LPG Measurement – How is LPG Measured?
The distinctive properties of LPG mean that it is measured in a number of ways.
LPG – Liquefied Petroleum Gas – measurement includes:
• Weight – kilograms or pounds
• Liquid Volume – litres or gallons
• Gaseous Volume – m3 of ft3
• Energy Content – megajoules (MJ) or BTU
• Vapour Pressure – kilopascals (kPa), Bar or PSI
• A pressure-temperature chart also shows temperature effects.
LPG Units of Measurement
When LPG is measured by weight, the units of measure are kilograms, pounds, tons or tonnes (metric tonnes).
LPG liquid volume is typically measured in litres or gallons.
When LPG is vapour (gas), the volumetric units of measure are cubic metres (m3) or cubic feet (ft3).
LPG energy content is expressed in Megajoules (MJ), Gigajoules (GJ) or Petajoules (PJ), with the unit of measure dependent on the quantities discussed.
BTUs and Therms are also units of measure for energy.
The units of measure for pressure are kilopascals (kPa), Bar or pounds per square inch (psi).
Measuring LPG by Weight
A common way of measuring and selling LPG is by weight, in kilograms.
There are different sizes of LPG gas bottles and the gas is sold based on the contents of a full bottle.
For example, LPG for homes is frequently delivered in 45kg gas bottles.
BBQ gas is typically supplied in 8.5kg gas bottles.
These numbers represent the weight of the gas inside the bottle, excluding the tare weight of the gas bottle.
Larger quantities of LPG are measured in tonnes (1,000kg).
In the US, the measurements would be in pounds and US tons (2,000 pounds).
For example, a typical BBQ gas bottle in the US contains 20 pounds of LPG.
Measuring LPG by Liquid Volume
LPG is a liquid when it is under pressure.
When delivered by tanker hose or LPG dispenser, for Autogas, the unit of measure is typically in litres.
As the specific gravity of LPG – propane – is about half that of water, there are roughly 2 litres of LPG per kilogram.
The exact weight to liquid conversion depends on the composition of the LPG, relative to the propane and/or butane content.
It is also temperature dependent.
The temperature of the LPG must be known to calculate measurement by volume.
LPG liquid meters have an automatic temperature correction process.
See the table below for propane conversion values.
Once again, the US measurement is different, this time being in US gallons.
One gallon equals about 3.8L.
Measuring LPG by Gaseous or Vapour Volume
When it is not under pressure, LPG exists in its gaseous or vapour form.
As with other gases, the volume of LPG is measured in cubic meters (m3).
In the US, they use cubic foot (ft3) as the unit of measure. One ft3 is equal to about 1/35 m3.
Measuring LPG by Energy Content
The energy value or calorific value of LPG is frequently expressed in Megajoules (MJ).
Larger quantities are expressed in Gigajoules (GJ).
Petajoules (PJ) are used when huge quantities are being discussed, as in national consumption or production.
In the US they use BTU. BTU, or British Thermal Unit, is another measure of energy based on the Imperial System.
One BTU is the energy required to raise the temperature of 1 pound of liquid water by 1°F, at sea level.
1MJ equals 948BTU, so to get an equivalent number of Megajoules, just divide the BTU rating by 948.
Measuring LPG by Vapour Pressure:
LPG Pressure Temperature Chart
As previously mentioned, when LPG is stored in a gas bottle, it is under pressure.
The term “pressure” refers to the average force per unit of area that the gas exerts on the inside walls of the gas bottle.
The pressure is measured in kilopascals (kPa).
“Bar” is yet another unit of measure for pressure.
1 Bar = 100 kPa, so it is metric based but not an SI unit of measure.
LPG vapour pressure can vary greatly based on temperature.
(LPG Pressure Temperature Chart shown)
The level of fill in the gas bottle comes into play when the LPG is being used, as it affects the rate of vapourisation.
As LPG is a liquefied gas, the vapour pressure inside the cylinder will remain the same from full until the last of the liquid LPG is vapourised.
Then the pressure will fall quickly as the last of the LPG vapour is used, as well.
The use of pressure as a measurement in LPG use is very limited.
A steady pressure only shows that liquid LPG is inside the cylinder, but not how much liquid is left.
In the US, vapour pressure is measured in pounds per square inch (psi).
Psi is defined as 1 pound of force applied per square inch.
As a comparison, 1 psi = 6.89476 kPa.
Final Thoughts
How you measure LPG relates to the context in which you are interested.
It also depends upon the country in which you live.
Most small LPG cylinders are sold by weight (kg or pounds) and most large tanks are filled and invoiced using litres (or gallons).