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Capactence fuel sender and GRT EIS question

Mike S

Mike S

Senior Curmudgeon
I am helping a buddy upgrade his plane from steam gauges and a Vision Microsystems engine monitor system to a GRT EFIS and GRT EIS.

The last (Hopefully last) issue is the EIS fuel level doesn't seem to be reading info from the sender from the Vision Micro .

The Vision Micro used a capacitance sender, with three wires coming out of a large threaded hex fitting. I have no idea if there is a built in converter in the actual probe or not, or if the Vision Micro unit was designed to read the sender output directly without any kind of converter.

Anybody out there dealt with this in the past, and what/how did you get the two units to play nicely with each other? I searched the archives, found multiple references to the problem, but not to the solution.

Any source or part number for the converter box------pretty sure one is needed-----to make this setup work.

Changing out the capacitance probe to a float is the last option, really do not want to even think about it.

Thanks in advance...
 
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The way capacitance senders work, inside the tank, uses only two wires - one is the airframe itself, and the other one is connected to the plates which are insulated from the airframe. If you have more than those two connections, then you already have some type of converter between those wires and the plates.

The trick may be figuring out which converter, and which wires do what. It's likely they are power, ground, and signal.
 
airguy said:
The way capacitance senders work, inside the tank, uses only two wires - one is the airframe itself, and the other one is connected to the plates which are insulated from the airframe.
Click to expand...


Well, the unit is a tube with a wire inside it, and the tank is fiberglass.
 
Mike - there is a frequent poster on this site, Reggie Smith (FasGlass or similar user name) who his a guru on Vision Microsystems stuff. Look him up - he will be able to tell you how the VM fuel probes work, and what adaptation may be required to go to the GRT EIS system.

His website: http://vmsupport.weebly.com/
 
Mike S said:
Well, the unit is a tube with a wire inside it, and the tank is fiberglass.
Click to expand...

Interesting design, I hadn't seen those - but the operating principle is the same. You essentially are building a capacitor consisting of two conductive elements, with a dielectric between them. That's only two wires, any way you cut it. The converter puts an AC signal on those two wires to charge/discharge the capacitor rapidly and deduces the dielectric constant by virtue of the amount of charge the capacitor will take. Part of the dielectric is air (or fuel vapor, which has a very similar dielectric constant) and part is fuel (which is quite different from air or fuel vapor). The net dielectric constant will change considerably with the amount of the capacitor that is submerged, which changes the amount of charge the capacitor will take. The converter figures this out, and converts it to a voltage level output to your instruments, similar to what a float gauge would give.
 
Canadian_JOY said:
Mike - there is a frequent poster on this site, Reggie Smith (FasGlass or similar user name) who his a guru on Vision Microsystems stuff. Look him up - he will be able to tell you how the VM fuel probes work, and what adaptation may be required to go to the GRT EIS system.

His website: http://vmsupport.weebly.com/
Click to expand...

Got it-------thanks.

BTW, just in case someone else needs to contact him, his login name is Glas467.
 
Mike,

Greg Niehues did a nice job of explaining the principle of operation of a capacitance probe.

Here's some info specific to the VMS VM1000 implementation:
1) 3 wire probe with an internal oscillator circuit. Red and black wires are for 5V DC excitation, white wire is the output signal which is a 5V AC square wave with frequency proportional to the capacitance and hence fuel level. The white wire also shares the black wire ground reference.
2) The VMS DPU outputs regulated 5V DC to the probe and converts the returning frequency to a fuel level based on a lookup table hard burned into the main program chip.
3) The conversion circuitry is inside the VM1000 DPU, not inside the probe.

First step I'd recommend is to test the probe to make sure it is good:

1. With power applied to VM1000 instrument system, perform the following DC voltage tests at the transducer wires:
a. BLK (meter com) to RED (meter +) (s/b = 5.00 VDC) . = ________
b. BLK (meter com) to WHT (meter +) (s/b > 1.9 VAC). = ________
c. BLK (meter com) to WHT (meter +) (s/b 1.9 to 2.4 VDC). = ________
2. With probe disconnected from EC100 or IO PCB, measure resistance between probe wires as follows:
a. BLK (meter com) and WHT (meter +) wires. (s/b approx 14500 ohms) . Resistance = ________
b. BLK (meter com) and RED (meter +) wires. (s/b approx 9100 ohms) . Resistance = ________
c. WHT (meter com) and RED (meter +) wires. (s/b approx 5500 ohms) . Resistance = ________

Next, determine if the GRT is able to read a frequency based capacitance probe. If so, it should be a matter of getting the settings just right. If the GRT requires 0-5V output (or other voltage based signal) from the probe, the only way to do this is through a frequency to voltage converter circuit.

Several folks here on VAF have had good luck using the Princeton converter mentioned previously in this thread.

Hope this helps,
Reggie
http://vmsupport.weebly.com/
 
Glas467 said:
Here's some info specific to the VMS VM1000 implementation:
1) 3 wire probe with an internal oscillator circuit. Red and black wires are for 5V DC excitation, white wire is the output signal which is a 5V AC square wave with frequency proportional to the capacitance and hence fuel level. The white wire also shares the black wire ground reference.
2) The VMS DPU outputs regulated 5V DC to the probe and converts the returning frequency to a fuel level based on a lookup table hard burned into the main program chip.
3) The conversion circuitry is inside the VM1000 DPU, not inside the probe.
Click to expand...

OK, so the circuitry is split between the probe and the DPU. The GRT does have a 5v output as I recall, so that should be good to drive the internals of the probe.

Glas467 said:
First step I'd recommend is to test the probe to make sure it is good:

1. With power applied to VM1000 instrument system, perform the following DC voltage tests at the transducer wires:
a. BLK (meter com) to RED (meter +) (s/b = 5.00 VDC) . = ________
b. BLK (meter com) to WHT (meter +) (s/b > 1.9 VAC). = ________
c. BLK (meter com) to WHT (meter +) (s/b 1.9 to 2.4 VDC). = ________
2. With probe disconnected from EC100 or IO PCB, measure resistance between probe wires as follows:
a. BLK (meter com) and WHT (meter +) wires. (s/b approx 14500 ohms) . Resistance = ________
b. BLK (meter com) and RED (meter +) wires. (s/b approx 9100 ohms) . Resistance = ________
c. WHT (meter com) and RED (meter +) wires. (s/b approx 5500 ohms) . Resistance = ________
Click to expand...

Looks like this is straight out of the manual he has-------cant do test 1, a b c as the VM1000 is no longer in the plane. Will have him do the others.

Glas467 said:
Next, determine if the GRT is able to read a frequency based capacitance probe. If so, it should be a matter of getting the settings just right. If the GRT requires 0-5V output (or other voltage based signal) from the probe, the only way to do this is through a frequency to voltage converter circuit.

Several folks here on VAF have had good luck using the Princeton converter mentioned previously in this thread.

Hope this helps,
Reggie
http://vmsupport.weebly.com/
Click to expand...

Hope it helps??? understatement of the day:D Immense help, and very much appreciated.
 
And the answer is--------

OK, to close the loop on this---

My buddy talked to Todd, at GRT. They make a converter box specifically for this combination:D

So, all should be working in a few days.

Many thanks to all who responded........appreciate the depth of knowledge available here.
 
Mike, you should also be aware that calibration of the capacitance senders will have to be done again if you change from 100LL fuel to something else, like Mogas. They have different dielectric constants and the calibration curve will be different. Zero fuel (empty tanks) should still be the same, but the quantity above zero will be different.
 
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