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Quantifiable assessment of gas is required for collecting, purifying, handling, transporting, marketing and using in different technologies, across various industries.
Metrics of Gas Measurements
The metrics of gas measurements depends on the information needed in applications and the convenience of transactions. The energy of a gas, the vapor pressure of a gas, its volume (or its liquid volume), or its mass may be measured. Factors such as temperature, pressure, and purity affect these properties of the gas.
It is intriguing how methods have evolved to measure up an invisible (usually) and a gaseous state of matter. The advanced science of gas measurement has state-of-the-art technology in this era.
Natural gas (methane) is measured in volume (cubic meters or cubic feet) at the resource well. One cubic foot of natural gas is the volume of gas contained in 1 cubic foot, at standard temperature and pressure. Generally, the gas production measured from the reserves ranges in thousands or millions of cubic feet. Thus, a trillion cubic feet of gas fill up a 2-mile sided-cube!
However, the value of natural gas is based on the energy content, measured in joules. About 4.2 joules is the heat needed to raise the temperature of one gram of water by one degree Celsius at standard pressure 101.325 kPa and standard temperature (15 degrees Celsius). British thermal unit or Btu is the imperial cousin unit of joules. 1 btu is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. The Btu content of gas is usually expressed as MMBtus (1 million Btus) or mcf (1000 cubic feet) or a therm (100 cubic feet)
“One cubic foot of methane gas at standard temperature and pressure (60 degrees Fahrenheit and 14.73 pounds per square inch) contains exactly 1,000 Btus.”
Impact of Chemicals, Gases, or Water Vapor on Energy Content of Natural Gas
The energy content of natural gas varies from different reserves, due to the presence of other chemicals, gases, or even, water vapor. When ethane, propane, butane, and other heavier hydrocarbons are present in the natural gas, the energy content increases. In the presence of other gases such as carbon dioxide or hydrogen sulfide, the energy content decreases.
Once the heating value of gas is known, its volume can be easily converted to Btus or joules. Companies also report the gas measurement in terms of “barrels of oil equivalent” or “boe,” which is equal to the heating value of 5.8 mcf of methane.
Natural gas when sold as a transportation fuel, compressed natural gas or CNG, it is measured in gasoline gallon equivalent or gge. According to NIST Handbook 44 Appendix D, “gasoline gallon equivalent (gge) means 5.660 pounds of natural gas.”
Exploration companies report the production of natural gas in mcf. However, without knowing the energy of the gas, in terms of Btu, the pricing of the gas cannot happen.
Overview of Liquefied Petroleum Gas
When LPG (Liquefied Petroleum Gas) is used for household purposes, it is measured in mass, in kilograms. LPG is weighed in gas cylinders/bottles, and the cost is based on its weight. During transportation, LPG is put under pressure and measured in large quantities, in tons (i.e. 1000 kg).
For small samples, gas is measured in volumes (liters), considering a stable temperature.
Thus, depending on the operation, equipment, its efficiency, commodity, and the consumer product, the method and the units of measuring gas are defined and it varies.
A frequently used list of unit conversions for gas measurements is given in this reference 1.
Sources and Further Reading
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