Hydrostatic Fuel Gauge

[BrumSaint]

Other than a wooden dip stick I don't have fuel gauge for my boat. I'm reluctant to fit one retrospectively because I don't want to drill a hole in the fuel tank. I recently came across a fuel gauge that measures the height of the fuel hydrostatically and is fitted into the fuel pipe (See diagram below). I don't know much about hydrostatics, but I understand that the pressure at the bottom of the fuel tank relates to the height of the fuel above it and if you know the measurements of the tank then the amount of fuel in the tank can be calculated.

 

My fuel pipe, like most others, exits the fuel tank at a higher level than the actual fuel level and because the engine is lower than the tank a syphon effect causes the fuel to flow to the engine. My knowledge of syphons is also limited, but I understand that for a syphon to work the pressure at the top of the tube is zero. What I'm not sure about is whether the gauge/sender unit will measure the pressure of the fuel at level A or at the height of Level B in the diagram below. I'm hoping there is someone here who knows about both hydrostatic measurement and the effect a syphon might have on the measurement.

[Jen-in-Wellies]

15 minutes ago, BrumSaint said:

What I'm not sure about is whether the gauge/sender unit will measure the pressure of the fuel at level A or at the height of Level B in the diagram

If it works, then level B. I wouldn't trust the reading with the engine running as the lift pump will affect the pressure. Engine off is more likely to be representative.

[Ex Brummie]

If you are referring to the LSK gauge, this is hydraulic as opposed to hydrostatic. From my work in the oil heating industry, many tanks had hydraulic gauges, typically with a sight tube. Hydrostatic gauges were used mainly in industrial usage where the gauge would be situated in a control room or boilerhouse and they had a manual pump which pressurised the sensing tube and then provided the reading as it levelled out.

The LSK sensing unit converts the hydraulic pressure into a digital readout, and would measure the 'weight' of fuel in the supply tube from its highest point, the top of the syphon loop. it is designed for gravity supply from a tank feed on a motor bike or scooter where the take off is at the base of the tank.

[nicknorman]

40 minutes ago, Ex Brummie said:

If you are referring to the LSK gauge, this is hydraulic as opposed to hydrostatic. From my work in the oil heating industry, many tanks had hydraulic gauges, typically with a sight tube. Hydrostatic gauges were used mainly in industrial usage where the gauge would be situated in a control room or boilerhouse and they had a manual pump which pressurised the sensing tube and then provided the reading as it levelled out.

The LSK sensing unit converts the hydraulic pressure into a digital readout, and would measure the 'weight' of fuel in the supply tube from its highest point, the top of the syphon loop. it is designed for gravity supply from a tank feed on a motor bike or scooter where the take off is at the base of the tank.

No I don’t think it would measure the level (pressure) from the top of the siphon loop. At the top of the loop, there is “pull” from both sides, I think it would measure the pressure from level B. Consider what would happen if you cut the pipe after the siphon at level B - nothing, because there is no additional resultant pressure contribution from the top of the siphon.

 

But because the sender is well below the bottom of the tank, it wouldn’t be very linear ie it would go from showing 1/2 full or whatever, to empty at the moment the inlet pipe was uncovered. Also you have to be careful not to introduce something that, in the event of a fire, would allow diesel to freely flow out of eg melted plastic (BSS issue).

 

Have you considered the MCS diesel gauge? Yes it does require drilling a small hole in the top of the tank but it’s not a big deal.

[Ex Brummie]

If you cut the pipe at B, then yes, the fuel would go back into the tank as the syphon is broken and the lower force prevails. 

Consider if you cut the pipe at A then the sensor would measure the weight of the contents over the whole length of the pipe back to A. 

However, as a complete picture, the gauge would not comply with BSS standards. 

Another option that could be considered would be the Atkinson Watchman or the Apollo fuel gauge which would still require a hole in the top of the tank, but also a 240v supply to the receiver which could be sited in the cabin, with the sender being battery powered.

Both are significantly cheaper than many alternatives quoted in chandlers.

[David Mack]

8 minutes ago, Ex Brummie said:

Consider if you cut the pipe at A then the sensor would measure the weight of the contents over the whole length of the pipe back to A. 

No. The sensor measures the pressure in the fuel, which is the height difference between the top of the fuel in the tank and the location of the sensor x the fuel density x the gravitational constant g. It does matter how long (or wide) the pipe is or how many siphons or dips there are - as long as the pipe is full of fuel without any air bubbles. As Jen says, flowing fuel may induce a pressure drop, so the most accurate reading will be obtained with the engine not running.

[Tony Brooks]

13 hours ago, David Mack said:

As Jen says, flowing fuel may induce a pressure drop, so the most accurate reading will be obtained with the engine not running.

