We have a buried 1000 gal (3785 l) propane tank used for heating.

When it was filled up recently the gauge went from about 49% to 88%. This would mean 490 gal to 880 gals, or an addition of 390 gals.

We were charged for 434 gals, which is an excess of 44 gals or about 11% extra. Both gauge readings were within about 24 hours, there's no way we consumed that much propane in that short of a time.

Are we interpreting this situation correctly? Could the gauge readings have as much as 11% error in them?

Note that this is the first time we have taken photos but we think this situation has happened before. So if the gauge is error prone would that error consistently under-represent the fill level? Usually measurement error goes both ways.

The supplier has said that they take readings from their truck, and that our gauge is really measuring pressure and not volume. I'm not sure if we should trust that or not. Our gauge is in gallons after all, not PSI or something pressure-related.

Here are before and after pictures of the gauge on the tank itself:

Before fill up:

enter image description here

After fill up:

enter image description here

FYI the tank is buried but there is a lid you open to view the gauge.

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    What is the temperature of the propane before the fillup? What is the temperature of the propane after the fill-up? There is enough information on the gauge to google it, what is its stated precision? – Harper - Reinstate Monica Jan 11 at 13:49
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    @Harper-ReinstateMonica we are going to talk to the gauge company actually. I'm not sure how to know the propane temperature... the ground temp itself should be fairly constant this time of year though I don't think that's what you meant. – UuDdLrLrSs Jan 11 at 13:53
  • The ground temp may be consistent, but what about the temp of the propane that is on the truck? It matters. Also, is your tank exactly the diameter that the gauge is made for? If the tank were a little larger, the gauge would fit but it would throw off the scale. – Harper - Reinstate Monica Jan 11 at 13:54
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    The propane company is charging you by the amount (i.e. volume) of propane metered through their dispenser. That meter should be tested and sealed by your local (in the US, usually the county or state) weights and measures authority. The gauge you are using is just an approximate measure and does NOT appear to be calibrated. – jwh20 Jan 11 at 15:00
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    Given the sparsity of indication marks on the gauge dial and the thickness of the gauge pointer, it is reasonable to assume that the gauge is meant only as a rough guide and not for accurate readings. Besides the gauge error another source of error is that your tank may not be precisely 1000 gallons as you have assumed. – Barry Jan 11 at 23:32

Your gauge is certainly not reading pressure.

The only thing pressure tells you is what temperature the propane is at (LPG is "liquified propane gas" and reflects the fact that propane is easily transported in bulk because it becomes a dense liquid rather than a gas a relatively low pressure. That pressure varies with temperature.)

So, there's probably a float in the tank reading the liquid level. Such things are not uncommonly in error - if you think about the shape of a typical buried LPG tank (a horizontal cylinder), there's a complex relationship between the distance the float travels and the percentage of the tank that is full, which is being "interpreted" by a mechanical linkage.

If you have a local authority in charge of weights and measures, they (probably) check on the metering of the supplier. You could inquire if they do and (if so) when the last test was, and what it showed.

  • in addition you don't need 11% error for this result, if the 49 reading is 3% high and the 88 is 3% low you could get that same result. – Jasen Jan 12 at 9:19
  • For example, here is the Texas government page saying that any LPG meters must be inspected/registered annually. The gas company should be able to provide you with documentation about when their trucks were last inspected (assuming your area has similar requirements). – JPhi1618 Jan 13 at 20:50

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