info for alternative connection with dual tank.

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bartlet
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info for alternative connection with dual tank.

#1 Post by bartlet » Sat Sep 07, 2019 11:44 am

Hello,

I have a car with two tank, one big and other one small, in the big tank I have the level gauge, in the smaller I do not have indicato level, the tanks are connected togheter by t pipe. can I connect the small tank to the bigger for permit accurate level indicato of lpg? I see the prins dual tank valve but I ask you technical advice for do a good setup.
Greetings.

Gilbertd
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Re: info for alternative connection with dual tank.

#2 Post by Gilbertd » Mon Sep 09, 2019 3:14 pm

The two tanks will empty at roughly the same rate so if one is half full, then other will be too (unless there is a big variation in temperature between the two). LPG gauges never are accurate, if it gives you a rough idea, that's as good as you are going to get, just reset the trip meter every time you fill up and go on how many miles you've done.
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Brian_H
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Re: info for alternative connection with dual tank.

#3 Post by Brian_H » Mon Sep 09, 2019 9:58 pm

You might get away with daisy chaining two level sensors together, depending what readings your getting on the existing sensor. Technically they should have a level gauge on each tank anyway even if it isn't connected to the dash readout. That said, I doubt its worth the bother as the gauges vary quite a bit in their accuracy anyway, you can allow for that a bit with adjustment, but typically I find on any of mine the most you see is about 1.8v across the sensor with a full tank, adding another sensor to this is likely to reduce the voltage so far that your unlikely to see any sort of useful value.

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Re: info for alternative connection with dual tank.

#4 Post by LPGC » Mon Sep 16, 2019 1:09 pm

Normally we expect tanks (on multiple tank setups) to empty at the same rate but they can empty one tank before much has been used out of the other.

As Bri says really. On some installs I have fitted a sender on each tank and a switch on the dash to flick between senders but it's hit and miss because each gauge really needs to be calibrated in software differently (so 4 lights for one sender might denote full as expected but full for the other tank might only be 3 lights). Wiring senders in series / parallel might work but probably wouldn't be as linear as with one tank (not that they're often linear with even one tank fitted!). The only real way of doing it with 2 or more tanks would be to fit something like a microcontroller that reads voltage from 2 or more senders, accounts for differences in calibration of each sender, averages the readings and outputs a voltage that more accurately reflects total gas left for the LPG ECU to base it's fuel gauge output on.
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bartlet
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Re: info for alternative connection with dual tank.

#5 Post by bartlet » Thu Sep 19, 2019 4:22 pm

LPGC wrote:
Mon Sep 16, 2019 1:09 pm
Normally we expect tanks (on multiple tank setups) to empty at the same rate but they can empty one tank before much has been used out of the other.

As Bri says really. On some installs I have fitted a sender on each tank and a switch on the dash to flick between senders but it's hit and miss because each gauge really needs to be calibrated in software differently (so 4 lights for one sender might denote full as expected but full for the other tank might only be 3 lights). Wiring senders in series / parallel might work but probably wouldn't be as linear as with one tank (not that they're often linear with even one tank fitted!). The only real way of doing it with 2 or more tanks would be to fit something like a microcontroller that reads voltage from 2 or more senders, accounts for differences in calibration of each sender, averages the readings and outputs a voltage that more accurately reflects total gas left for the LPG ECU to base it's fuel gauge output on.
thank you all for the answer, so my problem remains, The big problem is that the small tank it is 20 liters and the big tank it is 50 liters and the level gauge it is only on big tank so I can't see the gas that remain in the small tank, I thought you could connect the tanks together so that the bigger one pours the gas onto the smaller one and put the level gauge in the smaller tank. Apparently the best solution is to use a custom built microcontroller

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Re: info for alternative connection with dual tank.

