Bulk Propane Technical Question

Conan_the_Librarian · #1 22nd December 2017, 06:58 PM

Hoping there is a tech guru out there that can clear up a question I have.
House uses LPG from a bulk tank. Coming to the end of my contract thought I would look around for a new supplier.

With that in mind I looked at my usage over the last 19 months. This is what I found:

See attached SS.

http://forum.a8parts.co.uk/attachmen...1&d=1513972935

My question is, how can there be such a discrepancy in the number of litres to a single % between the first and last?

CtL the Confused

MikkiJayne · #2 22nd December 2017, 08:02 PM

That has to be ambient temperature, especially if you look at the dates - highest volume per 1% is in the winter where the density of the liquid propane is highest. There must be a difference in how the measure volume as it comes of the truck compares to how it behaves once its in the tank - perhaps it comes out as gas but the tank level is showing the liquid?

Conan_the_Librarian · #3 23rd December 2017, 06:41 AM

I can understand that for the small differences but the first delivery?

paulrstaylor · #4 23rd December 2017, 07:23 AM

Was the tank empty before the first fill?

The subsequent re-fills were all from about the same point, maybe the gauge isn’t as accurate at 1% or maybe the gas is more compressible as the tank is empty?

MikkiJayne · #5 23rd December 2017, 08:54 AM

I'm sure thats it ^ 1% level on the first fill so the vapour pressure will have been quite low and it'll have been almost all gas. It will have taken more volume to build up the pressure to get the gas to convert to liquid.

27litres · #6 26th December 2017, 11:21 PM

Is the volume measured at the tank?
Or off the truck?

The base measure of 1% will just be an inaccuracy of however they measure it (float or pressure measure).
It takes quite some pressure to liquify propane, which means quite a volume of gas, and a float measure (which is common in LPG tanks, so possibly in bulk propane tanks too?) will only start to measure once it can float on a liquid. That should explain the initial discrepancy in % full to volume.

You're dealing with a pressure vessel. Volume won't be accurate, as it varies with ambient temperature.
Off the truck is the most accurate measure, as it should be able to measure flow volume, much like an LPG pump does.

Does the tank sit on its side, or vertically?

Conan_the_Librarian · #7 13th January 2018, 08:29 AM

Finally got to the bottom of this one. 27litres was right; it's all down to tank shape. The float gauge is linear while the shape of the tank is not so from the bottom to the middle it take more liters per centimeter of rise the higher the float lifts. From the middle up, less. Most of my Fill started around the 40% mark......