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Heated pitot repairable?

rv6n6r

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My AN5812 heated pitot tube blew the fuse and is showing a dead short on the heater element. Repairable? A cursory look doesn't seem promising for a way to get the element out and replace. But new ones are darned expensive so I thought I'd check with the brain trust before plunking down for a replacement.

Also in case I pick up a used one, what should be the resistance be on the heating element for a working part? Perhaps someone with a new one can measure it with an ohmmeter.
 
My AN5812 heated pitot tube blew the fuse and is showing a dead short on the heater element. Repairable? A cursory look doesn't seem promising for a way to get the element out and replace. But new ones are darned expensive so I thought I'd check with the brain trust before plunking down for a replacement.

Also in case I pick up a used one, what should be the resistance be on the heating element for a working part? Perhaps someone with a new one can measure it with an ohmmeter.

Never measued one, but something that will draw 10-15 amps has to be less than 5 ohms, probably closer to 1 or 2. Obviously it will be greater than 0.

Larry
 
AN5814 electrical resistance

Not AN5812 (stubby) but rather AN5814 (long):

  • Bob Nuckolls measured heater resistance indirectly (measure current and voltage, calculate resistance) and got 280 mΩ at room temperature. Resistance goes up to ~800 mΩ when the heater gets hot.
  • I got this info from Bob's document Gauging Pitot Heater Performance

Can your measurement device distinguish between "dead short" and 280 mΩ?

I have a Dynon heated pitot that, judging by the wires, contains two heater elements, each of which measure 3 Ω, that connect to the controller. Maybe the Dynon has heater elements with lower thermal coefficient of resistance.
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Cessna PN 0721105-5

Thanks. But I'm still scratching my head how to get it apart to replace the element. Groping around on the internet I've found some old drawing that show the tube and tip unscrewing from the body but from all appearances this is one solid piece. Or at least chromed over to look like it.
 
I hope all reading this realize the difference between milliohms and megohms... little m versus BIG M. 9 orders of magnitude, easy to make the mistake.
 
Not AN5812 (stubby) but rather AN5814 (long):

  • Bob Nuckolls measured heater resistance indirectly (measure current and voltage, calculate resistance) and got 280 mΩ at room temperature. Resistance goes up to ~800 mΩ when the heater gets hot.
  • I got this info from Bob's document Gauging Pitot Heater Performance

Can your measurement device distinguish between "dead short" and 280 mΩ?

I have a Dynon heated pitot that, judging by the wires, contains two heater elements, each of which measure 3 Ω, that connect to the controller. Maybe the Dynon has heater elements with lower thermal coefficient of resistance.
.

It was some fraction of an ohm, as read by my Fluke. Blows a 10A fuse if I connect to power.
 
Rated power?

10 amps seems a low value to use for a pitot heater. I think you said it was a Cessna part. What value breaker does Cessna call for in that circuit? I think both of my former certified aircraft (Mooney) used 15 amp circuit breakers. I would not arbitrarily increase the fuse size but check the manufacturer rating for the pitot tube. Also check the gauge wiring in your plane on that circuit.
 
My AN5812 heated pitot tube blew the fuse and is showing a dead short on the heater element. Repairable? A cursory look doesn't seem promising for a way to get the element out and replace. But new ones are darned expensive so I thought I'd check with the brain trust before plunking down for a replacement.

Also in case I pick up a used one, what should be the resistance be on the heating element for a working part? Perhaps someone with a new one can measure it with an ohmmeter.

I can't help with the first part of your question because I've never tried to take one of these apart. I may be wrong, but I always just assumed that the elements were potted inside the tube. I'd be interested to find out one way or the other.

As to the second part of your question, a quick search for that part number on aircraft spruce pulls up a tube that says the it pulls 8 amps. Ohms law dictates that would result in about 2 ohms in a 14v circuit.

BUT- be careful if you get a used one. I'm confident that there are different dash numbers for this base part number and they almost certainly have different internals depending on their intended use. Case in point, the C-172 is a 28v system but puts the pitot on a 5a breaker. If it had the same element with only 2 ohm resistance, it would pull a whopping 14a and something would start to sizzle in short order. It likely has a much beefier element to pump up the ohms and ratchet down the current to something that will play nice with a 5a breaker.
 
Resolved

Thanks for all the responses. This has been resolved, details follow in case anyone is interested.

First to address some questions / responses: This is a TSO'd part that I got when I built the plane over 20 years ago. Presumably it is the same Cessna part that now sells for nearly $3200 (!) I couldn't find the original documentation but the specs on similar tubes say they draw 8A +- .5A and specify a 10A fuse. I'm fairly certain that's what was spec'd for this one and it's what I've been using for over 20 years with no problems until now.

Anyway I took it back out, checked the wiring in the plane for shorts, and re-measured resistance in the tube with a better multimeter, and it shows 1 ohm across the pins. I still don't know what's normal but in any case I then checked it on the bench by hooking it to an old (but still good) battery with a 15A fuse -- and it didn't blow and heats up just fine.

So I'm supposing that the 10A spec was good for the inrush current when it was new, but with age the resting resistance has increased (as I understand can happen with heating elements).

So I replaced the 10 fuse with a 15A and all is good. The wiring is #12 and less than 10' long which should be good for that.

Thanks everyone for helping me puzzle this out!
 
Last edited:
So I'm supposing that the 10A spec was good for the inrush current when it was new, but with age the resting resistance has increased (as I understand can happen with heating elements).
!

You mean, ‘resistance has decreased’. Lower resistance, higher current. IIRC when I owned a 182 the pitot heat was on a 15 amp CB.
If you ever decide to replace the pitot, consider one of the newer ones with a built in thermostat. Under most conditions they draw a lot less current, and if you forget it was on and touch it after landing you probably won’t burn the skin off your hand!
 
You mean, ‘resistance has decreased’. Lower resistance, higher current. IIRC when I owned a 182 the pitot heat was on a 15 amp CB.
If you ever decide to replace the pitot, consider one of the newer ones with a built in thermostat. Under most conditions they draw a lot less current, and if you forget it was on and touch it after landing you probably won’t burn the skin off your hand!
Yes of course you are correct that a decrease not an increase in resistance would make for higher current and blown fuses. Which leaves me a bit confused again since I believe heating elements tend increase not decrease in resistance over time. Maybe that's a red-herring and it was simply too close with the 10A CB all along -- and the 15A CB seems to have solved it so... good.
 
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