Verification that Heated Pitot is working

[iflycozy]

Question to you electrical designer gurus:

Here is what I would like to do and Jim Wier suggested that I post this
here. I want an indicator on my Cozy homebuilt airplane to show me
that my heated pitot is actually working when I turn on the switch on
the instrument panel. So, the light will go out (with the switch still
in the on position) if the heated pitot stops working for some reasons
(but not because it tripped the circuit breaker). So, how can I build
such a device or circuit? I would appreciate specific parts or
identification of parts as I am an amateur. This is a 12 volt DC
system and the heating element draws 7 amps. I know I can not use a
LED in series because it would blow the milli-second I turned the unit
on. I know I can't use a light or lamp in parallel because it would
not indicate if the heated pitot was on or off. So that is my dilemma.
Please help.

Reply to:

[John]

iflycozy wrote:

> Question to you electrical designer gurus:
>
> Here is what I would like to do and Jim Wier suggested that I post this
> here. I want an indicator on my Cozy homebuilt airplane to show me
> that my heated pitot is actually working when I turn on the switch on
> the instrument panel. So, the light will go out (with the switch still
> in the on position) if the heated pitot stops working for some reasons
> (but not because it tripped the circuit breaker). So, how can I build
> such a device or circuit? I would appreciate specific parts or
> identification of parts as I am an amateur. This is a 12 volt DC
> system and the heating element draws 7 amps. I know I can not use a
> LED in series because it would blow the milli-second I turned the unit
> on. I know I can't use a light or lamp in parallel because it would
> not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.
>
> Reply to:
>


Possible thermister bonded to the pitot with a second one insulated from it
but with basically the same air flow over it. Use a differential opamp to
measure the difference of output from each thermister use the opamp to
drive a simple transister amp to drive a led or lamp. I'm at work so I
can't look up specific parts and you'd need to do some testing anyway.
John

[Richard Isakson]

"iflycozy" wrote ...
> I want an indicator on my Cozy homebuilt airplane to show me
> that my heated pitot is actually working when I turn on the switch on
> the instrument panel.
>

The answer is easy, you just look at the ammeter. If it jumps when you turn
on the switch, the heater works.

The far more important question is: why would anyone fly a cozy into icing
conditions? Disturbing the airflow over a canard with ice is just asking
for controllability problems.

Rich

[RST Engineering]

The parts you will need are a 10 amp silicon diode, a 10 kilohm quarter watt
resistor, a small PNP transistor, a LED of the appropriate color, and a 470
ohm quarter watt resistor. All these parts are available from Mouser
Electronics, DigiKey Electronics, and the rest of the usual suspects.

The silicon (NOT schottky, plain silicon) diode goes in series with the wire
going to the pitot, anode to switch, cathode to pitot. THe diode will
probably require a heat sink.

THe emitter of the PNP transistor goes directly to the anode of the diode.

One lead of the 10K resistor goes to the cathode of the diode. THe other
lead of the resistor goes to the base of the transistor.

One lead of the 470 ohm resistor goes to the collector of the PNP
transistor. The other lead of the resistor goes to the anode of the LED.
The cathode of the LED goes to airframe ground.

HOW IT WORKS: When the pitot switch is turned on, 7 amps through the diode
will result in about 1 volt drop across the diode. A PNP transistor turns on
with 0.5 volts from base to emitter, so the transistor will turn on through
the 10K current limiting resistor. When the transistor turns on, it flows
current through the 470 ohm current limiting resistor to the LED.

If the pitot heater fails there will not be any significant voltage drop
across the diode and the PNP transistor will not turn on and the LED will
not light.

If you can't find a suitable 10 amp diode, a 0.1 ohm 10 watt resistor will
do the same job and will probably be cheaper. In either case, I suspect
that the pitot heater will be more reliable than the diode OR the resistor,
so a false heater-out alarm will be the predominant failure mode.

Be careful where you mount the diode or the resistor. Under normal
operating conditions, they will get hotter than hell.

