A true concentration is the amount of gas present per unit volume. For this reason, many gas concentrations are given in units of mg/m3. While this unit is correct, and is widely used in government regulations and industrial hygiene, it is more common to use relative units such as parts per million (ppm) or parts per billion (ppb). Relative units such as ppm and percent have the advantage that they do not change with pressure.
Oxygen in the air is about 20.9 % by volume (for any volume of air, 20.9 % of it is oxygen). Room air is 20.9% and so is compressed air in a scuba diver’s tank at ~ 2,000 psi. Similarly if I want to calibrate a gas monitor with 10 ppm of for example carbon monoxide, I can purchase a cylinder of 10 ppm carbon monoxide at 2,000 psi and deliver it to my gas monitor at close to atmospheric pressure and it is still 10 ppm. If one were working in mg/m3, the concentration at 2000 psi would be about 135 times as high as at ambient pressure because for every unit of volume there is 135 times more gas present. Since these relative concentrations are by volume of gas, they are sometimes written as ppmv, ppbv or % v/v in order to distinguish other relative measures such as ppm in liquids which are normally by weight.
One difficulty that arises is that many toxic gas sensors respond to the concentration of gas rather than the relative concentration. Thus if a gas sensor is calibrated at sea level and then shipped to Denver, (5,280 ft), the pressure is about 80% of sea level and so the absolute concentration will of any gas component will be lower than at sea level for the same ppm or % volume. Users of ChemDAQ products will be pleased to know that altitude is not an issue for their equipment since there is an automatic compensation parameter set in the monitor at the time of installation to correct for differences in altitude between ChemDAQ (where the sensor modules are calibrated) and the end user. LEL sensors also respond to the concentration and depending on how the sensor is constructed, some oxygen sensors respond to the absolute concentration, and others to the relative concentration.
Most of us however, happily live our lives at relatively constant atmospheric pressure and so the two units are readily interconvertible (see free ChemDAQ ppm to mg/m3 converter on the ChemDAQ website [bottom of page]) and we can use relative units without problem.
Another reason for using relative units occurs with flammable gases where percent LEL is widely used. Most flammable gases have a concentration range over which mixtures with air will burn. If there is too little fuel then there is insufficient heat produce for the flame to propagate and similarly if there is too little oxygen, again there is too little heat for the flame to propagate. A typical gas such as propane has flammability/explosive limits in air of 2.2 to 9.5 % by volume. [Matheson Gas Data Book, 6th Ed.], so any propane/air mixture in this range is flammable and so is potentially explosive. The lower limit of flammability is called the Lower Explosive Limit (LEL) and the other limit is the Upper Explosive Limit (UEL). The LEL and UEL values vary from gas to gas.
If one working in an environment where there is a risk of an explosive gas mixture forming, then no-one really cares if the concentration is 2.1% volume, they want to know whether the atmosphere is explosive or not and whether they should get out. Flammable gas monitors for workplace safety are therefore calibrated in % LEL which provides an immediate measure of the risk of forming an explosive atmosphere. The gas monitors typically have a range of 0 to 100% LEL. If the concentration reaches 100% LEL, then there is a potentially explosive atmosphere and so the alarms are set lower, typically 10% and 20% LEL.
To convert from % LEL to ppm, it is necessary to know the Lower Explosive Limit. Using ethylene oxide (EtO) as an example, the LEL is 3% by volume, which is equal to 30,000 ppm. It is common therefore in facilities that use large amounts of EtO to have two different types of monitor. One set of monitors measures EtO at parts per million concentrations to warn about potentially toxic exposures (OSHA Permissible exposure limit for EtO is 1 ppm calculated as an 8 hr Time weighted average) the other set of monitors is to detect potentially flammable gas mixtures that threaten not only people but also the building. Hopefully before the LEL monitors go into alarm the ppm levels alarms will have warned everyone to clear the air, but the LEL monitors are used to increase ventilation or turn on the sprinkler system.
There are even more units of gas concentration out there which are less common, but if anyone wants more explanation of these, please leave a comment.