pitot tube questions

The pitot heat has failed on our Aztec. No heat, not even warm, no amperage
draw. Circuit breaker, switch, leads, and ground all check ok.

Now here's the strange part. We get correct voltage across the leads and we
get continuity across the terminals on the tube. No continuity from the
terminals to the tube itself, so I don't think the element is shorted out
against the tube.

If there is continuity across the terminals, one would think that the
element is intact. If it's getting adequate voltage, why isn't it heating?

The only explanation I can come up with is that the leads are shorted out
inside the tube before they get to the element. However, it doesn't pop the
circuit breaker.

Any other ideas?? Can these old style tubes be repaired? Ours is similar
to an AN5812-12 but we have rubber hoses connected to the airspeed and pitot
lines rather than 1/4" fittings. The AN5812's seem to run about $1000!

Thanks
Jim




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Jim Burns wrote:
The pitot heat has failed on our Aztec. No heat, not even warm, no amperage
draw. Circuit breaker, switch, leads, and ground all check ok.

Now here's the strange part. We get correct voltage across the leads and we
get continuity across the terminals on the tube. No continuity from the
terminals to the tube itself, so I don't think the element is shorted out
against the tube.

Is this measured with the leads connected to the pitot tube terminals?
You have continuity across the terminals, but what is the element
resistance?


If there is continuity across the terminals, one would think that the
element is intact. If it's getting adequate voltage, why isn't it heating?

I don't know, but one possibility is that you have internal corrosion
that has greatly increased the resistance of the element such that there
isn't sufficient current flowing to heat up the element.


Matt

Jim Burns wrote:
: The pitot heat has failed on our Aztec. No heat, not even warm, no amperage
: draw. Circuit breaker, switch, leads, and ground all check ok.

: Now here's the strange part. We get correct voltage across the leads and we
: get continuity across the terminals on the tube. No continuity from the
: terminals to the tube itself, so I don't think the element is shorted out
: against the tube.

If the Aztec is 12V, the resistance of the heater in the pitot tube should be
between 0.75 and 1.2 Ohms. If it's greater than that the heater has failed
internally. (24V systems would be about twice that resistance, of course.)
The leads of most multimeters have 1 to 2 Ohms resistance, so checking
a low resistance is troublesome. I'd jumper the pitot tube heater to a 12V
battery and see if it sparks when connected or disconnected.
--
Aaron Coolidge

My fluke 189 is that sensitive! You need quality tools or a mechanic with
quality tools.

"Aaron Coolidge" wrote in message
...
Jim Burns wrote:
: The pitot heat has failed on our Aztec. No heat, not even warm, no
amperage
: draw. Circuit breaker, switch, leads, and ground all check ok.

: Now here's the strange part. We get correct voltage across the leads
and we
: get continuity across the terminals on the tube. No continuity from the
: terminals to the tube itself, so I don't think the element is shorted
out
: against the tube.

If the Aztec is 12V, the resistance of the heater in the pitot tube should
be
between 0.75 and 1.2 Ohms. If it's greater than that the heater has failed
internally. (24V systems would be about twice that resistance, of course.)
The leads of most multimeters have 1 to 2 Ohms resistance, so checking
a low resistance is troublesome. I'd jumper the pitot tube heater to a 12V
battery and see if it sparks when connected or disconnected.
--
Aaron Coolidge

The pitot heat has failed on our Aztec. No heat, not even warm, no amperage
draw. Circuit breaker, switch, leads, and ground all check ok.

Now here's the strange part. We get correct voltage across the leads and we
get continuity across the terminals on the tube. No continuity from the
terminals to the tube itself, so I don't think the element is shorted out
against the tube.

If there is continuity across the terminals, one would think that the
element is intact. If it's getting adequate voltage, why isn't it heating?

The only explanation I can come up with is that the leads are shorted out
inside the tube before they get to the element. However, it doesn't pop the
circuit breaker.

Any other ideas?? Can these old style tubes be repaired? Ours is similar
to an AN5812-12 but we have rubber hoses connected to the airspeed and pitot
lines rather than 1/4" fittings. The AN5812's seem to run about $1000!

Thanks
Jim



It sounds as though you have corrosion or some other resistance in the
circuit. I would check for a voltage drop across the circuit breaker and switch
while the circuit is under load.

It sounds as though you have corrosion or some other resistance in the
circuit. I would check for a voltage drop across the circuit breaker and
switch
while the circuit is under load.

Bingo.
voltage drops to about 0
played with it enough that the circuit breaker started popping
corroded terminal on the switch/lead to the circuit breaker

hooked a battery direct to the pitot heat terms and it works fine

will try a new switch & lead tomorrow

thanks all!

Jim


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NW_PILOT wrote:
: My fluke 189 is that sensitive! You need quality tools or a mechanic with
: quality tools.

Not just the sensitivity, though. To truly measure a low impedance path you
need a 4-wire meter. I traced the mOhms in a prototype PCB to locate the bridge with
a 4-wire meter.


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

"Jim Burns" wrote in message ...

It sounds as though you have corrosion or some other resistance in the
circuit. I would check for a voltage drop across the circuit breaker and
switch
while the circuit is under load.

Bingo.
voltage drops to about 0
played with it enough that the circuit breaker started popping
corroded terminal on the switch/lead to the circuit breaker

hooked a battery direct to the pitot heat terms and it works fine

will try a new switch & lead tomorrow

thanks all!

Jim

Bad breakers cost some folks a lot of money. We once replaced a
$600 strobe power supply because we got battery voltage at the strobe
power plug, but that was with the plug disconnected from the strobe.
The relatively high resistance of a bad breaker won't drop voltage
detectably with only a voltmeter drawing on it. Learned the hard way
that voltage drop measurements across suspect components are much more
informative.

Dan


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You may have a bad or corroded crimp somewhere. I fought a flap motor
problem for several hours before finding that one.

It was actually the switch. The switch was shorted out internally and one
terminal was shorted to the exterior case of the switch and ultimately to
the panel.

Jim

"Dan Thomas" wrote in message
om...
"Jim Burns" wrote in message
...

It sounds as though you have corrosion or some other resistance in
the
circuit. I would check for a voltage drop across the circuit breaker
and
switch
while the circuit is under load.

Bingo.
voltage drops to about 0
played with it enough that the circuit breaker started popping
corroded terminal on the switch/lead to the circuit breaker

hooked a battery direct to the pitot heat terms and it works fine

will try a new switch & lead tomorrow

thanks all!

Jim

Bad breakers cost some folks a lot of money. We once replaced a
$600 strobe power supply because we got battery voltage at the strobe
power plug, but that was with the plug disconnected from the strobe.
The relatively high resistance of a bad breaker won't drop voltage
detectably with only a voltmeter drawing on it. Learned the hard way
that voltage drop measurements across suspect components are much more
informative.

Dan


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