. Now
>you have a pitot that's about 1 ohm drawing 15 amps from 14 volts (0.933
>ohms but who's counting). Drop the current in this pitot to 7 amps and
>power is only 49 W. It was suggested by someone that the dropping
>resistor could be mounted on the pitot also which would help, but still
>not be equivalent.
>
>Bob W. (Not a thermodynamist but I learned ohms law at an early age.)
We touched on this in an earlier post. Recall that while you're reducing
the current through the heater by 1/2, this drops the voltage across it
by 1/2 also for a new dissipation value of 1/4 the original which comes
to 29.5 watts for the tube and 29.5 watts dropped across the companion
resistor for a system total of 49 watts.
|
Just a minor point - the 49 W is already 1/4 of the power consumption of
the 'new' pitot tube. The only point I was making was that "I assume
that an amp is an amp and is generally going to produce a certain
amount of heat" was not correct.
Bob W.
| Quote: |
If one is applying a rule of thumb that "7 amps was adequate" then the
goal is to drop the 15A tube (14 x 15 = 210 watts) down to what the
original tube ran (7 x 14 = 98 watts). Obviously this is not a drop
to 1/4 power but 1/2 power. So to get 98 watts dissipated in
the new tube at 1 ohm resistance then I(squared) x 1 = 98.
The new value for current drops NOT to 7A but to 10 amps. Given
14 volts applied and 10 volts needed for the new pitot heater then
we need to drop 4 volts at 10 amps in the power reduction resistor.
This calculates out to 4/10 or 0.4 ohms. The resistor will dissipate
4v x 10A or 40 watts.
So, to keep the same heat delivered to the pitot tube we can only
drop the current from 15A down to 10A. Total system draw will be
140 watts instead of the original 210 watts with 100 watts in the
tube and 40 watts in the resistor.
The foregoing assumption of equivalency holds only if the two
tubes distribute their energy over the same number of square inches
of tube surface. Equivalency for ice-melting abilities is measured
in watts per square inch of power density.
Bob . . .
|
--
N93BD - Rotary Powered BD-4 - http://www.bob-white.com
First Flight: 11/23/2006 7:50AM - 0.7 Hours Total Time
Cables for your rotary installation - http://www.roblinphoto.com/shop/
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stein(at)steinair.com
Guest
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| Posted: Thu Dec 07, 2006 8:33 pm Post subject: Pitot Current Draw |
|
|
Wow! This discussion has really taken on a mind of it's own....I see almost 41 posts in this thread! As usual and in my own sarcastic way, I'm really confused and boggled as to why we're even discussing this (and to the unbelievably detailed technical minutia that has arisen out of something like this)....when there is a perfectly good heated pitot on the market that does EXACTLY what you want it to do....it varies the current to the tube from .1 Amps to 7 Amps depending on it's own temperature. When it doesn't need the heat it just idles, when it does it shoves more power out there. Why re-invent the wheel...someone already has and his name is Warren Gretz! I swear, just reading all these posts will waste more time than the $400 and some odd bucks it takes to buy one brand new....no messing around with all kinds of crazy little electrical components...bolt it on and go!
Just my 2 cents as usual!
Cheers,
Stein.
[quote] --
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ceengland(at)bellsouth.ne
Guest
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| Posted: Fri Dec 08, 2006 12:22 am Post subject: Pitot Current Draw |
|
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If you want to vary the power to the heater, there are dozens, if not
hundreds of designs for adjustable switching power supplies. A switching
supply fed by ship's power that's capable of supplying full current to
the pitot at full voltage would be the most efficient way to vary the power.
This does not address weight issues or the wisdom of reducing the
effectiveness of the pitot heat.
Mike wrote:
[quote] The only way I can answer this question is to tell a story. First,
flying in the ice is something many pilot do every year and sometimes
often. If you know what you’re doing and are prepared then the risks
are minimal. I have been flying in ice with airplanes that are able
to handle it for better then 20 years. It’s not like flying on a
sunny day. But once you get use to it and prepare for it, it’s not
that big of a deal. A few words of advice, no two icing days are the
same. Only experience will help you here. My personal feeling on ice
in piston powered planes, you don’t have many out when the weather
gets bad. When in ice greater then light find a place to land ASAP.
As for you electric question; First the easy answer is to put a bigger
battery in the airplane. Second, why would you think that using less
power for pitot heat is going to be any better then turning the pitot
heat off all together? Unless you run a thermal equation on the
speed, temperature, and heat (layman’s terms) required to guarantee
the anti-ice properties needed, then everything your doing is just a
big guess anyway. And I guessing that since I have to go to the book
to do said equation thoroughly, I would guess the electrical formula
by comparison would be a piece of cake. I know this is not what you
want to hear but the facts are the facts. The reason that the pitot
uses so much power (in general) is that is what it takes to keep ice
off the probe in 100% humidity at temperatures < 10dec C.
Sorry for the poor and somewhat sarcastic answer, but I think Chuck
brought it out in me.
Regards,
Mike Larkin
--
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