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JLuckey(at)pacbell.net
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 Posted: Sun Jan 01, 2012 9:17 pm Post subject: Capacitive Fuel Level Sensor - How do they work? |
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How do capacitive fuel sensors work?
Can someone give me the general theory of operation, what type of signal
does the transducer produce? Why do I want one? Etc?
TIA
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Mon Jan 02, 2012 1:31 am Post subject: Re: Capacitive Fuel Level Sensor - How do they work? |
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http://www.discovercircuits.com/PDF-FILES/capgage.pdf
A capacitor is constructed of two parallel conductors insulated from each other. The amount of capacitance is related to the distance between the conductors and the type of insulation. The insulation between the conductors of a capacitance fuel level probe consists of air and/or fuel. As the fuel lever changes, so does the capacitance. The capacitance is part of an oscillator circuit. As the capacitance changes, so does the oscillator frequency. The frequency is converted to a voltage that is displayed with a fuel gauge.
The advantage of a capacitance type fuel level probe is that there are no moving parts. The disadvantage is that the gauge must be calibrated each time that the type of fuel is changed because unleaded fuel and leaded fuel and gasohol each have different insulating properties. When the fuel tank is almost full, the capacitance fuel gauge will display different levels depending on the type of fuel in the tank. When the tank is almost empty, the fuel gauge will be more accurate because air is now the insulating medium and the probe does not know or care what type of fuel used to be in the tank.
Joe
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Joe Gores |
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sarg314(at)gmail.com
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| Posted: Mon Jan 02, 2012 6:34 am Post subject: Capacitive Fuel Level Sensor - How do they work? |
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You just make a capacitor where the gap between the two plates of the cap (or the gap between a central wire and an outer tube, or whatever the geometry of the cap is) is open to the gasoline in the tank. The gasoline serves as the electrolyte in the capacitor. If the cap. is completely submerged, then the gap between the 2 parts of the cap is completely full of electrolyte and the capacitance of the the thing is maximized. As the gasoline level drops, the portion of the gap that is filled with fuel , and hence has elevated capacitance, decreases. So the overall capacitance of the capacitor changes with fuel level.
So you put this cap. into a circuit that allows you to measure the capacitance. The easiest way is to make an oscillator circuit and measure the frequency of the oscillator. The frequency of the oscillator will change with the fuel level.Â
The problem is that it's hard to make a cap. whose capacitance varies a lot with the presence or absence of the gas. The variation in capacitance is usually in the range of a few hundred picofarads - not much. So the circuit that measures the capacitance has to be right there at the capacitor because if it was 10 feet away, the wires going to the cap would have way more capacitance than the capacitor itself. (I understand they actually made this mistake on the A-10 fuel systems. Made them very flaky and failure prone.)
I have the Princeton capacitive fuel sensors in my 6A and wish that I didn't. (They're the tube with central wire type of design) They show full until you get down to the last 5 or 6 gallons of gas. So if I see them "off the peg" much at all it means I have very little fuel left. Fortunately my fuel flow integrator in the VM-1000 engine monitor I have is accurate. Also, I understand they get badly confused if any water gets into them. If I were to change from 100LL to auto gas they would have to be recalibrated because autogas has significantly different electrolyte properties. (Recalibrating is doable but kind of a pain since you have to drain the tank.) If I had it to do over, I'd use the ancient float with potentiometer style of fuel level transducer.Â
The Princeton probes sold by Aircraft spruce were designed by a guy named Todd who actually works for Grand Rapids, or at least did the last time I talked to him. Well, I inferred that he designed them. I'm guessing he did that before he joined (or formed?) GRT and still performs as "Princeton" on the side. - just a theory on my part, having been in the electronics business.
On Sun, Jan 1, 2012 at 10:13 PM, Jeff Luckey <JLuckey(at)pacbell.net (JLuckey(at)pacbell.net)> wrote:
| Quote: | --> AeroElectric-List message posted by: "Jeff Luckey" <JLuckey(at)pacbell.net (JLuckey(at)pacbell.net)>
How do capacitive fuel sensors work?
