Some other way of measuring fuel level

[Avocet]

Ok, this isn't brought on by a boaty shortcoming, but SURELY after all these years, there's a better way of measuring fuel level than either a sight tube that needs the boat (car) to be still and level gets dull and hard to see through; a float and variable resistor that also suffers from fuel slosh as the vehicle moves; or a dipstick?! The floats are also pretty rubbish if your fuel tank is a funny shape, and end up with very non-linear readings.

Isn't there a chaep-as-chips ultrasonic (or something like that) volumetric sensor that can be connected to a box of electrical gubbins that is programmed with the capacity of the tank, and just continuously measures the volume of the air space above the fuel and subtracts it from the known volume of the empty tank? OK, I know some tanks are baffled and we might need a sensor for each "bay" in the tank. Is there any other way of doing it? I wondered about weighing in-motion but that seem too messy.

 

[lw395]

There are level sensors with no moving parts, that work by thermistors. A small current is passed through the thermistor, if it is immersed in fuel it does not warm up, if it is in vapour, it warms up and the resistance changes.
I believe that is how the sensor on my bike works.
When it works!
It is about 12 years old and starting to act randomly.

Measuring the volume of air (vapour) by its acoustic properties is interesting, but with volatile fuel you might get some odd answers as temperature changes?
It

 

[prv]

The floats are also pretty rubbish if your fuel tank is a funny shape, and end up with very non-linear readings.

They don't have to be. When Tek-Tanks made us a new fuel tank for Kindred Spirit, with a complex cross-section, they calculated the levels for different volumes and had Wema make a customised sensor. The gauge read linearly, and the difference in cost was about a tenner.

Ariam's water tank is a flattish triangular prism plus a cuboid top, to fit the hull side. Soon after we got the boat, I attached a flow meter to the galley tap and emptied a full tank in ten-litre increments, noting the gauge position each time. This gave us an accurate figure for useable capacity, and also showed that the gauge was completely linear - obviously customised for the builder in a similar way.

Pete

 

[sailorman]

we have a sight tube with a cock @ the bottom. the tank capacity is marked every 10 lts beside the tube. simples & works with no battery either.
The tank is a flat shape 5` long under a bunk

 

[Avocet]

There are level sensors with no moving parts, that work by thermistors. A small current is passed through the thermistor, if it is immersed in fuel it does not warm up, if it is in vapour, it warms up and the resistance changes.
I believe that is how the sensor on my bike works.
When it works!
It is about 12 years old and starting to act randomly.

Measuring the volume of air (vapour) by its acoustic properties is interesting, but with volatile fuel you might get some odd answers as temperature changes?
It

That's interesting. How does it cope with fuel sloshing around when driving? Does it just rely in the damping in the gauge (or software)? The application I was thinking of was a car, but a car with a very wide, flat fuel tank, so the resolution of any float or level-based gauge would be rubbish (only about 100mm from top of the tank to bottom but maybe something like 12 gallons of capacity). I know you can compensate for a varying tank cross section with software (or even just irregularly spaced markings on the gauge face), but the problem with moving fuel level still persists. Putting the float as close as possible to the geometric centre of the tank helps with that, up to a point, but doesn't solve it (and does nothing for the poor resolution).

 

[Concerto]

My Westerly Fulmar has a very accurate method of checking the tank - a dip stick! Very reliable and will never go wrong - unless I loose it. A perfect KISS solution.

 

[LadyInBed]

"SURELY after all these years, there's a better way"
Yes, you could use a flow meter to measure how much you've used.

 

[prv]

"SURELY after all these years, there's a better way"
Yes, you could use a flow meter to measure how much you've used.

Two flow meters, you mean

Pete

 

[Bilgediver]

My Westerly Fulmar has a very accurate method of checking the tank - a dip stick! Very reliable and will never go wrong - unless I loose it. A perfect KISS solution.

+! Wooden with a file handle . Marked in old fashioned gallons. I am lucky the tank filler is in the cockpit floor.

 

[Avocet]

Thanks all, remember this isn't a boat though (it's a modified car), so an accurate "in-use" fuel level display is much more important and a dipstick is definitely a no-no!

 

[Avocet]

Two flow meters, you mean

Pete

Interestingly, the car already does that (sort-of_ by counting the injector pulses and deriving the amount that must have gone in from a load of engine parameters. it does so for the trip computer (which, unfortunately, is also relatively inaccurate - always a few MPG optimistic for some reason, though you'd have thought it should be absolutely spot-on using that method!

 

[lw395]

Thanks all, remember this isn't a boat though (it's a modified car), so an accurate "in-use" fuel level display is much more important and a dipstick is definitely a no-no!