 

I don't think there is any "may" about it because there is a filter between the tank and sensor. That, plus the fuel returned to the tank on most diesels should ensure a pressure drop of some sort across the filter, but as said, once the engine is stopped the reading should be consistent, but possibly not accurate unless carefully sited height wise, or calibrated.

[Jen-in-Wellies]

So as you are merrily boating along, you know that you can't believe the low level the fuel gauge is now reporting. When the engine splutters to a halt, the fuel gauge now accurately shows that the tank is empty. 😀

[Chewbacka]

There is a reason most use a stick to measure the level,  It is -

Cheap

Accurate

Reliable

Foolproof

Doesn’t need batteries or wiring up

 

and for most using once a week is enough

[nicknorman]

11 minutes ago, Chewbacka said:

There is a reason most use a stick to measure the level,  It is -

Cheap

Accurate

Reliable

Foolproof

Doesn’t need batteries or wiring up

 

and for most using once a week is enough

That’s all very well if you can easily use a stick. In our case the tank is below the engine and there is a flexible filler pipe with bends from the tunnel deck fitting - which does not form a straight line down to the tank bottom. Which is why I installed a gauge.

[BrumSaint]

Hi, Everyone

Thank you for the responses, but unfortunately there is not a clear opinion whether the gauge measures at level A or B so I'm not much further forward.

 

For Ex-Brummie the manufacturers website https://www.lskelectronics.com/fuelgaugepro1 refers to it as a hydrostatic measuring device.

 

For those who mentioned the BSS requirements although it is sold with a plastic T-piece, if I purchase it I would of course use a metal T-piece.

 

For those who mentioned the inaccuracy whilst the engine was running, thank you that hadn't occurred to me, but as I measure now (with the stick) when the engine is not running there would be no difference (apart from dragging the stick out from the engine bay, unlocking the fuel cap, inserting the stick, wiping the stick clean and then putting the stick away).

 

The kit can be purchased in the UK https://www.sip-scootershop.com/en/product/fuel-gauge-lsk-fuelgaugepro1_15892000 (and as as Ex-Brummie says it is marketed for a motor bike or scooter where the take off is at the base of the tank), but at £140+ I'm reluctant to buy it unless I'm sure it will measure accurately the contents of the tank and not be affected by the height of the syphon pipe. I have written to the manufacturer about the issue, but so far I've not received a reply.

 

Thanks again for the replies.

 

 

[blackrose]

26 minutes ago, BrumSaint said:

Hi, Everyone

Thank you for the responses, but unfortunately there is not a clear opinion whether the gauge measures at level A or B so I'm not much further forward.

 

For Ex-Brummie the manufacturers website https://www.lskelectronics.com/fuelgaugepro1 refers to it as a hydrostatic measuring device.

 

For those who mentioned the BSS requirements although it is sold with a plastic T-piece, if I purchase it I would of course use a metal T-piece.

 

For those who mentioned the inaccuracy whilst the engine was running, thank you that hadn't occurred to me, but as I measure now (with the stick) when the engine is not running there would be no difference (apart from dragging the stick out from the engine bay, unlocking the fuel cap, inserting the stick, wiping the stick clean and then putting the stick away).

 

Continuing with the stick definitely sounds like the most reliable option for you. One wouldn't normally need to read a fuel gauge whilst actually cruising, assuming a measurement had been taken before setting off and there was adequate fuel in the tank. Anyway, nothing in this thread has induced me to change from the stick method.

[Tony Brooks]

1 hour ago, BrumSaint said:

Thank you for the responses, but unfortunately there is not a clear opinion whether the gauge measures at level A or B so I'm not much further forward.

 

I thought it had been adequately explained so I will have another go.

 

The fuel in the pipe dipping into the tank has weight so is trying to pull the fuel out of the pipe back into the tank, but as there is no way fort air to get into the top of the bend the fuel will stay put and just try to pull the fuel up the down pipe and into the tank

 

At the same time the fuel in the down pipe running to the filter also has weight so it is trying to pull fuel out of the tank, but as there is no outlet no fuel can flow.

 

So we have a weight fuel in the relatively short down pipe in the tank pulling one way and the weight of the fuel in the longer pipe pulling the other so one will try to cancle out the other but as the down pipe in the tank is shorter the weight of fuel in it will be less so the resultant "force" will be equivalent of level B as long as the engine is not running.

[jonesthenuke]

3 hours ago, BrumSaint said:

Hi, Everyone

Thank you for the responses, but unfortunately there is not a clear opinion whether the gauge measures at level A or B so I'm not much further forward.

 

For Ex-Brummie the manufacturers website https://www.lskelectronics.com/fuelgaugepro1 refers to it as a hydrostatic measuring device.

