What is mass flow?
Most Alicat devices measure and control the mass flow rate of a gas. As gas flows through an Alicat, it passes through a flow channel with temperature and pressure sensors. These sensors measure the instantaneous temperature and pressure of the gas stream, and use that information in combination with known gas properties to calculate a mass flow rate.
This mass flow rate is really a volumetric flow rate standardized to a specific set of temperature and pressure conditions.
Calculating mass flow using instantaneous temperature and pressure readings
Imagine flowing argon through an Alicat mass flow instrument. The sensors within the flow stream read 23°C and 20 PSIA, and these values are shown on the screen of the device. These values are not directly used to calculate the mass flow rate, however.
Each Alicat device is loaded with gas tables containing values from 3-dimensional gas properties data. These tables chart NIST-traceable gas compressibility and viscosity data against pressure and temperature across the entire usable range of the mass flow device. The mass flow rate shown on screen is the volumetric flow rate of the gas if it was flowing at standard temperature and pressure (STP) conditions.
The simple steps are as follows:
- The device uses the actual temperature and pressure of the gas to calculate the instantaneous volumetric flow rate.
- The device uses an algorithm to calculate what the flow rate would be if the gas were at 25°C and 1 atm.
- The device outputs a standardized volumetric flow rate, or mass flow rate.
Whether flowing at 23°C and 20 PSIA, or -10°C and 56 PSIA, or 42°C and 14 PSIA… the mass flow rate shown on screen will always be the same value. All measurements are converted to a standardized volumetric flow rate using STP conditions. The default STP conditions are 25°C and 1 atm (14.6959 PSIA), however this is adjustable.
Going back to the example above, a standardized volumetric flow rate will be calculated for argon at 23°C and 20 PSIA. However, at 25°C and 1 atm, argon has the following properties:
- Absolute viscosity: 226.23990
- Density: 1.63387
- Compressibility: 0.0003656
No matter what temperature or pressure you are flowing the argon, the value shown on screen will always be as if the gas was flowing at 25°C and 1 atm.
Units of mass flow
Because mass flow rate is a standardized volumetric flow rate, the units are typically expressed as such. Examples include SLPM (standard liters per minute), SCCM (standard cubic centimeters per minute), and SCFH (standard cubic feet per hour).
These are volumes per time, not units of mass. However, a mass flow rate can easily be converted to a true mass flow rate using the following equation: True mass flow = (mass flow rate)(gas density at STP or NTP). All you need is the mass flow rate and the gas density.
Some mass flow instruments output a true mass flow measurement directly, such as Coriolis instruments.
When to use mass flow
A mass flow rate is standardized such that you can confidently flow exact numbers of particles through your system without having to worry about temperature or pressure fluctuations. Mass flow devices are commonly used for applications requiring high accuracy, precision measurement and control of specific amounts of gas.
Imagine you need to flow argon through a mass flow controller at exactly 5 SLPM. When you begin flowing, the temperature and pressure of the gas are 23°C and 20 PSIA. However, someone turns on a furnace nearby and the temperature climbs up to 27°C. The device will always correct the values to STP conditions, 25°C and 1 atm, and the exact right amount of particles will be flowed.
Does mass flow rate change with density?
Mass flow rate depends on the density of the gas that is being flowed. For laminar differential pressure devices, it is important to know the exact composition of the gas being flowed so that precise density corrections can be made.
Fortunately, Alicat differential pressure meters and controllers are pre-loaded with 98+ gases containing gas density information. A bonus of calculating and reporting at standard conditions is that it is fairly easy to apply correction factors to your flow data if you accidentally flow with the wrong gas selected.
True mass flow measurements also depend on density. Coriolis mass flow meters and controllers, however, do not use known gas properties to calculate flow rates. They are able to measure fluid densities themselves and accurately output true mass flow rates, even when flowing process fluids of unknown composition.
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