Mileometer and low level warning light?
I always zero the trip on my bike, then fill up before 200 miles.
Older bikes often have a reserve tap. A mate had a Triumph Herald like that.

 

[Yngmar]

I've seen a boat that had an elaborate pneumatic measuring system. You have a tiny hand pump in the instrument console which pumps air into a flexible tube, and an air pressure gauge with a scale on it. The other end of that tube is connected to a thin pipe that is inserted from the top of the tank going down to the (almost) bottom, with an opening at the bottom.

So whatever liquid (fuel, water) is in the tank will normally be at the same level inside that little pipe. When you start pumping air into the pipe, it pushes the liquid out the bottom of the pipe, until it's all out and air just bubbles out. On the gauge this means you pump until the gauge stops rising. When the gauge settles, read off the tank level. This works because the more liquid is in the tank, the higher the pressure is on the air filled pipe.

Now on this boat it also had several push-buttons that effectively connected the pump and gauge to the different tanks, allowing you to read any fuel, water or holding tank level (fuel and water had different scales on the gauge due to different density).

All this sounds way complicated, but in use and operation it is fairly simple, requires no electricity and unlike the electronic sensors it hasn't broken down in the time I've fiddled with that boat.


The same boat also has a (now broken - see spare parts request in Wanted forum) Navman DIESEL 3200 fuel flow sensor. This consists of two accurate (positive displacement meter) fuel flow sensors, one in the supply line and one in the fuel return line. You tell it once what the size of the tank is and when you filled it up to the brim. From there it simply counts how much fuel went into the engine, subtracts how much came back out (via the fuel return) and can thereby give you a highly accurate measurement of how much is left in the tank (and fuel consumption rate). It's quite expensive, very accurate, works at any angle of heel and sea state. At least one sensor is now broken and spare parts no longer available. Nobody else seems to make one suitable for diesel engines (there's a bunch for petrol ones).

 

[Mistroma]

Interestingly, the car already does that (sort-of_ by counting the injector pulses and deriving the amount that must have gone in from a load of engine parameters. it does so for the trip computer (which, unfortunately, is also relatively inaccurate - always a few MPG optimistic for some reason, though you'd have thought it should be absolutely spot-on using that method!

I did a bit of digging on a previous car as mpg was quite a bit out. You could tweak a specific register via CAN bus adapter and get pretty good accuracy. I supplied about 6 months of refill and mileage data for every refill. However, the pillock in the garage explained that he was a master technician and couldn't use my data. It wasn't possible to use an average of averages and he'd need to fill it, drive about 30 miles and measure amount used before tweaking the register. He wanted money for time driving around and I'd need to return on another day (60 mile round trip) to get it done. Couldn't possibly just adjust it using my figures while connected to diagnostic kit for current service.

I gave up trying to explain that although he was correct about averages of averages it was irrelevant. Every reading at every fill was almost the same (51.1, 51.1, 51.2 and so on). Pretty easy to see that just using anything around 51.1 will greatly improve matters when system always reports about 57mpg.

So it may well be possible to improve reading on your car by tweaking a register in the control gubbins.

Interesting thought about air volume, it's about the only thing in the system that won't be put out by tank shape. However, I think measuring volume out and in via flow meters might be easier to implement. You'd need to do some calibration over several fills and check it matched amount used to refill from time to time. It should be pretty accurate once you have the capacity of your tank when full.

 

[Mistroma]

I had not heard of the NavMan 3200 system, not surprised it was expensive.

I've seen the air pump system and that works with fuel or water. However, you need to calibrate for density of liquid being used and it doesn't help with odd shaped tanks. It lets you know height of liquid above the dip leg but you still need to make a calibration curve and use it when boat is upright and steady.

The tank on my own boat is a bloody awful shape, deeper at front vs. back, centreline vs. side and curved to fit the hull. My own solution was to remove the sensor and dip at various fill levels. I then bent the float arm into a curve to match the tank profile better. Not ideal but it's a lot better than the straight arm and now gives a good indication over a wide range of fuel levels. I still keep a log of hours since last fill, so never 100% confident it won't just stick at some point.

A well designed, simple, accurate, cheap system without moving parts would be very nice. I think most of these requirements are mutually exclusive. In fact, marine stuff mostly comes in poorly designed, complex, expensive versions.

 

[Avocet]

I've seen a boat that had an elaborate pneumatic measuring system. You have a tiny hand pump in the instrument console which pumps air into a flexible tube, and an air pressure gauge with a scale on it. The other end of that tube is connected to a thin pipe that is inserted from the top of the tank going down to the (almost) bottom, with an opening at the bottom.

