Industrial Gas Flow Measurement: Mass Flow or Volumetric Flow?
Simply put, if you only need reasonable (~5%) accuracy and a visual indicator, there are many relatively inexpensive volumetric flow measurement instruments, such as those offered by Kobold USA (contact us for help). However, because gas volume is significantly effected by temperature and pressure, if your application calls for higher accuracy you should be looking at an instrument that measures mass flow.
Let's get into some more detail:
Temperature - When a gas is heated, the molecules have more energy and more velocity, therefore they crash into each other and spread further apart from each other, thereby increasing in volume.
Example: Air density @ 0°C is 1.29g/liter
Air density @ 25°C is 1.19 g/liter
Although the difference is only 0.1 g/liter, at 100 lpm the variance is 10 g/min.
Pressure - As pressure increases or decreases the volume of gas changes conversely. Think of a balloon filled with 1 liter of He. As you submerge the balloon under water, increasing the external pressure exerted on the balloon, its internal volume will shrink. Conversely, if you allow the balloon to float away high into the atmosphere, reducing the external pressure, the internal volume will expand.
The characteristic that will not be affected by temperature or pressure is the mass of the media. Although “Mass” Flow is measured as the weight of the gas flowing at a specific rate (pounds per hour, kilograms per sec), this can be converted to volumetric measurements (e.g. liters per minute (LPM) or cubic centimeters per minute (CCM). However, when doing this, a “Standard” temperature and pressure must be specified (e.g. 0°C and 760 Torr are frequently used). Now you have Standard flow units (e.g. SLPM, SCCM).
Most common Mass Flow Meters utilize two wire-wound thermistors that are part of a bridge circuit which is balanced at zero flow but outputs a differential signal as gas is flowing. This temperature difference is dependent on the mass flow of the gas. The result is much higher accuracy, improved repeatability and better linearity.
This post was authored by Jim White of IES Technical Sales. Jim can be reached at JimW@iestechsales.com, or 978-777-2900. IES offers expert assistance on mass flow measurement and control applications in the northeast region of the U.S.
IES Technical Sales is a value added technical sales, distribution, and solutions provider serving the high technology vacuum, plasma/thin film, temperature, fluid handling and metrology markets.