Vapor Mass Flow Meter
Vortex meters are often used for vapor flows. The formula m= (MW*P*V)/(RT) allows for the direct calculation of mass of a gas stream with constant composition from volume flow, pressure and temperature measurements.
m = mass
MW = Molecular Weight of gas
n= number of moles of gas
P = Pressure
T = Temperature
r = “universal” gas constant, which is a function of the units for P, V, and T.
Until recently calculating gas mass flow required multiple sensors however there are now two manufacturers (Krohne http://www.krohne.com/OPTISWIRL_4070C_Vortex_Durchflussmessgeraet_en.11250.0.html and Honeywell http://hpsweb.honeywell.com/Cultures/en-US/Products/Instrumentation/flow/VersaFlowVortex/default.htm ) who have integrated pressure and temperature compensated vortex meters so you can calculate mass flow for vapours/gases with a single meter. Several other manufacturers also have this calculation capability but require an external pressure signal to be connected to the vortex meter.
The preferred way to measure steam flow is in kg/hr and fortunately, because the composition of steam does not change, the calculation of steam flow with a temperature and pressure compensated vortex flow meter is relatively straightforward.
I cannot understand why these companies do not aggressively market this unique ability to measure vapor mass flow but hopefully this post at least raise the awareness for you should you have need to make this measurement.