Can someone give me the general theory of operation, what type of signal
does the transducer produce? Â Why do I want one? Etc?
TIA
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Tom Sargent
[quote][b]
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jonlaury
Joined: 06 Nov 2006
Posts: 336
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| Posted: Mon Jan 02, 2012 8:41 am Post subject: Re: Capacitive Fuel Level Sensor - How do they work? |
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The electronics have been explained by Joe and Sarg, and Sarg elaborated on his experience with Princeton probes.
But the reason for installing cap probes has gone unanswered.
Cap probes allow for fuel measurement of irregularly shaped tanks. My fuel bays in the wing change in taper, both in depth and width, in addition to the whole tank sitting at an angle (dihedral), making any kind of accurate measurement impossible over the tank range when using a linear type (float) sensor.
I have installed 8' Centroid probes. Haven't fueled them yet.
They come with two small potentiometers built in to adjust the empty and full points. Available in 0-5v output and various resistance output so you can use them with your existing gauge.
http://www.centroidproducts.com/tableofc.htm
Not a recommendation, just a resource, as I have no real world experience with this product, . YMMV.
John
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Wed Jan 04, 2012 9:19 am Post subject: Capacitive Fuel Level Sensor - How do they work? |
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The sensors are in fact just two rings of metal like a sleeve one inside the
other. Fuel has a particular dielectric value so with increase in fuel
level the capacitance of the sleeves one to the other changes.
Why you would want one? There are several reasons. One is by varying the
shape of the rings you can make a sensor that will compensate for an odd
shape of tank. You can also wire more than one sensor in parallel to give
accurate readings on a tank that can hold a fair amount of fuel and still
have one end of the tank completely dry. For instance in an aircraft that
has considerable dihedral and wet wings.
Finally in all capacitive systems there is a calibration probe which has to
be installed at the bottom of the tank. Water has a completely different
dielectric constant from fuel so if any water gets into your tanks and even
partially submerges this probe a light will immediately alert you.
Noel
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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email(at)jaredyates.com
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| Posted: Wed Jan 04, 2012 3:58 pm Post subject: Capacitive Fuel Level Sensor - How do they work? |
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For those of us with a high-wing, we also get to use a third option of optical quantity indication.
On Wed, Jan 4, 2012 at 4:41 PM, Noel Loveys <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)> wrote:
[quote]--> AeroElectric-List message posted by: "Noel Loveys" <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)>
When a boat or ship leaves port the skipper and the navigator know exactly
how much fuel is on board...All they need is a flow meter to calculate how
much fuel they have on board. Aircraft however also like to know exactly
how much fuel is in each tank even if there is a break in a fuel line. That
requires an accurate method of measuring the fuel in the tank, in the air.
There are only two recognized methods; Capacitance fuel gauges and drip
sticks.... Drip sticks don't work in the air.
Noel
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Wed Jan 04, 2012 4:49 pm Post subject: Capacitive Fuel Level Sensor - How do they work? |
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True if your seat is close to the wing as in my Kitfox...You got me!
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Jared Yates
Sent: January 4, 2012 8:24 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Re: Capacitive Fuel Level Sensor - How do they work?
For those of us with a high-wing, we also get to use a third option of optical quantity indication.
On Wed, Jan 4, 2012 at 4:41 PM, Noel Loveys <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)> wrote:
--> AeroElectric-List message posted by: "Noel Loveys" <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)>
When a boat or ship leaves port the skipper and the navigator know exactly
how much fuel is on board...All they need is a flow meter to calculate how
much fuel they have on board. Aircraft however also like to know exactly
how much fuel is in each tank even if there is a break in a fuel line. That
requires an accurate method of measuring the fuel in the tank, in the air.
There are only two recognized methods; Capacitance fuel gauges and drip
sticks.... Drip sticks don't work in the air.
Noel
--
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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