fuel level sender for aircraft


I have a fuel level sender and LED gauge from a privately built
aircraft. It uses a strange implementation of an LM3914 dot/bargraph
LED display driver IC. The sender is a standard type and is
constructed from linearly wound resistance wire with a range from 10R
to 100R. I'll post the circuit later, but in the meantime does anyone
know of any design rules covering fuel tank senders? I ask this
because the wiper arm and spring loaded rotating contact are very
intermittent, making me wonder how it is possible for electrical
contacts to safely operate in the presence of petrol vapour.
Presumably there is oxygen in the tank, too.
- Franc Zabkar
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Reply to
Franc Zabkar
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I guess most cars used that system in the past, long time ago I fixed one on my VE Valiant (made in 1969) and it was just as you describe. I'm pretty sure I've seen a lot of other car manufacturers using same or similar senders - just check some old Gregorys manual in your local library for details.
Tom
Reply to
Tom
On Wed, 18 May 2005 13:18:45 +1000, Tom put finger to keyboard and composed:
I'm reasonably familiar with the resistive senders. I believe most vehicles use these. There was a time when Ford Australia released capacitive senders in some of their models but these were susceptible to moisture and were discontinued.
I would have expected aircraft to be fitted with better instrumentation than the average car, but this home built airplane continues to surprise me. The pilot tells me that the electronic fuel gauge was designed and built by a Queenslander, but unfortunately he is no longer interested in supporting it.
Here is the circuit:
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Notice that the IC's negative supply rail, which is normally connected to circuit ground, is left to "float" on the sender input. Notice also where the IC gets its signal input. The fact that pin 4 connects to pin 8 means that the sender's normal signal range of 0-5V is reduced to only 1.25V. I can only imagine that this design is attempting to linearise the sender's input in some novel way, but I can't see how. If this were my design, I would have used a 50mA current source to drive the sender, and a standard 0-5V meter circuit straight from National Semiconductor's datasheet
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.
BTW, I'm also a Mopar lover from way back.
- Franc Zabkar
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Reply to
Franc Zabkar
Fascinating. I'm sure that Frank is right, there must be standards for them, perhaps an aviation ng or similar. Either that or a quick phone call to the appropriate people. If there aren't any standards I'm never flying again :))
Reply to
Jim
Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:
I would have used a 50mA current source to
Seems to me a current source is the exact wrong way to go, especially as high as 50 mA. If the there was a break in the sender with a small air gap, a current source will try to force 50 mA through the gap until it hits a voltage limit or a spark jumps. I know the voltage is relatively low, but I would be surprised if the concept of a current source is used in car or plane.
Reply to
Geoff C
I used to have a Cessna one and one off a Bell 47 and yes they are just rheostat types like you'd find in a car, with a gauge needle driven by a heater winding on a bi-metallic strip. I understand some of the recent models are of capacitance type too. It's identical to older car fuel gauges and I don't know why they don't explode either.
Reply to
Mark Harriss
There are standards alright (and tons of manuals(literally) for civil aircraft).
Homebuilt are a differnt kettle of fish all together. A LAME - Licensed Aicraft Maintenance Engineer should know maybe not about homebuilt.
If in doubt try having a look on the CASA website or try contacting them.http://www.casa.gov.au/
Probably better off contacting them or talking to someone who knows the rules and regs covering homebuilt.
Some quite tought penalties covering aviation and airworthiness and safety as well as a shitload of paperwork regardless of what you are doing.
Alex Gibson
Alex
Reply to
Alex Gibson
On Wed, 18 May 2005 23:17:47 GMT, Geoff C put finger to keyboard and composed:
The existing circuit uses a 180 ohm resistor in series with a 15V supply. This allows a current of 79mA to flow when the tank is full and 54mA when it is empty. In the event of a broken sender, the OC voltage would be 15V, the same as for an OC current source. AFAICS a 50mA current source should be safer.
BTW, my current source would use a series limiting resistor, say 180R. The sender voltage would range from 0 to 5V, and the series resistor would drop 9V. This leaves a range of 1V to 6V for the current source.
- Franc Zabkar
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Reply to
Franc Zabkar
On Thu, 19 May 2005 18:40:05 -0800, Mark Harriss put finger to keyboard and composed:
All I know is that the plane was built from a commercially supplied kit.
- Franc Zabkar
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Please remove one 's' from my address when replying by email.
Reply to
Franc Zabkar
Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:
Well that is interesting. I suppose then that must be the standard way to do it. I did not know that. If that is the way it has been done for the last 80 years then it must be a proven method.
Reply to
Geoff C
Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:
Well that is interesting. I suppose then that must be the standard way to do it. I did not know that. If that is the way it has been done for the last 80 years then it must be a proven method.
Reply to
Geoff C
You know Franc, for less than $140 USD your friend could have a new gauge and sender unit for his plane, thats pretty good for plane prices. I'd be very wary of touching anything on planes without the necessary approvals, even if it is a homebuilt plane.
Is it a homebuilt with normal certification or a homebuilt with an experimental certificate?, the regs could be very different. A call to an avionics shop could put you in the picture.
Reply to
Mark Harriss
I am fairly certain that the resistive element in fuel gauge senders is completely enclosed thus making the risk of explosion from arcing impossible. The main problem with resistive senders is that they eventually become unreliable due to wear and tear on the sliding contact elements. One of the most reliable types is that using a set of reed relay switches and a float magnet such as shown in these examples
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Reply to
Ross Herbert
Not on the ones I've attended to, on Nissans and Commodes. They are simply a wire-wound linear pot with a wiper. Anyway, any exclosure would need to be vapour-sealed to have any impact on the prospect of pyrotechnic events.
which causes intermittent contact and presumably - to the extent permitted by the 10V reg and meter impedance - some sparking.
Reply to
budgie
The one I pulled apart was made in the 50's and I don't think it was from a car.
In order to get any sort of arcing you first need the circuit carrying the current to go open circuit. Since the wiper is tied to one end of the resistive element it can never go open circuit so there is no arcing. The only effect is that the full resistance of the element is applied.
Reply to
Ross Herbert
On Fri, 20 May 2005 00:58:37 GMT, Geoff C put finger to keyboard and composed:
The "standard" way utilises a mechanical gauge with a fixed series resistance, and a bimetallic strip for damping. The rheostat in the sender forms the bottom half of a potential divider. This arrangement is highly nonlinear. For example, the difference between "full" and half" may be 30L while the difference between "half" and "empty" may be only 20L. Of course part of the problem may be the uneven cross section of the tank.
- Franc Zabkar
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Please remove one 's' from my address when replying by email.
Reply to
Franc Zabkar
Well, I suppose anything is possible even if it is remote. Generally, the resistance wire is of reasonable thickness such that the tension of the wiper will not wear through it for at least 50 years or more. Not only that but the wiper is made of softer material than the resistor wire so it will wear out faster. An open circuit due to wiper wear will be the most likely event to occur.
Reply to
Ross Herbert
It seems that the risk of a petrol tank explosion is next to impossible according to this document
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so my theory for having an enclosed sender doesn't come into it where petrol is concerned.
Reply to
Ross Herbert
Most of the non-linearity comes from two sources:
. the meter responds to current, the sender is a avariable resistor - partly a hyperbolic function.
. the sender arm traverses say 70 degrees as the fuel level goes from full to empty. The angle at half-tank isn't half-travel or half-resistance.
Obviously these factors can be "aligned" to partially compensate, but the result is always going to be non-linear with such a configuration.
Reply to
budgie
The ones I've seen are all have the rheostat element exposed to the fuel. They just have a half can to mount the element on the inside of
Reply to
Mark Harriss

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