Alicat PCU Portable Calibration Unit - Manual (Page 19)

19
Other Gases: PCU Flow Meters can easily be used to measure the flow rate of gases other than those
listed as long as “non-corrosive” gas compatibility is observed. For example, a flow meter that has been
set for air can be used to measure the flow of argon.
The conversion factor needed for measuring the flow of different gases is linear and is simply determined
by the ratio of the absolute viscosity of the gases. This factor can be calculated as follows:
Q
og
= Q
1
[η
1
/ η
og
]
Where: Q
1
= Flow rate indicated by the flow meter
η
1
= Viscosity of the calibrated gas at the measured temp.
Q
og
= Flow rate of the alternate gas
η
og
= Viscosity of the alternate gas at the measured temp.
Say we have a meter set for air and we want to flow argon through it. With argon flowing through the
meter, the display reads 110 SLPM. For ease of calculation, let us say the gas temperature is 25°C.
What is the actual flow of argon?
Q
og
= Actual Argon Flow Rate
Q
1
= Flow rate indicated by meter (110 SLPM)
η
1
= Viscosity of gas selected or calibrated for by the meter at the
measured temp.
η
og
= Viscosity of gas flowing through the meter at the measured temp.
At 25°C, the absolute viscosity of Air (η
1
) is 184.918 micropoise.
At 25°C, the absolute viscosity of Argon (η
og
) is 225.593 micropoise.
Q
og
= Q1 (η 1 / η
og
)
Q
og
= 110 SLPM (184.918 / 225.593)
Q
og
= 90.17 SLPM
So, the actual flow of Argon through the meter is 90.17 SLPM. As you can see, because the Argon gas
is more viscous than the Air the meter is set for, the meter indicates a higher flow than the actual flow.
A good rule of thumb is: “At a given flow rate, the higher the viscosity, the higher the indicated flow.”
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 the 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 meter 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.