Jim







"iflycozy" > wrote in message
ups.com...
> Question to you electrical designer gurus:
>
> Here is what I would like to do and Jim Wier suggested that I post this
> here. I want an indicator on my Cozy homebuilt airplane to show me
> that my heated pitot is actually working when I turn on the switch on
> the instrument panel. So, the light will go out (with the switch still
> in the on position) if the heated pitot stops working for some reasons
> (but not because it tripped the circuit breaker). So, how can I build
> such a device or circuit? I would appreciate specific parts or
> identification of parts as I am an amateur. This is a 12 volt DC
> system and the heating element draws 7 amps. I know I can not use a
> LED in series because it would blow the milli-second I turned the unit
> on. I know I can't use a light or lamp in parallel because it would
> not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.
>
> Reply to:
>
>

[Richard Lamb]

Ok, as Devils Advocate, why do you think you need a heated pitot
in the first place?

Are you Instrument Management Certified?
Will the airplane be instrument certified?
Are you _really_ going to fly a _canard_ in icing conditions?
Without De-Icing systems on the canard (and possibly wings too?)\

Sounds iffy to me...

Richard

[Richard Lamb]

Ok, as Devils Advocate, why do you think you need a heated pitot
in the first place?

Are you Instrument Management Certified?
Will the airplane be instrument certified?
Are you _really_ going to fly a _canard_ in icing conditions?
Without De-Icing systems on the canard (and possibly wings too?)\

Sounds iffy to me...

Richard

[Dave S]

Richard Isakson wrote:

>
> The far more important question is: why would anyone fly a cozy into icing
> conditions? Disturbing the airflow over a canard with ice is just asking
> for controllability problems.
>
> Rich

Why would you have a heated pitot on a Cessna 172 or a piper arrow then?
Neither of THEM are certified for entry into known icing conditions, but
its nice to have the ability to safely fly OUT of them if you get INTO them.

Dave

[Richard Lamb]

Hi Jim,

Question..

Why put the diode in series with the pitot heater power.
(Ok, it senses that power if flowing in the heater circuit, but is that
necessary?)

Couldn't the same thing be accomplished by using the diode as a
temperture
sensor (thermometer) to decide if the pitot tube was above a certain
temperature?

No self-heating on the part of the sense diode, and no pitot heat
failure if the diode
opens up (burns up?).

Richard

[Richard Lamb]

Hi Jim,

Question..

Why put the diode in series with the pitot heater power.
(Ok, it senses that power if flowing in the heater circuit, but is that
necessary?)

Couldn't the same thing be accomplished by using the diode as a
temperture
sensor (thermometer) to decide if the pitot tube was above a certain
temperature?

No self-heating on the part of the sense diode, and no pitot heat
failure if the diode
opens up (burns up?).

Richard

[Dan, U.S. Air Force, retired]

Richard Lamb wrote:
> Ok, as Devils Advocate, why do you think you need a heated pitot
> in the first place?
>
> Are you Instrument Management Certified?
> Will the airplane be instrument certified?
> Are you _really_ going to fly a _canard_ in icing conditions?
> Without De-Icing systems on the canard (and possibly wings too?)\
>
> Sounds iffy to me...
>
> Richard
>
Maybe he wants to add it instead of installing more important systems
like cup holders.


Dan, U.S. Air Force, retired

[RST Engineering]

Of course that is another way of doing it, but it involves bonding the diode
thermally to the pitot tube, putting a reference diode somewhere in the same
general vicinity of the pitot tube to sense ambient temperature, insuring an
ambient airflow over the reference diode, an opamp to sense the 2.5
millivolts/°C change, and all of that stuff. I prefer dumb when I can get
away with it.

As I recall, the pitot heater gets about 50°C above ambient, so you are
messing around trying to detect a little over a tenth of a volt change,
which isn't rocket science, but trying to explain it in this newsgroup is.

Jim




"Richard Lamb" > wrote in message
oups.com...
> Hi Jim,
>
> Question..
>
> Why put the diode in series with the pitot heater power.
> (Ok, it senses that power if flowing in the heater circuit, but is that
> necessary?)
>
> Couldn't the same thing be accomplished by using the diode as a
> temperture
> sensor (thermometer) to decide if the pitot tube was above a certain
> temperature?
>
> No self-heating on the part of the sense diode, and no pitot heat
> failure if the diode
> opens up (burns up?).
>
> Richard
>

[Richard Isakson]

"Dave S" wrote ...
> Why would you have a heated pitot on a Cessna 172 or a piper arrow then?
> Neither of THEM are certified for entry into known icing conditions, but
> its nice to have the ability to safely fly OUT of them if you get INTO
them.

