Flow Tracker Series 1000/2000
Reference Manual
17
Mass flow rate
The mass flow rate is the volumetric flow rate corrected to a
standard temperature and pressure (14.695 PSIA and 25 °C).
This parameter is located in the lower middle portion of the
display over the dynamic label “Mass”. The engineering unit
associated with the mass flow rate is called a standard milliliter
per minute (SmL/min). The flowmeter uses the measured
temperature and the measured absolute pressure to calculate
what the flow rate would be if the gas pressure was at
1 atmosphere and the gas temperature was 25 °C. This allows a
solid reference point for comparing one flow to another.
Volume flow vs. mass flow
At room temperature and low
pressures the volumetric and mass flow rate will be nearly
identical, however, these rates can vary drastically with changes
in temperature and/or pressure because temperature and
pressure of the gas directly affects the volume. For example,
assume a volumetric flow reading was used to fill balloons with
250 mL of helium, but the incoming line ran near a furnace that
cycled on and off, intermittently heating the incoming helium.
Because the volumetric flowmeter simply measures the volume
of gas flow, all of the balloons would initially be the same size.
However, if all the balloons are placed in a room and allowed to
come to an equilibrium temperature, they would generally all
come out to be different sizes. If, on the other hand, a mass flow
reading were used to fill the balloons with 250 standard mL of
helium, the resulting balloons would initially be different sizes,
but when allowed to come to an equilibrium temperature, they
would all turn out to be the same size.
This parameter is called “corrected mass flow” because the
resulting reading has been compensated for temperature and
pressure and can therefore be tied to the mass of the gas.
Without knowing the temperature and pressure of the gas (and
thus the density), the mass of the gas cannot be determined.
Volumetric and mass flow conversion
In order to convert volume
to mass, the density of the gas must be known. The relationship
between volume and mass is as follows:
Mass = Volume x Density
