Voltage of Fuel Sender/Float

Pulsating? Why pulsating? Anyway, if the wiring is like this, how can it be limited?

http://www.moellermarine.com/sites/moellermarine/files/gauge_sender_wiring_diagram.jpg

I'd say you can't, that limiting resistor serves the purpose of dividing the voltage so you have less chances of sparking inside your fuel tank, BUT also serves as a current limiter in case of turns shorted within the sender's winding, avoiding heating of the same.

Safety first, common sense first... I can't tell which one goes before the other.

Regards

The current pulsates because of the way the fuel gauge works.

Read this article: http://auto.howstuffworks.com/fuel-gauge.htm

I did not observe any pulsating current, just a normal DC voltage (i.e. straight line voltage would mean fixed current, I think....)

No pulsed supply required. Simple voltage divider setup. refer to post #4.

Well now that the principal of operation is understood.... I am wondering if it is practical to use the sender output as input to a vehicle tracking unit meaning to get fuel level. Why I say I wonder if it is practical, because yes of course capturing an analog voltage is not an issue, but converting the signal in to accurate tank volume information with all the movement/sloshing/etc might not be that easy. Does anyone have any experience?

The problem, as you say, is not the analog voltage capture (actually an analogue current capture by the sound of things). The problem is calibration. You will need to empty the tank and then refill it litre by litre, noting the output readings as you go.

Yes, one can read the voltage across the sending unit to get a secondary fuel level reading. However, there will be two critical concerns in doing this that an engineer must keep in mind. First, bubbapebi brought up the most vexing problem in his link. Most vehicle fuel tanks today are very irregular shaped vessels. The fuel level (what a float actually measures) will not be linearly proportional to the volume of fuel present in the tank. Second, the circuitry sensing the voltage must have an input impedance greater than the largest resistance of the sender by at least a factor of ten. This is usually not a difficult task but it is the reason that you cannot just use a second identical fuel gauge.

As for fluid sloshing, this is easily solved by some form of bandwidth limitation (i.e. rolling average, low pass filtering). What is a problem that cannot be easily corrected is related to the geometry problem of the irregular shaped tanks. This fuel gauge is for a vehicle and not a static engine. As such the tank and its contents will experience changing accelerations in both magnitude and direction. Thus the same volume of fuel will present different voltage levels at the sensor depending on the slope of the hill being climbed, the centripital acceleration of a turn, etc.

That is why I said you have to fill it litre by litre when calibrating. As far as sloshing fuel is concerned, I understand all vehicle fuel tanks these days have internal baffles to prevent this happening.

You don't get it. Maybe a little humor will help to explain. Notice the level in the glass.

That's too funny and a great example!!

Yes, I know what you are saying with regard to tilt, but sloshing is a problem of acceleration, not of tilt. That is what the baffles are good at preventing. Getting back to tilt, though, has anyone actually observed the tank contents to change dramatically between a level surface and a steep slope?

I try to make it a point to keep my head out of the tank while in motion, unless I'm tanked-up on the fumes!

I have seen this many times and I live on one of the largest glacial wash plains on this planet. There are no mountains here. I see my fuel gauge drift up and down nearly daily from lateral acceleration when I go through a clover leaf turn. No, I don't own an agile sports car with a lateral skid pad greater than 1.0G. I drive an eleven year old Nissan Sentra. A performance sports car IMHO is wasted on this suburban sandbar.

I also wonder how you propose a fluid responds differently from acceleration than from tilt?

Fair enough. If you say it happens, then it happens. It's just that I haven't noticed it.
With respect to your second point, there is no difference as far as the fluid itself is concerned. It is the behaviour of the fluid in a baffled tank which differs. Acceleration is a temporary phenomenon, whereas tilt may be steady state. The effect of the baffle is to limit the fluid excursions during acceleration, even though it allows fluid to assume a fully new level under steady state conditions.

As for fluid sloshing, if I remember right is one of the main reasons they used a pulsating signal, to buffer the readings and slow down the erratic gauge movements from sloshing effect.

Now, if the OP is so worried about the sloshing effect, he or she should maybe look at converting their fuel quantity sensing unit to a capacitive fuel probe system.

I will also caution the OP about modifying anything in or around a fuel system, without skills and knowledge dealing with combustible fuel, as this has the potential of being nominated for Darwin's Award!

Why do you want to send tank level information to a vehicle tracking unit?

What external I/Os does your tracking unit have?

Can you tell us more about your ultimate goal?

@Wal

Well, the goal is, to know the level of fuel in the tank..... And more specifically, to be able to calculate the mileage for a trip (distance is known by the GPS).

You know the level in the tank by looking at the fuel gauge now.

Can you calculate your mileage now, manually, with the information being relayed to you with your current setup?

I would not say that I know the level in the tank by "looking" at the gauge. Firstly, it has to be in a machine readable format (i.e. the vehicle tracker is "looking"). Secondly, it is not a simple issue of performing an A/D conversion since the voltage is constantly changing because of all the sloshing.Yes in theory if I knew the level (i.e. voltage) I could look it up in my calibration table and know the level/volume. But in reality, that signal coming from the fuel sender is very noisy and not stable.

You can't work out your mileage with that information.

Using level is a very indirect way of determining the actual amount of fuel consumed.

You can infer the amount consumed if you know your tank volume, record a level reference, detect a level change and note it, subtract that from the reference, multiply the fractional level change by the total volume, hope your level signal is linear, and remove any noise in the data to get an estimate.

Sounds tough to me.

If you could measure the amount of fuel consumed "directly" you would at least have some data that could be used directly.

Then there's the question of exactly how you would combine your GPS data and the fuel consumption data. You mentioned handing the fuel data to a vehicle tracking unit. What interface is available?

Now that we have an idea what your trying to do, I would suggest installing a fuel flow transmitter, calibrated to the SG of the fuel, then do what you want with the signal. That's about the surest way of eliminating the sloshing effect and erratic gauge movement. This would give the actual fuel consumption either in gals/liters or lbs per hour. Then knowing your tank volume, you can calculate what's left in the tank