 

For those who mentioned the BSS requirements although it is sold with a plastic T-piece, if I purchase it I would of course use a metal T-piece.

 

For those who mentioned the inaccuracy whilst the engine was running, thank you that hadn't occurred to me, but as I measure now (with the stick) when the engine is not running there would be no difference (apart from dragging the stick out from the engine bay, unlocking the fuel cap, inserting the stick, wiping the stick clean and then putting the stick away).

 

The kit can be purchased in the UK https://www.sip-scootershop.com/en/product/fuel-gauge-lsk-fuelgaugepro1_15892000 (and as as Ex-Brummie says it is marketed for a motor bike or scooter where the take off is at the base of the tank), but at £140+ I'm reluctant to buy it unless I'm sure it will measure accurately the contents of the tank and not be affected by the height of the syphon pipe. I have written to the manufacturer about the issue, but so far I've not received a reply.

 

Thanks again for the replies.

 

 

 Taking your comment  "there is not a clear opinion whether the gauge measures at level A or B".

 

Level A is irrelevant in terms of measuring the tank level in the way proposed, the siphon section has no impact on the pressure as the two sides cancel each other out. This assumes the upstaged section is full of fuel with no air/vapour spaces (which should be the case as the engine would be pulling fuel through  when running).

 

The pressure at B under static conditions will be proportional to the difference in level between the top surface of the liquid in the tank and level of the sensor and the density of the liquid. As it's  easier to measure positive pressures, the general approach is to place the sensor below the bottom of the tank so that the pressure is always positive. Where this is not possible then the measurement is limited to tank levels above the height of the sensor and this would need to be taken into account when calibrating the system (noting it would match the usable tank contents in the diagrams shown.  If the sensor can work with both positive and negative pressures then this restriction is overcome but the sensor will be more complex/expensive.

 

The significant problem with etc design in the sketch is the location of the sensor on the fuel suction line. When the engine is running The pressure at the sensor will be reduced due to the pressure drop along the pipework and the additional pressure drop across the filter.  This may be significant, depending on flow rate and the degree to which the filter is fouled. Consequently the measurement will be of little use when the engine is running, however, the reading would be correct when the engine is shutdown, thus its not all bad. I would note that the fuel flow rate for a narrowboat will be low (say 1 litre per hour) and the filter capable of much higher flows the error due to engine running may be limited.

 

If you want to proceed with this system, install the sensor low down and below the  bottom of the suction line in the tank if this is practicable. Also install it between the tank and filter if this is possible. This will reduce the problems noted above

 

The best solution would be to connect to the tank directly. Do you have a drain line or fitting that would allow this to be done?

Edited November 29, 2022 by jonesthenuke
Typo

[Victor Vectis]

Yes.

 

Stick with the stick.

 

(Like wot I does)

[BrumSaint]

 

18 hours ago, Tony Brooks said:

 

I thought it had been adequately explained so I will have another go.

 

 

16 hours ago, jonesthenuke said:

 Taking your comment  "there is not a clear opinion whether the gauge measures at level A or B".

Thanks to both Tony & JonestheNuke, when I said there wasn't a 'clear opinion' I meant that at that time arguments had been made to support the level being read at both A & B.

 

Both of your explanations however have helped clarify the issue to my 'lay' mind, to the extent that I will probably buy one.

 

Also as stated above I hadn't considered whether the engine running may make a difference, but thinking about it the gauge I use for my fresh water tank (Which is an MSC one and is placed between the tank and the pump, but no syphon is involved) also uses hydrostatic pressure and drops to zero when a tap is turned on and the pump is running. 

 

16 hours ago, jonesthenuke said:

The best solution would be to connect to the tank directly. Do you have a drain line or fitting that would allow this to be done?

Unfortunately, no.

 

Thanks again to everyone for the comments and the advice, all very helpful.

[BrumSaint]

As a final comment on this thread. I've received a reply from the manufacturer and they have said that it will not work with a syphon.

 

I also thought about the comments made in relation to the BSS. I originally only considered these in relation to the plastic T-piece sold with the kit, I have since looked a bit more closely at the actual sensor/sending unit and am not convinced that this is fire resistant and would therefore be a BSS fail.

 

So back to the good old dip-stick.

[Tracy D'arth]

23 minutes ago, BrumSaint said:

As a final comment on this thread. I've received a reply from the manufacturer and they have said that it will not work with a syphon.

 

I also thought about the comments made in relation to the BSS. I originally only considered these in relation to the plastic T-piece sold with the kit, I have since looked a bit more closely at the actual sensor/sending unit and am not convinced that this is fire resistant and would therefore be a BSS fail.

 

So back to the good old dip-stick.

Good idea. Let us know if it breaks down or goes inaccurate.