Dave,

A 172 can take a bit of icing and get away with it. Trust me and please
don't ask any questions I won't answer. In selecting a canard type
aircraft, the owner has to realize that there are some huge differences in
the way the horizontal controls react to airflow disturbances. In the case
of icing, he needs to take extreme measures to avoid those conditions. Even
if that means grounding the airplane for an extended period. That's what he
bought into when he got the airplane.

Having said that, if anyone knows of a canard icing study that says
differently please point me to it.

Rich

[[email protected]]

Nothing wrong with an old-fashioned ammeter. Dead simple, and tells you
everything
you want to know. I have one that was used for prop de-icer amps (not
that I use it for that - I actually use it to test solar panels).

David Johnson

[John_F]

The easiest way is a glass encased reed switch that turns on a LED in
series with a resistor. Wrap a large enough wire to carry 8 amps
around the reed switch enough times until it will close the contacts
at about 5 amps. Total of 4 parts required: wire, reed sw, LED, and
resistor of about 560 ohms.
PS Do not mount this near the compass.

I think it would be easer and weigh less to check the heater with your
hand as a pre flight item.
John


On 19 Oct 2005 11:20:01 -0700, "iflycozy" >
wrote:

>Question to you electrical designer gurus:
>
>Here is what I would like to do and Jim Wier suggested that I post this
>here. I want an indicator on my Cozy homebuilt airplane to show me
>that my heated pitot is actually working when I turn on the switch on
>the instrument panel. So, the light will go out (with the switch still
>in the on position) if the heated pitot stops working for some reasons
>(but not because it tripped the circuit breaker). So, how can I build
>such a device or circuit? I would appreciate specific parts or
>identification of parts as I am an amateur. This is a 12 volt DC
>system and the heating element draws 7 amps. I know I can not use a
>LED in series because it would blow the milli-second I turned the unit
>on. I know I can't use a light or lamp in parallel because it would
>not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.
>
>Reply to:

[Marc J. Zeitlin]

Richard Isakson wrote:

> Having said that, if anyone knows of a canard icing study that says
> differently please point me to it.

While I won't disagree with you that icing conditions are certainly to
be avoided in canard aircraft, probably more than in conventional
aircraft, I will say that I know two folks that have flown in icing
conditions, collected some ice on the canard, and had minor issues.

One of those people is me, during my instrument lessons near
Schenectady, NY in April of this year. I got about 1/16" - 1/8" of ice
on the canard, and my stall speed went up about 10 Kts. I flew around
under the clouds for one circle of the pattern, the ice melted, and we
landed. Of course, having the stall speed go up and not realizing it is
far less dangerous in a canard, since I can't stall the whole plane or
spin it - I noticed the speed difference because the nose started
bobbing at 90 mph in the downwind to base turn.

Another aquaintance with a COZY has had ice up to 1/8" - 1/4" on his
canard, and he says about the same thing.

That being said, I still agree that icing conditions are to be
strenuously avoided - 1/8" of ice is certainly not a lot, and I wouldn't
want to be experimenting with more.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2005

[nooks!!]

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http://www.tpub.com/aviation1.htm
u might find this one useful. i hope that one helps! they got infos
there regarding electrical and aviation and more!. try visiting it!






iflycozy wrote:
> Question to you electrical designer gurus:
>
> Here is what I would like to do and Jim Wier suggested that I post this
> here. I want an indicator on my Cozy homebuilt airplane to show me
> that my heated pitot is actually working when I turn on the switch on
> the instrument panel. So, the light will go out (with the switch still
> in the on position) if the heated pitot stops working for some reasons
> (but not because it tripped the circuit breaker). So, how can I build
> such a device or circuit? I would appreciate specific parts or
> identification of parts as I am an amateur. This is a 12 volt DC
> system and the heating element draws 7 amps. I know I can not use a
> LED in series because it would blow the milli-second I turned the unit
> on. I know I can't use a light or lamp in parallel because it would
> not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.
>
> Reply to:
>

