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Converting Mass Flow to Volume Flow

******** See this application note for some background info on mass vs. volume flow that will help understand the info in the article also: **********

To convert a mass flow to volume flow we must first determine the density of the gas. This is done by knowing the gas properties and the density of the gas at the actual temperature and pressure of the volume. This temperature and pressure is ideally obtained by measuring it at the DUT. The Unit of Measure Converter can be used to obtain the density as follows:

In the above example we are flowing Air and measured our DUT pressure to be 14.323 PSIa (absolute pressure is required, cannot use gauge pressure) and DUT temperature to be 74.543F at our test point. This produces a density of 1.15672 kg/m3. 

We now take our mass flow rate from the molBox and convert it to units of kg's per unit of time. We want to use kg's so that our units match up to the units used for density previously to make the math simpler. Similar, we usually select a unit of time to match the unit we are ultimately trying to achieve in the end, but this is not required. In this example we are ultimately trying to get the volume flow in units of actual lm (actual liters per minute).

Lets say our mass flow rate on the molBox is 10.193 ulm and that we entered 70F as the reference temperature when selecting this unit (the equivalent to slm@70.F in our software products). Based on the above recommendations, this converts as follows:

We can now take the .011828 kg/m and divide it by the density of 1.15672 kg/m3 which equals .010225 m3/m. We can now convert this to lm (alm ... actual liters per minute) using the measured temperature and pressure from above to produce the conversion below:



The end result is our mass flow rate of 10.193 slm@70F converted to a DUT volumetric flow rate of 10.54821 lm at the measured pressure of 14.323 PSI and 74.543 F.


In some testing it might also be necessary to then convert the volumetric lm to a standardized pressure and temperature. Lets say are standardizing to 14.7 PSI and 70F. Here is this additional conversion:

 *** It is interesting to note that the above conversion is very close to to the same as selecting a unit of slm@70F to convert to, but the slight difference in pressure (14.7 PSI vs 101.325 kPa) causes the small difference.

If want to convert volumetric flow to mass flow it would be the same process but instead of dividing by the density you would just multiply by the density.  

Note, the molBox and our software has measures to also convert to volumetric rates/units and this is just an example to show how this can be done manually or to better understand the conversion. 

Keep learning

Four Steps to More Effective Gas Flow Calibration - App Note

Gas Flow Calibration Fundamentals - Webinar

Gas Flow calibration using molbloc/molbox - Paid Course

See related products

molbox1+ Flow Terminal

molbox RFM Reference Flow Monitor

molbox RFM Gas Flow Calibrator Kits

COMPASS® for molbox™ software

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