#6 Post by Gilbertd » Thu Sep 19, 2019 8:39 pm

But they should empty equally, so when the big tank is down to 25 litres, the small one will be down to 10 litres. There isn't really a simple answer but as the gauges are so inaccurate, just do when everyone else does and reset the trip meter when you fill up. You'll know when it is getting empty from the number of miles you have travelled and when it really is empty, it'll swutch back to petrol.
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bartlet
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Re: info for alternative connection with dual tank.

#7 Post by bartlet » Thu Sep 19, 2019 9:14 pm

Gilbertd wrote:
Thu Sep 19, 2019 8:39 pm
it'll swutch back to petrol.
this is the problem, my car don't have petrol tank it is monofuel

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Re: info for alternative connection with dual tank.

#8 Post by LPGC » Fri Sep 20, 2019 2:43 pm

The simplest way would be to fit 2 gauges and 2 senders (you have one of each already), so you'd have a sender on each tank wired to separate gauges.

Gas doesn't 'pour' from one tank to another but unless your setup has one way valves fitted in the tank outlets (in the pipe from tank outlets) or the outlets are joined using a special T which only allows gas to flow from one tank at a time gas can go flow from the tank with the higher pressure into the tank with the lower pressure (usually only when the electrovalve is open but also at other times in extreme cases because electrovalves are designed to allow gas to flow backwards into a tank if pressure at the outlet side of the electrovalve is high enough - pipes don't have headroom to allow for expansion of the liquid gas)

As gas is drawn from a tank the liquid gas level goes down and to maintain the pressure in the growing headroom in the tank some gas has to boil off inside the tank, which has a cooling effect on the tank which lowers the tank pressure. In theory this has a balancing effect that means both tanks will empty in the same time... but other factors are at play so it doesn't always go the way the basic theory would suggest and one tank can empty before the other. Other factors can be very subtle such as if tanks are at different temperatures for other reasons (one nearer a heater / aircon outlet inside the car, or nearer the exhaust or radiator air wash outside the car), maybe even differences in piping layout from tanks, even a slight difference in height could make a very subtle difference.
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DodgeRover1
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Re: info for alternative connection with dual tank.

#9 Post by DodgeRover1 » Mon Sep 30, 2019 8:18 pm

Possibly a bit late but if I was going straight LPG then I would run the smaller tank as a reserve, with a seperate switch

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Re: info for alternative connection with dual tank.

#10 Post by LPGC » Tue Oct 01, 2019 9:31 am

DodgeRover1 wrote:
Mon Sep 30, 2019 8:18 pm
Possibly a bit late but if I was going straight LPG then I would run the smaller tank as a reserve, with a seperate switch
That can be done, if done it makes it a bit more critical to fit a one way T in outlets between tanks - With one tank empty the other tank full as soon as the full tank solenoid is opened gas will flow from the full tank to the empty tank for a while during which the liquid feed pressure/flow to the reducer can be low. Gas can flow from a full tank with open solenoid to an empty tank with closed solenoid because the solenoids allow backflow to prevent pipes getting over-pressured when the solenoid is closed. A one way T will prevent the back/cross flow (as would a second solenoid fitted to the empty tank outlet in reverse flow to normal fitting). Even after tank to tank flow has stopped (tanks pressure balanced) the was empty tank may be cold due to incoming gas that evaporated to pressurise that tank, and the full tank can be cold due to gas flowing out of it at high rate increasing headroom causing gas to evaporate in that tank to pressurise the bigger headroom, 2 cold tanks lower pressure again. The effects are more likely to cause problem if the full/reserve tank isn't really full but a small amount of gas has been left in it as a reserve (e.g. gas drawn from both tanks until both are quite low and the driver thinks by turning one off when it still has a bit of gas left in it he will run on the other tank until it runs out and still know he has a bit of gas left in the one he turned off) because the same cooling effect is acting on a smaller mass when the tank isn't as full, also more likely if the vehicle uses a lot of power (not necessarily booting it, more likely the vehicle is big and heavy so needs a good flow of gas to the reducer for normal driving).
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