30 CFR § 90.206
Approved sampling devices; equivalent concentrations.
The concentration of respirable dust shall be determined by dividing the
weight of dust in milligrams collected on the filter of an approved sampling
device by the volume of air in cubic meters passing through the filter and
then converting that concentration to an equivalent concentration as measured
with an MRE instrument. To convert a concentration of respirable dust as
measured with an approved sampling device to an equivalent concentration of
respirable dust as measured with an MRE instrument, the concentration of
respirable dust measured with the approved sampling device shall be
multiplied by a constant factor prescribed by the Secretary for the approved
sampling device used, and the product shall be the equivalent concentration
as measured with an MRE instrument.