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Gas cylinder contents (gas remaining)

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    Josef KonradParticipant

    Can anyone tell me where I can get a pressure gauge that will tell me how much gas I have left in my bottles?

    When I clamp my thumb over the outlet the pressure in a full cylinder doesn’t seem that much different to one that’s nearly empty. How can that be?

  • Profile Photo
    Fyko
    Fyko on

    With LPG there is no relation between the pressure in the cylinder and the fullness of the cylinder. A gas such as oxygen for instance, that will never liquefy under any pressure if the temperature is above -119°C, will show a pressure that is proportional to the quantity in the container.

    Pure butane (one component of LPG) at 20°C will boil in the cylinder until the pressure reaches 220 KPa and then stop. Bleeding some pressure off will restart the butane boiling until the pressure in the container is up to 220 Kpa again. Pure propane at 55°C on the other hand will boil until the pressure reaches 2200 KPa and then stop.

    So the pressure in the tank can vary from 220 KPa to 2200 KPa whether the tank is 1% or 100% full. The pressure depends on the temperature and the composition of the liquefied gas, not on how full the container is.

    The only way to assess how much remains in the gas cylinder is to weigh it. The empty weight of the cylinder is stamped on it, and so anything over and above that weight is the liquid within. The maximum capacity of the cylinder is also clearly marked, and the difference between the actual weight of the cylinder and its empty weight is the contents. These are the same parameters used to fill the cylinder.

    We do have the facility to monitor the weight of up to two gas cylinders and display this weight on the screen of a mobile phone using our Caravan Pilot app. It requires a load cell to be placed under the gas cylinder. The load cell(s) are wired back to, the Caravan Mover, the MotorPac, or the VanSecure control unit from where the data is transmitted to the mobile phone via Bluetooth communication.

     

    Profile Photo
    Josef Konrad
    Josef Konrad on

    Does this thing of yours actually work?

    I’ve looked everywhere on the Internet  I can think of and I can find nothing similar anywhere in the world.

    Profile Photo
    Simon Tasman
    Simon Tasman on

    Hi Josef,

    You’ll find everything from these guys works 100%.

    They cut their teeth designing defence-related stuff in the 1970s-1990s. Now they’re just having fun and helping their pals out with clever gadgetry.

    The reason that nobody else has thought of it is because nobody else thinks.

    Profile Photo
    Fyko
    Fyko on

    Hi Josef,

    Above is the load cell that weighs the gas cylinder. You loosen the clamp that holds the gas bottle and place the load cell underneath so the bowl at the bottom of the cylinder rests on the silver button of the load cell. The load cell is permanently wired back to the control unit through a 4-wire signal cable.

    When it’s time to pack up you just pull the load cell out and tighten the clamp the around the cylinder.

    This thing gives you sufficiently accurate warning that you’re running out of gas that you don’t need a second bottle as a spare, which saves some space and expense. If you do have 2 bottles in use simultaneously you can attach a monitor for each one and get separate read-outs on the mobile phone.

    A low level of LPG gas doesn’t trigger an alarm like the gas leak detector does because there’s not the same degree of urgency but you can see your LPG gas levels anytime you’re in Bluetooth range of your caravan.

    Profile Photo
    Fyko
    Fyko on

    One interesting and unexpected discovery that we made during the field trials and calibration exercises that we did with various new and partially discharged gas cylinders is that the gas cylinders as supplied by the various dealers are often not full.

    The very accurate weighing equipment that we used to calibrate and test our weight monitor gadget revealed the 9Kg cylinders actually often only contain 8Kgs of gas.

    Something to watch for if you object to getting ripped off at every opportunity.

    Profile Photo
    Simon Tasman
    Simon Tasman on

    What about the empty weight of the cylinders?

    Won’t that be confusing to some?

    Profile Photo
    Fyko
    Fyko on

    The empty weight of a gas cylinder is always very clearly indicated by stamp or stencil.

    To find the weight of the remaining gas contents you can do one of two things:

    1. You can read the empty weight and subtract it from the indicated total, or
    2. You can enter the empty weight into the system using a screen-based slide control. This will record any value up to 20Kgs in 0.1Kg increments, and this amount will be deducted from the measured total weight of the cylinder prior to display. More convenient for some, but assumes that you have entered the correct empty weight. (Which is easily checked).

    Nothing there to get confused about.

    Profile Photo
    Fyko
    Fyko on

    The advantage of using the Tare offset feature is that the app displays any gas remaining weight of 2Kgs or less with a red background, ad more than that with a green background. So you can easily see when your cylinder is running low.

    If you read only the gross weight you’ll never get this warning.

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