[Roger]

On 19 Oct 2005 11:20:01 -0700, "iflycozy" >
wrote:

>Question to you electrical designer gurus:
>
>Here is what I would like to do and Jim Wier suggested that I post this
>here. I want an indicator on my Cozy homebuilt airplane to show me
>that my heated pitot is actually working when I turn on the switch on
>the instrument panel. So, the light will go out (with the switch still
>in the on position) if the heated pitot stops working for some reasons
>(but not because it tripped the circuit breaker). So, how can I build
>such a device or circuit? I would appreciate specific parts or
>identification of parts as I am an amateur. This is a 12 volt DC
>system and the heating element draws 7 amps. I know I can not use a
>LED in series because it would blow the milli-second I turned the unit
>on. I know I can't use a light or lamp in parallel because it would
>not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.

Find one of those out door thermometers with a remote read out. Some
of the older ones had a meter. The newer ones have a battery powered
digital read out which could probably be adapted to the plane's
electrical system, but you wanted simple.

Just drill s small hole into the pitot tube metal where is it thick,
preferably from the inside of the mount. (stay away from the heaters
and the actual tube) Put a little heat sink compound in the hole,
stick the remote sensor in, clamp it in place and run the wires to the
inside where they hook to the sensor. Neat, easy, simple.

As a side note, you don't need accuracy. Fire up see what it reads.
Go flying and see what it reads. It won't be near as hot on cold day
and even less so in rain, but I doubt you'll be flying the cozy in the
rain. Put a sticker or just a piece of tape above, below, or beside
the read out with what you'd expect to see for normal readings.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>Reply to:

Roger

[Bushy Pete]

> Find one of those out door thermometers with a remote read out. Some
> of the older ones had a meter. The newer ones have a battery powered
> digital read out which could probably be adapted to the plane's
> electrical system, but you wanted simple.
>

KISS, the best approach........

It will even tell you if the heater is not up to the job regardless of it's
on or off state.

Peter

[Kyler Laird]

>>system and the heating element draws 7 amps. I know I can not use a
>>LED in series because it would blow the milli-second I turned the unit
>>on. I know I can't use a light or lamp in parallel because it would
>>not indicate if the heated pitot was on or off. So that is my dilemma.

You should be all set if you put an LED in parallel with a resistor
in series with the heater. Basically you're making a high-current
ammeter. The values of the LED and resistor depend most on what's
available at your local Radio Shack or equivalent.

>Find one of those out door thermometers with a remote read out.

You'd mount that out on the pitot tube?! That seems a bit extreme.

--kyler

[Roger]

On Fri, 21 Oct 2005 14:17:02 GMT, Kyler Laird >
wrote:

>>>system and the heating element draws 7 amps. I know I can not use a
>>>LED in series because it would blow the milli-second I turned the unit
>>>on. I know I can't use a light or lamp in parallel because it would
>>>not indicate if the heated pitot was on or off. So that is my dilemma.
>
>You should be all set if you put an LED in parallel with a resistor
>in series with the heater. Basically you're making a high-current
>ammeter. The values of the LED and resistor depend most on what's
>available at your local Radio Shack or equivalent.
>
>>Find one of those out door thermometers with a remote read out.
>
>You'd mount that out on the pitot tube?! That seems a bit extreme.

Inserting a second wire into the assembly is extreme? In addition it
lets you know how well the system is working, particularly if it's one
of those two heater types.

The Deb has a digital read out for a number of parameters (Includes
temp in F and C, Density altitude, battery voltage, etc...). All I'd
need to do with something like that would be hook up the TC leads.

Any time I'm in rain, or visible moisture the pitot heat is on.

BTW, I've flown loose formation to Oshkosh with a Cozy a number of
times. One day there were scattered showers all along the path. I
thought I was going to wear out the vernier throttle on the Deb trying
to stay in position.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>--kyler

[iflycozy]

My DAR requires a heated pitot and an indicator if I want my airplane
to be able to fly IFR. It is that simple. There is no need to discuss
whether the FARs require this under Part 91.205(c) because my DAR
interprets it to include it and won't change his mind.
Matt

[RST Engineering]

I don't know why you are using a DAR, but I'd find another DAR that knows
what 91.205(c) really says. That interpretation is busslhit. Heated pitot
isn't required for standard airworthiness IFR flight; I have no idea where
this requirement for an experimental comes from.

