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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: https://us.flukecal.com/literature/articles-and-education/flow-calibration/application-notes/mass-flow-versus-volume-flow-ga **********

 

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 using the actual temperature and pressure. This temperature and pressure is obtained by measuring the gas directly within the volume/device/DUT that is currently being tested. It is not the temperature of the lab, standardized conditions, or similar and can change at different points in the system and/or at each different flow rate that is set. We already have the gas properties built in so once have the measured temperature and pressure values for the current state of the volume/device/DUT we can use our Unit of Measure Converter tool to calculate the gas density as follows:

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In the above example we are flowing N2 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 current test point. This produces a density of 1.12133 kg/m³. Note that humidity is N/A as it is used only for Air since it is not a "dry gas" but would be another component to measure if flowing Air.

 

To help visualize the calculations, we will take the mass flow rate from the molBox and convert it to units of kg's (kilograms) per unit of time. We like to use kg's so that our units match up to the unit that density is calculated in (kg/m³) to keep the math simple. It is also a pure mass unit (kilogram based) rather than a volumetric unit (ex. liters based) standardized to a specific pressure/temperature to produce a mass value. We also usually select the unit of time to match the unit of time we are ultimately trying to achieve in the end. In this example, we will convert from a mass flow of slm@70F (standard liters per minute) to a volumetric flow of alm (actual liters per minute). We will thus initially convert our mass flow to kg/m (kilograms per minute).

 

Lets say our current mass flow rate on the molBox is reading 10.193 ulm (user defined liters per minute) and that we entered 70F when prompted on the molBox for the user defined (also known as the standardized or reference temperature) when selecting this unit. This is the equivalent of selecting slm@70F in our software products or similar. Based on the above recommended unit, we can convert to kg/m as follows:

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Note that the Gas Pressure and Gas Temperature values in the above conversion are not manually entered as both these units are mass flow and not volume flow. Only when going to a volume flow unit will these values need to be entered. You will see that the unit slm@70F shows it has been standardized to 101.325kPa (14.696 PSIa) and 70F since it is a volumetric based unit being used for mass flow. The unit of kg/m is a pure mass flow unit so it does not have any pressure or temperature associated with it.

 

We can now take the .011828 kg/m (kilograms per minute) mass flow and divide it by the density of 1.12133 kg/m³ (kilograms per cubic meter) which equals .010548 am³/m (actual cubic meters per minute) of volume flow. 

                      0.011828 kg/min ÷ 1.12133 kg/m³ = 0.010548 am³/min

 

The last step is to convert the am3/m volume flow to our final desired volumetric unit of alm (actual liters per minute). Since both units are now volumetric units we need to enter the same pressure and temperature for both units as was measured initially from the DUT for a proper conversion:

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The end result is our mass flow rate of 10.193 slm@70F converted to a DUT volumetric flow rate of 10.548 alm at the measured conditions of 14.323 PSIa and 74.543 F.

 

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

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On a related topic, it is interesting to note that the above conversion of 10.18979 alm at 14.7 PSIa and 70F is very close to the same as selecting the unit of slm@70F to convert to, but it calculates as 10.19260. A slight difference in pressure is what causes this as it is very common to round up the true standard atmospheric pressure of 101.325kPa which equals 14.696 PSI to 14.7 PSI. It is no coincidence in this full circle example that we are now back to a mass flow value essentially the same as our original molBox mass flow of 10.193 slm@70F !!!!

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

 

Note, that the molBox (using the unit vlm) and our software has inputs for the pressure and temperature values to automatically calculate volumetric flows and the above is just an in depth example to show how this can be done manually or to hopefully 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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