# Gas Laws & their influence on Natural Gas Flow Measurement

For any natural gas flow measurement instrument to provide accurate gas flow, gas laws must be considered, including Boyle’s Law, Charles’ Law, Avogadro’s Law, Gay-Lussac’s Law, Combined Gas Law, and the Ideal Gas Law. By doing so, we can see how temperature, pressure, and volume impact gas instrumentation.

## Boyle’s law The amount of force pressing on a particular area is known as pressure and is expressed in a variety of units, such as psi (pounds/inches2), atmospheres, bars, mm of Hg, inches of water, and pascal (Pa).

P Î± 1/V or P1V1 = P2V2

Boyle’s Law suggests that when the temperature of the gas is constant, the relationship between the pressure (P) of a given mass is inversely proportional to the volume (V). In other words, if the pressure doubles, the volume is reduced by half. Alternately, if the pressure is cut in half, the volume of gas is doubled.

## Charles’s Law

V Î± T

V1/T1 = V2/T2 or V2/V1 =T2/T1 or V1T2 = V2T1.

Charles’s Law is also known as the law of volumes and designates that gases expand when heated, and the volume of gas is directly proportional to the absolute temperature (Kelvin scale) if the pressure of a gas remains constant. If the absolute temperature (T) of a gas doubles, the volume is doubled, and vice versa.

## Gay-Lussac’s Law

Pi/Ti = Pf/Tf

Gay Lussac’s Law (ideal gas law) states that the pressure of a gas is proportional to the absolute temperature. In other words, if we double the pressure of a gas, the absolute temperature will double, and vice versa.

Pi = initial pressure
Ti = initial absolute temperature
Pf = final pressure
Tf = final absolute temperature

V Î± n

V1/n1 = V2/n2

Avogadro’s Law suggests that for a given mass of an ideal gas, the volume and amount of gas (n – moles) is proportional if the temperature and the pressure are constant. As the moles (n) increase, the volume also increases proportionately.

## Combined Gas Law

pV/T = k

When we combine Boyle’s Law, Charles’s Law and the Gay-Lussac’s law we result in a single expression known as the Combined Gas Law equation which states that the ratio between the pressure-volume and the temperature of a system remains constant (k) and is expressed as units of energy divided by temperature.

## Ideal Gas Law

Based on kinetic principles, the pressure of a gas is created by gas molecules colliding with the walls containing it. The more molecules (per unit volume) generate more collisions, which raise the pressure. The kinetic energy of gas molecules is generally proportional to the temperature (absolute). While the molecules are at rest, at absolute zero, at high temperatures, the molecules increase in speed, and pressure rises. When combining these concepts, the Ideal Gas equation is created.

PV = nRT

The Ideal Gas Law is similar to the Combined Gas Law. It is an equation of the state of an ideal gas (R) and conforms to Boyle’s, Charles’s, and Avogadro’s Laws. This is hypothetical and estimates the behavior of gases based on specific conditions. It suggests that the state and amount (n) of a gas is determined by the volume (V), pressure (P) and temperature (T)

By reviewing these gas laws, we demonstrate how simple variables, such as temperature and pressure, influence gas measurement, and impact gas instrumentation. This is why some metering applications require temperature and pressure compensation to ensure accurate measurement and minimize waste. 