Flows referenced to STP (SLPM, SCFH) may have to be converted to NTP. Flows referenced to NTP (most rotometer readings) may have to be converted to STP.
The Ideal gas law is used to make the conversion:
If PV1 = nRT1 describes the fixed mass of gas at one condition, and PV2 = nRT2 describes the same mass of gas at the other condition then:
P: pressure is the same (1 ATM)
N: Number of molecules is the same
R: Gas Constant
T: defined by the reference conditions (absolute temp)
V1/T1 = V2/T2
V2 = V1*(T2/ T1)
For the case of a sample at NTP, the volume would decrease by a factor of (273.15)/(20+273.15),
NLPM * 0.93 = SLPM
Mg/NM3 / 0.93 = Mg/SM3
The same approach may be used to convert to one of the reference conditions from any other condition (from room temperature, for example).