This document discusses different units used to measure gas concentration and the importance of understanding how to convert between units. It explains that gas concentration is expressed in volume fraction, parts per million by volume (ppm), or mass per unit volume depending on whether measuring flammability, toxicity, or oxygen levels. The key units discussed are parts per million by volume (ppm), volume fraction, mole fraction, and mass per cubic meter. Tables provide examples of conversion factors and exposure limits for several gases. Being able to convert between units is important for comparing toxicity and flammability limits on a common scale.

1. October 21, 2015
Understanding Units of Gas Concentration
analyticexpert.com/2015/10/understanding-units-of-gas-concentration/
By Edward Naranjo, Marketing Director, Emerson
Gas concentration is one of the most important determinants of a substance’s hazard
potential. Flammability, toxicity, and oxygen deficiency are often determined by
concentration. For combustible gas detectors, gas concentration is expressed as a volume
fraction of combustible gas or vapor in air known as the lower explosive limit (LEL), while
for toxic gas detectors, the signal output is read as a percent by volume (% vol.), parts per
million by volume (ppm (vol.)), or mass per unit volume. Countries and jurisdictions use
different units of measurement to define maximum permissible combustible and toxic gas
concentrations in the workplace. The choice of the units to use depends on the chemical
and its abundance under ambient conditions. As a result, it is necessary to become familiar
with the units used and methods for converting between units of measurement.
Volume and Mole Fractions
Units of volume fraction and mole fractions are frequently used for gas concentration. The
most common value fraction is ppm (vol.), defined as the ratio between the volume of a
constituent V and the total volume V :
As an example, 10,000 ppm = 1% (v/v) or a volume
fraction of 0.01. The volume fraction of a constituent φ is
defined as the volume of constituent V divided by the
volume of all constituents of the mixture V :
Similarly, the mole fraction of constituentX is the moles of a target
substance n divided by the total number of moles in a mixturen :
The values of volume fraction and mole fraction are identical under
the ideal gas law:
The advantage of volume/volume or mol/mol units is that gas
concentrations reported in these units do not change over temperature and pressure. By
contrast, atmospheric concentrations like mass per unit volume (ex. mg/m ) decrease as
gas is expanded since the component’s mass remains constant as the volume increases.
Mass Concentration Units
Concentration units based on mass include mass fraction (ex. mass chemical per total
mass) and mass per unit volume. Like ppm (vol.), mass/mass concentrations are
commonly expressed as parts per million, where m is the mass of constituent i and m is
the total mass:
i total
i
i
total
i
total
3
i total
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2. Note: Where a gas concentration is expressed simply
as ppm, it is unclear whether a volume or mass basis
is intended.
In the atmosphere, it is common to express
concentrations of mass/volume air like milligrams per meter cubed (mg/m ). Thus, the
United States’ National Institute for Occupational Safety and Health (NIOSH) Pocket Guide
for Chemical Hazards reports worker exposure limits in ppm (vol.) and mg/m . To convert
from ppm (vol.) to mg/m , it is assumed the ideal gas law applies under standard
temperature and pressure where MW equals molecular weight:
Note that at standard conditions (p = 760 mmHg, T = 273°K), one mole of any pure gas
occupies a volume of 22.4 L.
Another useful formula is one that converts from units of ppm (vol.) to ppm (m) at 760
mmHg and 25°C:
For convenience, NIOSH recommended exposure limits (REL’s) for several toxic gases are
shown in Table 1 below.
3
3
3
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3. Table 2 shows the LEL’s of several combustible gases in ppm (vol.), ppm(m), and volume
fraction.
Last, conversion units for common industrial gases are illustrated in Table 3.
Situations Where Unit Conversions May Be Useful
It is tempting to think that converting units of measurement is hardly necessary. After all, in
most countries, combustible gas concentrations are measured either in volume fractions or
its derivatives, while toxic gas exposure limits are established in ppm (vol.) or mass volume
units. Yet one cannot gauge a gas’ hazard potential without comparing attributes in a
common plane. Consider that several toxic gases like ammonia and hydrogen sulfide are
combustible. The graph in Figure 1 below showing gas concentration in ppm (vol.) gives a
sense of toxic and combustible limits in relationship to one another.
Similarly, most hydrocarbons are harmful long before they are at combustible
concentrations. As shown in Figure 2 below, the IDLH is often at approximately 10% LEL.
Based on those results, an analysis of hydrocarbon toxicity recommended that alarm levels
be set at 10% LEL to protect workers from hydrocarbon narcosis (Gardner 2012).
References
ISO 10156, Determination of Fire Potential and Oxidizing Ability for the Selection of
Cylinder Valve Outlets. 2010. Geneva, Switzerland: ISO.
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4. Gardner, R. 2012. Use of Reciprocal Calculation Procedures for Setting Workplace
Emergency Action Levels for Hydrocarbon Mixtures and their Relationship to Lower
Explosive Limits. Ann. Occup. Hyg. 56 (3): 326–339.
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