How do I convert mg/m3 to ppm at different temperatures and pressures?
The number 24.45 in the equations above is the volume (liters) of a mole (gram molecular weight) of a gas or vapour when the pressure is at 1 atmosphere (760 torr or 760 mm Hg) and at 25°C.
To convert mg/m3 to ppm at other temperatures and pressures, one must calculate the volume of 1 gram molecular weight of an airborne contaminant (e.g. 92.13 grams of toluene) by using the formula:
V = (RT/ P)
where R is the ideal gas constant; T, the temperature in kelvins (273.16 + T°C); and P, the pressure in mm Hg. This information can be substituted in the formulas for converting between mg/m3 and ppm.
| TLV in mg/m3 | = | (P/RT) x MW x (TLV in ppm) |
| = | P x MW x (TLV in ppm)
62.4 x (273.2 + T°C) |
and
| TLV in ppm | = | 62.4 x (273.2 + TX) x (TLV in mg/m3)
P x MW |
where the value of R is 62.4 when the temperature (T) is in kelvins, K (=273.16 + T°C), the pressure is expressed in units of mm Hg and the volume is in liters. There are different values for the gas constant R if the temperature is expressed degrees Fahrenheit (°F) or if other units of pressure (e.g., atmospheres, kilopascals) are used.
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