Gas Cylinder Markings

Gas cylinder markings for most US marketed cylinders. 

WC: The water capacity is how much water the cylinder will hold in pounds. The "WC" stamped on the bottle followed by a number such as "23.8" means the bottle will hold 23.8 pounds of water.

Multiply by 0.42 to convert the water weight to LPG weight  23.8 x 0.42 = 9.996 pounds.  Divide, pounds by 2.2 to convert pounds to kilograms.   9.996 / 2.2 convert to KG = 4.54 KG so a 4.5Kg cylinder

So using the same formular a WC marking of "47.6" converts to a 9.08KG cylinder.  47.6 x 0.42 = 19.992. lb, convert to Kg 19.992 / 2.2 = 9.08 Kg

As a side note , WC is used to measure cylinder capacity because water is not compressible its the best medium to use, whereas LP gas or even air is. Thus WC is a reliable and repeatable measure of capacity. I have read where some people talk about the capacity of the gas can vary due to pressure, in this case there is no room for the variables. 

TW: Tare weight of empty cylinder, in pounds (US cylinders), so if you have a set of scales its possible to weigh the full cylinder and then subtract the TW (Tare Weight) to check if the fill weight is OK. 

DT: Is the length of the dip tube, which is where the bleeder value opening inside of the cylinder is located. This determines how full the tank will get before liquid starts coming out of the bleeder valve, when open. Generally as discussed earlier a WC of 47.6 lbs can be filled with 20 lbs  (9.08 Kg) of LP gas. 

A replacement LPG cylinder valve, the thin tube with the bulb is the dip tube 

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Gas cylinder markings for most Australian marketed cylinders. 

WC: Water Capacity. It is the internal volume at test. This is also the volume of LPG it contains when full. This can be used to work how many kgs of LPG it can hold. In this case due to the markings being in metric, find the "W.C." marking on the bottle, for a 9 kg bottle, the WC is 22.0 Kg. However this is not always the case, this 22.0kg is a generalisation and I have seen a variety of values around this value plus or minus. The normal propane or propane/butane mix at the specified pressure will weigh approximately 41% of the water capacity. Doing the math 41% of 22kg = 9kg  or simply put 22 x .41 = 9.02 

TW: (occasionally TM) is short for Tare Weight, it is the weight of the empty cylinder with no valve. (lets not discuss mass and weight difference)

EW: Is not always there. It is the TW (Tare Weight) plus the valve weight. i.e. the weight of empty cylinder (EW = Empty Weight) before filling, keep in mind the change over of cylinder valve types from POL to LCC27 is under way and could mean the value of the EW of an older cylinder may not be 100% correct but should still be close. 

TP: Is the Test Pressure. You also find the test date and the testers registered number. Usually valid for 10 years.

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OK how to work out the number of kgs of LPG is in the cylinder or the cylinder can hold  ? 

A really rough and ready way to calculate capacity. Half the WC and then round down to the closest size cylinder to get the Kg capacity of LPG. So a WC of 22 calculates to 11 Kgs of LPG  the closest size bottle to this is 9 Kg so its more than likely the cylinder you have is 9 Kg  a rough and ready calculation

A better way is get out your phone calculator and multiply the WC value by .41, As an example a WC value of twenty two.  22 x .41 = 9.02 so the cylinder can hold 9 Kg or a WC of eleven, 11 x .41 = 4.5 so a 4.5 Kg capacity cylinder. 

Got a partly used cylinder and want to know how much LPG is left.

The simplest way to check the contents of your cylinder is to weigh it. Weigh the bottle with a set of bathroom scales is normally the easiest, how ever we have found hand held luggage measuring scales work a treat. (doing it on a moving boat underway or at anchor isn't very accurate)

As an example if your partly used 9 Kg bottle weighs in at 11 Kg. 

Take the EW (empty weight) in this case 8.1 Kg away from the of the value of the partly used bottle (11 Kg) as measured by your scales. 

E.g. 11kg – 8.1kg = 3.9kg.   You have 3.9kg of gas remaining from your 9kg capacity cylinder = 43%

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This bit will is like throwing a cat amongst the pidgions. I say this because it may take some getting your head around. 

Storing LPG in cylinders allows it to be transported in a smaller physical volume than would be required if the gas was distributed in its natural state. Gas cylinders have internal pressures that are higher than atmospheric pressure. LPG must have a boiling point below 20 degrees Celsius in order to be considered a gas.

When using the actual WC (water capacity) figures stamped into your cylinder to work out your cylinder storage the DT (length of Dip Tube) also plays into the final calculation. The cylinder will usually only be filled 80% of the full volume as there needs to be an area for the gas to form in the cylinder. So any calculations using the WC marking should take into account a full cylinder is 20% less volume. (hence the multiply the WC value by .41)

So doing the calculation a different way, (keep in mind this isnt an exact science due to several variables)

WC = 22      20% of 22 =  4.4        22 - 4.4 = 17.6  so the volume for a fill will be 17.6L.   So the weight of the fill will roughly be 8.979591836734694Kg  I would round this up to 9Kg   

Gas volume can be calculated in litres with some basic mathematics.

For LPG (propane) 1kg = 1.96L. Therefore, 14.2kg = 27.832L

For LPG (butane) 1kg = 1.724L. Therefore, 14.2kg = 24.48L

For a 75:25 butane/propane mix, 1kg = 1.783L.

Unlike water, 1kg of LPG doesn’t equal 1L of LPG. This is because the density of LPG is less than the density of water. In Australia, where LPG is propane, 1kg of LPG has a volume of 1.96L.

Conversely, 1L of LPG weighs 0.51kg.

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 If the valve is being replaced trim the dip tube to match valve being replaced, this should then mean the stamped info remains current.

Swap & Go 

As mentioned earlier the filling of cylinders isn't an exact science due to several variables, so to keep legal (weights and measures) the Swap and Go people have reduced the volume of the fill to take account of the fact not all cylinders are created equal and have reduced the amount of gas they fill the cylinders with. This doesn't reduce the cost of course but you know you get a the fill weight they are advertising it is.  

The 9kg gas cylinder is the larger cylinder and is filled with 8.5kg of LPG. The smaller 4kg gas cylinder is filled with 3.7kg of LPG. So what sucks for me is I have a 4.5 Kg cylinder so I always look for a refill station, and unfortunately they are getting fewer every day.