So whatever liquid (fuel, water) is in the tank will normally be at the same level inside that little pipe. When you start pumping air into the pipe, it pushes the liquid out the bottom of the pipe, until it's all out and air just bubbles out. On the gauge this means you pump until the gauge stops rising. When the gauge settles, read off the tank level. This works because the more liquid is in the tank, the higher the pressure is on the air filled pipe.

Now on this boat it also had several push-buttons that effectively connected the pump and gauge to the different tanks, allowing you to read any fuel, water or holding tank level (fuel and water had different scales on the gauge due to different density).

All this sounds way complicated, but in use and operation it is fairly simple, requires no electricity and unlike the electronic sensors it hasn't broken down in the time I've fiddled with that boat.


The same boat also has a (now broken - see spare parts request in Wanted forum) Navman DIESEL 3200 fuel flow sensor. This consists of two accurate (positive displacement meter) fuel flow sensors, one in the supply line and one in the fuel return line. You tell it once what the size of the tank is and when you filled it up to the brim. From there it simply counts how much fuel went into the engine, subtracts how much came back out (via the fuel return) and can thereby give you a highly accurate measurement of how much is left in the tank (and fuel consumption rate). It's quite expensive, very accurate, works at any angle of heel and sea state. At least one sensor is now broken and spare parts no longer available. Nobody else seems to make one suitable for diesel engines (there's a bunch for petrol ones).

Thanks, that's the sort of "outside-the-box" thinking I'm after. I'm pretty certain I have seen such a system on a car before (a 1936 Alvis, in fact)! but you can't use it while you're moving. Also, in my case, because of the shallow tank depth, the "head " of pressure method wouldn't be great as far as the resolution goes.

 

[pvb]

... a float and variable resistor that also suffers from fuel slosh as the vehicle moves...

My last boat had a tubular VDO sensor, not subject to sloshing variation.

 

[Avocet]

I did a bit of digging on a previous car as mpg was quite a bit out. You could tweak a specific register via CAN bus adapter and get pretty good accuracy. I supplied about 6 months of refill and mileage data for every refill. However, the pillock in the garage explained that he was a master technician and couldn't use my data. It wasn't possible to use an average of averages and he'd need to fill it, drive about 30 miles and measure amount used before tweaking the register. He wanted money for time driving around and I'd need to return on another day (60 mile round trip) to get it done. Couldn't possibly just adjust it using my figures while connected to diagnostic kit for current service.

I gave up trying to explain that although he was correct about averages of averages it was irrelevant. Every reading at every fill was almost the same (51.1, 51.1, 51.2 and so on). Pretty easy to see that just using anything around 51.1 will greatly improve matters when system always reports about 57mpg.

So it may well be possible to improve reading on your car by tweaking a register in the control gubbins.

Interesting thought about air volume, it's about the only thing in the system that won't be put out by tank shape. However, I think measuring volume out and in via flow meters might be easier to implement. You'd need to do some calibration over several fills and check it matched amount used to refill from time to time. It should be pretty accurate once you have the capacity of your tank when full.

I had wondered about that, but you still need some sort of level sensor for when the punter doesn't fill the tank to the brim. I know that in theory, it should be able to keep track of what's gone through the injectors (and back along the return line) with a fair degree of accuracy, but it always falls down when the punter just put's "about a gallon" in, rather than brimming it.

 

[pvb]

Thanks, that's the sort of "outside-the-box" thinking I'm after. I'm pretty certain I have seen such a system on a car before (a 1936 Alvis, in fact)! but you can't use it while you're moving. Also, in my case, because of the shallow tank depth, the "head " of pressure method wouldn't be great as far as the resolution goes.

Sterling offer a pneumatic tank gauge; it's programmable for tank depth and fluid density - http://sterling-power.com/products/pneumatic-tank-guage

 

[prv]

The tank on my own boat is a bloody awful shape, deeper at front vs. back, centreline vs. side and curved to fit the hull. My own solution was to remove the sensor and dip at various fill levels. I then bent the float arm into a curve to match the tank profile better. Not ideal but it's a lot better than the straight arm and now gives a good indication over a wide range of fuel levels.

The tubular float sensors I described in post #3 work by a magnet in the float passing reed switches that connect and disconnect different sized resistors. So it's easy for the manufacturer to move the reed switches up and down to produce a non-linear sensor, hence their willingness to do one-offs to your own figures for only £10 over the standard price.

The only downside is that you don't get a smooth continuous movement of the gauge, it jumps from one specific point to the next as the float passes each switch. I don't find that a problem on either our fuel or water tank.

Pete