Pitot Ammeter technical assistance?

[curiouspurchases]

Hi all,

This may be a slightly long post, but I'll try and get all the relevant info out of the way before posting :)

OK, I'm just bought a pair of pitot ammeters off eBay. The seller claimed they came out of a 707, btu couldn't offer any further info. Now they've arrived, I can see all the bits that weren't in the pictures, including the very solid-looking connector on the back. This connector has 5 pins arranged in pentagonal form, with 3 locking studs protruding from the surround. Two of the pins are numbered 1 and 5, obviously indicating the convention. The front of the meter is marked 'PITOT AMPS' and graduated from 0 to 3 (centre of the dial) in half-amp increments, then to the other end of the dial (up to 10) in smaller increments. The side of one of the meters has white-printed text reading:

'PITOT AMMETER
WACLINE, INC. DAYTON, O.
WACLINE P-N LKCC-18024
WACLINE SERIAL NO. 225'

There are also some other printed marks, none of which are very readable except for one reading 'July 68'.

My question is this: what's the pinout for the connector at the back? I already know the following:

pin1 - connected to pin2, and meter chassis
pin2 - connected to pin1, and meter chassis
pin3 - connected to pin4
pin4 - connected to pin3
pin5 - ?

My guess was that pins 1 and 2 are supposed to be the aircraft's 0V, with pin5 the positive supply, but hooking it up to 12Vdc did nothing, so I'm leaving that well alone for the moment. Applying a voltage across pins 3 and 4 made the meter register a current however (3A...all I could bear to give it before my [thin] wiring melted).

Any help would be much appreciated - I'm positive there's some way to make then light up (everything in the cockpit does, right? :)) which would be cool beyond belief :cool:

I'm also slightly concerned by an amateur-pilot friend mentioning that they aren't supposed to be used as inline ammeters...so what *are* they for, and how can I make them do it?

Many thanks.

[Golden Rivet]

Dont know anything about your purchase but I can concur with your friend in that most ammeters found on large aircraft are not designed to be operated in line, but across a shunt.

A shunt is fundamentally a resistor of very low resistance and connected external to the ammeter in parallel with its moving coil.

Therefore what this type of ammeter is measuring is a voltage drop across a resistance.

With regard to the lighting, you may need to apply a bit more voltage - would imagine the backlighting is 28v

GR

[curiouspurchases]

OK, brilliant; that gives me a good starting point :)

Any ideas for the shunt resistance, or should I just trial-and-error for it? Could just use a trimmer and a known current until it matches the dial calibration...

And thanks for the lighting tip, I wasn't sure what voltage everything ran at inside the big a/c. (assumed it was 12V like the smaller ones I guess :))

Thanks for your help.

CP

[rwm]

Large aircraft have several voltages running different systems. DC voltage is normaly 28vdc and the AC is 120 at 400hz. Many of the non primary instrument lights are 3vac. Primary lights and instuments are normaly off the battery bus, and 28vdc. Your pitot ammeter I believe is used to show the draw for the anti-ice heater built into the pitot head. This is most likely to be 120vac.