Volumetric Flow Versus Mass Flow Technology

When systems integrators and engineers require the measurement of gas flow rate in scientific research or in an industrial process, they have the choice of several types of technologies. The challenge that is presented before them is choosing the right gas flow meter. Apart from technology, engineers must also look into many other parameters: cost of ownership, price for value, application flexibility, size of footprint, accuracy with changing application conditions, calibration method and repeatability.

The question is which flow metering technology should be used to get the highest accuracy, repeatability and lowest cost of ownership. For this question to be answered, engineers must first come to a conclusion whether they want a flow meter technology that can deliver gas mass flow rate or simply gas volume flow rate measurement.

Put in simpler terms, the mass flow rate is the WEIGHT of the sample, while the volumetric flow rate is the SIZE of the sample.

While volumetric flow and mass flow look similar on the outside, Sierra Instruments’ Chief Engineer, John Smitherman demonstrates the extensive differences between volumetric flow and mass flow metering in the latest Minute Flow Tip Video.

Minute Flow Tip: Mass Flow Versus Volumetric Flow Technology

What is Gas Mass Flow Rate?

Gas mass flow rate is the actual mass of a gas which moves through a measurement instrument per unit of time. Its units are usually in standard liters per minute (slpm) or standard cubic centimeters per minute (sccm) in SI units. The most common English units include standard cubic feet per minute (scfm). In both these cases, “Standard” refers to standard reference conditions for pressure and temperature. This allows more usual industrial measurement terms to easily be converted to mass flow.

What is Gas Volumetric Flow Rate?

On the other hand, gas volume flow rate or volumetric flow rate is the volume of fluid which passes per unit time. Volumetric flow rate has several units of measure. Some of the common examples are cubic centimeters per minute (ccm) and cubic feet per minute (cfm).

Inherent Advantages of Mass Flow Versus Volumetric Flow

Mass flow technology has an inherent advantage over volumetric flow technology by its very nature since it accounts for absolute measurements. Common process gases like Argon, Air, N2, CO2, and Oxygen follow the principles of the “Ideal Gas Law.” While not all gases are “ideal”, these common gases work on the principle that variations in the density are caused by the variation in pressure and temperature.

Volumetric flow is less reliable than mass flow to account for absolute measurements because variations in temperature and pressure (T&P) influence the gas density, reducing the accuracy. Volumetric flow measurement needs T&P compensation to determine true gas mass flow rate.

Insertion-type thermal mass flow meters and direct thermal mass flow controllers also offer engineers direct gas mass flow measurement without flow computers or pressure or temperature sensors. Coriolis flow meters are the only other technology that measures mass flow rate directly with no secondary flow computers.

Sierra’s thermal mass flow controllers are true digital MFCs based on capillary thermal mass flow technology which directly measures molecular flow (counts gas molecules). Mass flow provides greater application flexibility because the mass flow rate is unchanged by upstream gas temperature and pressure fluctuations like other differential pressure devices. There is no need to buy extra temperature and pressure devices to “infer” the mass flow. In a single compact mass flow meter, SmartTrak provides direct gas mass flow control which improves repeatability, accuracy, adds application flexibility, and reduces cost of ownership.

In addition to these benefits, SmartTrak’s digital mass flow meters and controllers have the following advantages:

  • Available PID valve tuning for full-range stability
  • Technology to knock out upstream pressure fluctuations
  • Low flow size for any range between 0 to 50 slpm
  • 10-point NIST calibration based on primary standard
  • Up to +/- 0.5% of full scale accuracy
  • Up to 1000 slpm and higher available flow rates

 

Sierra

This information has been sourced, reviewed and adapted from materials provided by Sierra Instruments.

For more information on this source, please visit Sierra Instruments.

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