Mass flow measures just what it says, the mass or weight of the gas flowing through the instrument. Mass flow (or weight per unit time) units are given in pounds per hour (lb/hour), kilograms per sec (kg/sec) etc. When your specifications state units of flow to be in mass units, there is no reason to reference a temperature or pressure. Mass does not change based on temperature or pressure.
However, if you need to see your results of gas flow in volumetric units, like liters per minute, cubic feet per hour, etc. you must consider the fact that volume DOES change with temperature and pressure. To do this, the density (grams/liter) of the gas must be known and density changes with temperature and pressure.
When you heat a gas, the molecules have more energy and they move around faster, so when they bounce off each other, they become more spread out, therefore the volume is different for the same number of molecules.
Think about this: The density of Air at 0°C is 1.29 g/liter. The density of Air at 25°C is 1.19 g/liter. The difference is 0.1 g/liter. If you are measuring flows of 100 liters per minute, and you don’t use the correct density factor then you will have an error of 10 g/minute.
Volume also changes with pressure. Think about a helium balloon with a volume of 1 liter. If you could scuba dive with this balloon and the pressure on it increases. What do you think happens to the weight of the helium? It stays the same. What would happen to the volume (1 liter)? It would shrink.
Why is the word “standard” included with the volume terms liters and cubic feet in mass flow applications?
A mass flow meter measures mass …and we know we can convert to volume. To use density we must pick one (or standard) temperature and pressure to use in our calculation. When this calculation is done, the units are called standard liters per minute (SLM) or standard cubic feet per minute (SCFM), for instance, because they are referenced to a standard temperature and pressure when the volume is calculated.
Using the example at the bottom of this page, we can see a standard liter can be defined differently. The first balloon contains 0.179 grams of Helium at 0°C and 760 Torr (density of 0.179 grams/liter). Heat up that balloon to room temperature and the volume increases, but the mass has not changed. The volume is not 1 liter anymore, it is 1.08 liters.
So to define a standard liter of Helium at 25°C we must extract only one liter from the second balloon and that liter weighs only 0.164 grams.
If a mass flow meter is set up for STP at 0° C and 760 Torr, when it measures 0.179 grams of Helium, it will give you results of 1 SLM. If a second meter is set up for STP at 25° C and 760 Torr, when it measures 0.164 grams, it will give results of 1 SLM.