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measuring petrol/ethonol accurately Answered

G'day All,

If this has been done before, I apologise, but I have not found anything when I have searched.

I am a team manager for a team of students who need to design, construct a three wheeled vehicle. The test is a 24 hour "trial" (more like a race ;-) )

We have been competing in a category for "Hybrid Vehicles" which an additional power source to asssist the rider.  In our case it is a 50cc engine using a friction drive arrangement so we can disengage it if required or if we desire.  The reason for this is that we only recieve 3 litres of petrol for the event............  This is my problem.  I have not been able to measure petrol in the tank accurately enough.  Over the last two years I have had one litre left or ran out after 21 hours.   

I have been thinking of using a strain gauge or load cell to measure the weight of the tank.  I'm thinking it could be as simple as powering up the load cell with the appropriate voltage, and measuring voltage drop across it, maybe with an added resistor. Or it could be made more complex by using a circuit to drive a number of LED's giving an indication of the fuel in the tank.

If anyone outhere has any ideas to do this simply and reliably, I would love to hear from you.  If it is more complex then so be it.

Here is a link to the event to give you an idea of what it is about



6 years ago

My first motorbike had two fuel lines from the tank. The inlet for the first was higher in the tank than the second, as the level dropped the bike started coughing and spluttering and I knew I'd used most of the tank, turning a valve changed to the second lower line and I had ~30 miles of fuel left.
With three or four lines sequentially lower you'd have a rough idea of your rate of fuel usage and how much fuel was left. More low-tech than previous suggestions but it would be relatively inexpensive and fairly robust.

How about weighing the tank in real time? Have all connections and supports being flexible or attached to the weighing pan. This should give an accurate reading of the amount of fuel on-board unless you really start to tilt the whole vehicle allot.

Would the tank need to be secured such that it wouldn't swing? If not, could the tank be allowed to pivot, thus keeping it in line with gravity regardless of the roll (angle) of the vehicle itself? With a two-wheeled vehicle, this would dramatically compromise performance, but perhaps not with a three wheeler, if placed close to the two-wheeled axle. Then visual confirmation or a float would be more effective. If you've got the two wheels in the back, then maybe a mirror would allow an easy view of the tank.

If you can fill-up a petrol-can at a petrol station, the pump-measure should be close enough to 3L.
Otherwise you should be able to mark-off on glass bottles to 3L


Prior to the race start, the officials use a flow meter to allocate exactly 3 litres to each team before the tank is sealed using a lock wire on the cap. I need to know what fuel has been used during the race, hence trying to work out something more accurate than a line drawn on the tank every 100ml. I have found that the slope of the pit lane makes this a bit hit and miss.

if the pit lane is at the same angle all the way along, just account for that and put the markings at that angle

A flow-meter would be a good idea, but that's not practical.
You might have a go at seeing whether you can measure depth accurately with a light-source & light-sensitive receiver, but I wouldn't.

If the tilt of the pit-lane is putting the tank-reading-out, you could make a device to measure the angle and construct a chart by degrees to adjust the reading.


tossed the idea of a flow meter but as there is a fuel retun to the tank, I would need two.

If you please sir,what is the capacity,configuration,and composition ofthe tank in question? Photographs?

Currently it is a rectangular tank made from plastic. It is mounted solidly at the ends to the frame of the vehicle at a slight angle towards one corner to help use all of the fuel. Capacity is around 4 litres.

I said currently as the rules are changing to force us to use an ethonol blended fuel (85% ethonol) so the tank may change to suit, depending on the rules next year.

The simplest ( I hope we are on the same page ) solution that comes to mind is a sight glass. A square dowel perhaps of Lexan half in and half out of the side of the tank,sealed with a two part petrol insensitive epoxy putty.Graduated on the outside with a known amount of liquid on the inside.Next would be a conventional sensor/float type,installed in the tank and attached to a guage. Perhaps the latter from a motor cycle or go cart, could be modified to suit. I hope I have been some help...

We currently have the tank, which is sorta see through enough to see the fuel, is marked in 50ml or 100ml intervals. But this is a bit hit and miss as the pit lane isn't level so any markings are out straight away......

I'm after something where I can be close to 100% certain on the amount of fuel left. So I can determine how hard we can go.

If this is still an active question, my immediate response to the problem of tilt throwing off your sight glass readings are either
- to have two sight glasses, one on each side of the tank, and average the readings
- get the pit crew to construct a level area- a sheet of plywood, shimmed under the corners until it's level in both axes (or even just one if both is too hard) should do

Coln,I think the retro fit "float" device is going to be the best compromise. 100% accuracy under all circumstances,I have to think,is almost unatainable.Going completely over the top(for your amused consideration) I suggest a multi dimensional gimbaled tank holder.This will get you close to 100%,cost a fortune,and earn you the name "lawn mower man".A tough nut to crack Coln.I wish you the best of luck....


6 years ago

ultrasonic ranger for depth, some sort of tilt sensor for the angle of the tank. Do some software to compute the fill level based on the current geometry. That should make it insensitive to nonlevel places. Probably couldn't make it go while driving, the surface would be very uneven, even with filtering.

I am assuming that you could get a ranging sensor to not explode the gas. :)

Weigh it. Its a bit position sensitive, but you would probably be close enough - some analysis needed.

Hoping to weigh it in the bike at aeach pit stop. Sorta have to since refueling is not allowed during the 24 hours of the race. As I said else where, after 10am Saturday, the tanks are sealed.

As L. says - weigh it consistently in the same place - you could always use a smartphone's accelerometer to sense the vehicle attitude and correct the weight for it.