Aircraft thermometer gauge question - Page 1
Forum section: electronics / projects

author: compet17
author_rank: Regular Contributor
timestamp: January 04, 2015, 01:50:12 pm
content: I got this old aircraft outside temperature indicator. I have the idea to make it work again to tell me the temperature of something (maybe the outside air??)The thing is quite simple, just a bunch of handmade precision resistors (that look like inductors) a pot and an electromechanical instrument.... which puzzles me a bit.Why does it have four wires?Does someone have infos for this instrument?I made a diagram and I assume that pin 2 is ground, 4 is 12/24V (as written on the nameplate) and pin 3-4 are connected to the thermistor.Manufacturer: Thomas A. Edison Inc. W.Orange N.J. Instrument DivisionModel: 150-9Part: 34520YFAh yes and in case someone wants to know: This instrument was installed in a Lockheed Super Constellation from 1955. The scanned image is from the "Flight Manual" of that aircraft. It's only a simplified schematic but it shows the principle.

author: max_torque
author_rank: Super Contributor
timestamp: January 04, 2015, 02:04:23 pm
content: The unit has 4 wires because as you has said, 2 for power / ground, 2 for the sensor. The schematic shows it is using a wheatstone bridge type arrangement, where the current flowing across the meter coil depends upon the "Balance" of the sum of the resistances on either side of the bridge:http://en.wikipedia.org/wiki/Wheatstone_bridgeBy measuring the resistances of the bridge resistors in the meter, and the resistance of the meter coil you can easily work out the range of sensor resistances that relate to a max / min meter reading.(ps, the unit uses a bridge type arrangement to make it virtually imune to input voltage fluctuations)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 02:25:14 pm
content: Mmm... it's not independent of supply voltage, in fact it is proportional... I suppose it could be constructed in a way that minimizes that, however. Example: perhaps the yellow wires entering the movement are to *de*magnetize a permanent magnet, thus tending to reduce the movement's gain when the input signal is made larger by the elevated supply. This would only work over a modest range, because deflection goes as the product of magnetic fields. If the movement (rotor) field is proportional to Vsupply, by making the stator field proportional to (1-k*Vsupply), the product becomes Vsupply - k*Vsupply^2, where k can be adjusted to null the fluctuations around nominal supply.Just a guess. There might be other, better ways of doing it.The "resistance bulb" I would guess is something with a well-defined tempco, like nickel or constantan. A PTC or RTD type thermistor: fairly weak (resistance not quite proportional to Tabs), but reliable. A thermocouple, of course, would require cold junction compensation, which would be more complicated.The nice thing about a Wheatstone bridge is, its output (i.e., the voltage across the "span" of the bridge) is zero when the circuit is balanced. If the meter movement is not just a movement, but a feedback mechanism which adjusts one resistor until the voltage reads zero, it would indeed be independent of supply voltage again; a variation in supply would modulate how strongly the mechanism is able to correct, and therefore control the linearity and error of the reading. I do see what looks like an adjustable rheostat on top, but it looks like the movement is enclosed, so there would be no way to adjust a bridge resistor; and there's no indication on the schematic that there is such a mechanism.Tim

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2015, 02:42:34 pm
content: The sensor will likely be a Pt100 sensor, connected to 2 pins.I will guess pin 2 is common for power and sensor, and the power is applied to pin 4 for 12V, and pin 1 for 24V, with the sensor between pins 3 and 2.The meter is ratiometric, having 2 coils arranged so the one generated the field, and the other the moving coil. I relies on the ratio of the currents to drive the pointer, not the absolute value.

author: compet17
author_rank: Regular Contributor
timestamp: January 05, 2015, 07:46:49 am
content: Thanks for the replies but there seems to be a misunderstanding. My question was: why does the instrument itself (the needle movement) have 4 wires?

author: SeanB
author_rank: Super Contributor
timestamp: January 05, 2015, 02:39:56 pm
content: Quote from: compet17 on January 05, 2015, 07:46:49 amhttp://www.myclassbook.org/megger/How it works and why there are 4 wires in it as well.

author: compet17
author_rank: Regular Contributor
timestamp: January 05, 2015, 07:24:55 pm
content: That could be the answer why it works on 12 and 24 Volts because the ratio is constant even when the supply voltage changes. And that makes sense because the aircraft power supply in those old airplanes was not electronically regulated (like in a car 12 to 15 Volts). So they needed something that indicated an accurate value even if the supply was instable.Very ingenious!

author: compet17
author_rank: Regular Contributor
timestamp: April 28, 2015, 07:13:47 pm
content: A follow up to this strange mechanical meter.... I disassembled it now totally:You can see the yellow needle with a counterweight and at the end of the axle is a magnet. The magnet is inside a massive copper housing with a copper cover.There are four coils total. 2 red and 2 yellow, each pair is connected in series. The red and yellow coils were mounted around the copper housing with an angle of about 30 Deg between each other.There was no spring to move the needle or position it.When playing with this movement, I found that when both coils (red and yellow) had the same current, the needle was centered. Any difference in the currents lets the needle move in one or the other direction.The only thing where I don't have an explanation for is the second part from the left. It is mounted on the two main screws and has a movable "c" shaped ring with a soldered piece of wire that looks like a tiny antenna. This is definitively something to adjust something but how?The only explanation I have at the moment is that is has magnetic properties of some sort. Maybe a steel wire that influences the field from the coils...Ah yes and the whole movement was inside a steel drum which provided magnetic shielding.Any ideas??

author: jeroen74
author_rank: Frequent Contributor
timestamp: April 29, 2015, 08:02:08 pm
content: I guess it's a type of air core movement:http://en.wikipedia.org/wiki/Air_core_gaugehttp://www.mikesflightdeck.com/instruments/diy_aircore_instruments.html