Fuel gauge current consumption - SailNet Community
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Fuel gauge current consumption

I'm trying to figure out how much current a fuel gauge consumes. Typically, a sending unit is 33 to 240 ohms, so in a 12V system that would mean a maximum current draw of 364ma, which I suspect is not true. I'm guessing there is some electronics inside the gauge to account for voltage changes and such and that this changes the consumption.

Since we can pretty much replace brand x gauge with brand y gauge (the analog ones) I assume there is a standard. Can anyone explain? Thank you.
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Old 10-07-2019
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Re: Fuel gauge current consumption

Actually I think that's not that far off. The gauge is just a voltmeter, and I suspect it has a fixed resistor in it and is measuring the voltage drop across it. So current would depend on the sum of the sender and gauge resistances.

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Re: Fuel gauge current consumption

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Actually I think that's not that far off. The gauge is just a voltmeter, and I suspect it has a fixed resistor in it and is measuring the voltage drop across it. So current would depend on the sum of the sender and gauge resistances.
I would be surprised if it's that simple. If it were, then the gauge reading would be very dependant on the battery voltage. Fuel gauges probably are mostly turned on when the engine is running, but water and waste tank gauges tend to be on quite bit. If each gauge used 300+ ma that would be an amp burned up and the readings dependant on the battery state.
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Re: Fuel gauge current consumption

The gauge is not a simple voltmeter or ammeter. There are two coils in the instrument and their resistance and the circuit configuration must be included in your calculation of the amount of current that the gauge circuit draws. The gauge is also damped.

Google around a little bit or measure the resistance of your particular gauge.

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Re: Fuel gauge current consumption

I do know the gauge and sender are just connected in series across the 12 volt supply. I have no idea what the resistance of the gauge is, but if it is similar to that of the sender, you are looking at maybe 300 ohms at half level, which would be 40ma. Again, all dependent on the resistance of the gauge,

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Re: Fuel gauge current consumption

The gauge has two coils. One in series with the sender and one in parallel. The magnet on the armature is balanced between them. This eliminates battery voltage from influencing the reading. The resistances of two individual coils in the gauge can be measured by the resistances between the three connections on the gauge. Google may also answer the question for your gauge.

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Re: Fuel gauge current consumption

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Originally Posted by jarcher View Post
I'm trying to figure out how much current a fuel gauge consumes.
a gnats fart worth

Why do you really what to know? (fusing or wire size maybe)

Don't blow air up my rear, be useful and blow it at the sails!
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Re: Fuel gauge current consumption

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a gnats fart worth

Why do you really what to know? (fusing or wire size maybe)
I want to know exactly what I asked.
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Re: Fuel gauge current consumption

I bought an inexpensive one on Amazon and played with it. It's hooked up between 12VDC on one side, and on another side it goes to 12VDC with the tank sensor in line. This makes sense if you think about the system with two coils as posted earlier.

Measuring the current, the side with the sensor varies roughly between 5ma and 35ma while the other side seems to want 30ma. Not a lot of current, especially when the engine is running, which is when you want the fuel gauge anyhow. But that's the fuel gauge. Water and waste tanks have the same type of sensors and are typically turned on under battery power. Of course, other brands gauges will probably be different but probably not dramatically so. Maybe older bimetal ones will use more power.

I want to simulate a tank sensor. I was able to simulate the variable resistor based sensor with resistors of course, but also with a constant current power supply. I suspect that if I can have a constant current supply I can vary from 0ma to 50ma I'll be able to control most gauges.

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Re: Fuel gauge current consumption

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Originally Posted by jarcher View Post
But that's the fuel gauge. Water and waste tanks have the same type of sensors and are typically turned on under battery power.
For the record there is no need to have any tank reading it's level all of the time. A momentary switch to check the level anytime curiosity strikes is what I would do.

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