Sorry I answered in the first place; I thought this was an academic
exercise. Had I know that some ahhsole authority figure with his thumb up
his hiney was making it a requirement, I'd have declined to respond.

Jim



"iflycozy" > wrote in message
ups.com...
> My DAR requires a heated pitot and an indicator if I want my airplane
> to be able to fly IFR. It is that simple. There is no need to discuss
> whether the FARs require this under Part 91.205(c) because my DAR
> interprets it to include it and won't change his mind.
> Matt
>

[John McDoe]

iflycozy wrote:
> Question to you electrical designer gurus:
>
> Here is what I would like to do and Jim Wier suggested that I post this
> here. I want an indicator on my Cozy homebuilt airplane to show me
> that my heated pitot is actually working when I turn on the switch on
> the instrument panel. So, the light will go out (with the switch still
> in the on position) if the heated pitot stops working for some reasons
> (but not because it tripped the circuit breaker). So, how can I build
> such a device or circuit? I would appreciate specific parts or
> identification of parts as I am an amateur. This is a 12 volt DC
> system and the heating element draws 7 amps. I know I can not use a
> LED in series because it would blow the milli-second I turned the unit
> on. I know I can't use a light or lamp in parallel because it would
> not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.
>
> Reply to:
>
>


I would use an ammeter as the most simple solution.

If you must have a light, it must be something that senses current
because current is what generates the heat, and you want an indication
if your heated pitot is working.

There are some simple circuits that would allow you to turn on a light
when current is flowing in a circuit. You will need to put a sense
resistor in the circuit.

[jls]

"iflycozy" > wrote in message
ups.com...
> My DAR requires a heated pitot and an indicator if I want my airplane
> to be able to fly IFR. It is that simple. There is no need to discuss
> whether the FARs require this under Part 91.205(c) because my DAR
> interprets it to include it and won't change his mind.
> Matt
>

On our campus one of the mechanics has this neat infrared reader device he
bought from the Snap-On man. You point it at anything, like a warm pitot
tube, or a black Cadillac in the sun, or a cold cylinder, and get a
temperature reading.

Otherwise, fly through known ice and see if it works.

Check the contacts for resistance?

[Don Hammer]

I know we are talking homebuilts here, but aircraft certified under
Pt23, that have a heated pitot, must have an indication that it is off
or inoperative. (See Pt23.1326)

I remember years ago, we had to retrofit all our corporate aircraft
with pitot heat warning systems. There was an AD that required it
after a crash where the heat switch was left off. All of the systems
I'm aware of measure the current to the probe(s) and give a light or
ICAS message if the current is low. Some larger aircraft I'm familiar
with pass the power wire through a coil to measure the current, which
I'm sure is a bunch higher on a turbine aircraft. Maybe the same
concept would work at 7 amps.

I think if I was building an aircraft, I'd work towards certified
standards, so I personally think having a heated pitot and warning
system is a good idea.

[abripl]

Another simple way is to put in a miniature panel ammeter so you can
actually see the current used. I got one of them (real tiny) for my
inflight adjustable IVO prop so I can tell when it is at extremes. More
info on the tiny panel ammeter at http://www.abri.com/sq2000/hints.html
Would be a much simpler circuit and probably safer.

iflycozy wrote:
> Question to you electrical designer gurus:
>
> Here is what I would like to do and Jim Wier suggested that I post this
> here. I want an indicator on my Cozy homebuilt airplane to show me
> that my heated pitot is actually working when I turn on the switch on
> the instrument panel. So, the light will go out (with the switch still
> in the on position) if the heated pitot stops working for some reasons
> (but not because it tripped the circuit breaker). So, how can I build
> such a device or circuit? I would appreciate specific parts or
> identification of parts as I am an amateur. This is a 12 volt DC
> system and the heating element draws 7 amps. I know I can not use a
> LED in series because it would blow the milli-second I turned the unit
> on. I know I can't use a light or lamp in parallel because it would
> not indicate if the heated pitot was on or off. So that is my dilemma.
> Please help.
>
> Reply to:
>