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Propane / LPG Measurement

Why a seperate section on the measurement of Liquified Petroleum Gas (LPG) or propane? When you purchase LPG or propane, you cannot see the product and you must rely on the operator of the filling equipment to ensure that you receive the proper measurement. As a consumer, you are completely reliant on the operator to ensure you get the volume of fuel you pay for. The information contained below will help you become an informed consumer.

For most of us, the only time we will purchase LPG is when we fill our barbeque tanks, which is a relatively small volume. However, for commercial and industrial customers, the volumes of LPG consumed in a year can be quite large as can the potential errors if the fuel is not measured properly.

Properties of Propane

So what is LPG? Most of us refer to the fuel in our tanks as propane. While this is not incorrect, it is also not entirely accurate. In fact, the fuel in your tank will contain a high percentage of propane, mixed with several other fuels such as butane, methane, etc. Propane is stored and measured as a liquid and used as a vapour (except in some special applications). Contrary to popular belief, propane is not cold. Propane, like all liquids has a boiling point. The boiling point for propane is -42° Celsius (approx -44° F) at atmospheric pressure. At temperatures below the boiling point, propane will be in a liquid state. Since we cannot easily keep our propane tanks below these temperatures, we simply increase the pressure. Liquids will not boil, even well above their boiling point temperatures, if the pressure is increased (same concept that keeps the coolant in your cars radiator from boiling away). When you allow propane to escape from the cylinder, it will immediately begin to boil. To do so, it must steal heat from its environment. If liquid propane touches your skin, it will take the residual heat from your body to assist it in boiling. This is what causes frostbite and is why most people assume that propane is cold.

When propane boils, it increases in volume at a rate of approximately 270 times the original liquid volume. Since the vapour is also about 1.5 times as heavy as air, it will sink to the ground and fill in low spaces. The increase in volume and heavy ground hugging properties of propane are what combine to make it dangerous to store in closed spaces. If the air/fuel concentration reaches approximately 2.4% to 9.5% propane, the fuel will ignite in the presence of an ignition source. While this is a relatively narrow flamability range, you still do not want to allow escaping propane to concentrate in buildings or other enclosed spaces.

Propane is stored under pressure in a tank. The pressure is directly related to the temperature of the propane. Remember that propane boils at -42°C - at this temperature, the pressure will be 0 kPa. As the temperature increases, so does the pressure, until at about 30°C, the presssure will be 966 kPa (or about 140 psi). Tanks and cylinders are equipped with relief valves. These valves will open and vent propane if the pressure inside the tank exceeds the pressure setting of the valve. Large storage tanks and tanks mounted in vehicles usually have relief valves set at 1723 kPa (250 psi), tanks on forklifts will be set at 2151 kPa (312 psi) and all other cylinders will be set at 2585 kPa (375 psi). In order to ensure there is adequate room in the tank for the liquid propane to expand if the temperature increase, tanks are only filled to 80% of the overall volume. Overfilling a tank can leave insufficient room for expansion which will result in increased pressure and venting through the pressure relief valve.

Tanks and Cylinders

Propane is supplied in tanks or cylinders. Although these terms are often used interchangeably, if mounted to a frame or exceeds 420 lbs capacity, it is a tank, while a cylinder is portable and relatively small (below 420 lbs of propane). Tanks and cylinders both have pressure relief valves to ensure that internal pressures do not increase to unsafe levels.


Many tanks are filled using a meter. The tank can be taken to a dispenser or a refueling truck may come to the tank. In both cases, the meter will display the actual amount of propane dispensed into the tank (in litres or gallons). The operator knows the tank is full by using a spit valve. The spit valve is a small valve which opens into the vapour space of the tank at the 80% fill level. When the liquid level in the tank reaches this level, the valve will begin to spit liquid propane which will look like a white fog as it begins to boil. At this point, refueling is stopped and the spit valve is closed. The tank is now safely filled to the 80% fill level.

Small cylinders are usually filled by weight. To fill these cylinders properly, the attendant must know the empty (tare) weight of the cylinder, the "water content" (WC) of the cylinder and if the cylinder currently contains propane or is empty. The water content of the cylinder is multipled by 42% (0.42) to calculate the appropriate amount of propane that can be safely placed in the tank. WC is stated in pounds for imperial cylinders and in litres for metric cylinders. The same formula works in both cases but for imperial cylinders, the result is the amount of propane in pounds, while for the metric cylinders, the result is the amount of propane in kilograms. Small one use cylinders must not be refilled. These cylinders are usually marked 39 or 2P.

Water Content (WC) and Tare Weights (TW or T) will vary among cylinders. Many fill stations will set their equipment to an average value for the most common cylinders. This will not guarantee that you are getting the proper amount of propane in your cylinder. Make the attendant do the calculations using the values from you cylinder. The proper formula to ensure a properly filled cylinder is:

Filled Weight = TW + (42% of WC) + Nozzle Weight*
* - include nozzle weight if weighing while cylinder is still connected to the nozzle. Do not include if weighing the cylinder without the nozzle.

To simplify the process, we have provided an LPG Calculator for your use.

Last modified: 24 January 2010 22:11:16