Performing High Accuracy Mass Flow Measurements on Low Mass Flow Instruments
Robert Collins, InnoCal® Manager,
Measurement and control of the low mass flow of gases (<1 sccm to 30 slm) is critical in many processes where yield and product quality are dependent on the accurate introduction of small quantities of gases at certain points in the production process. The instruments used to measure and control mass flow must be verified and/or calibrated regularly to meet their performance potential and to conform with formal quality assurance requirements. A typical thermal mass flow controller (MFC) has an accuracy specification of ±1% of full scale so, following the conventional 4:1 guideline, a standard of at least ±0.25% is needed to calibrate it.
Traditionally, the only calibration methods capable of providing the necessary accuracy to calibrate MFCs in a reasonable amount of time have been based on the measurement of displaced volume or pressure change in a fixed volume (Bell Provers). These methods do not provide real time measurement and offer limited performance at very low flow rates.
Studying the requirements for a mass flow calibration system, it becomes apparent the application of traditional laminar flow theory to laminar flow elements offers interesting possibilities. The flow elements can be purely mechanical and very stable, they can be of a relatively small size, response times should be limited only by pressure measurement speed, and, theoretically, very low flow ranges can be covered. Laminar flow measuring techniques are known to be very linear and the rangeability is excellent since flow is directly proportional to differential pressure. However, the accuracy of around ±1% typically associated with laminar flow elements is far from the ±0.2% needed to meet today's requirements for a calibration standard.
Traditional application of laminar flow techniques shows that accuracy has been limited primarily by the poor knowledge of pressure, poor knowledge of gas temperature, failure to account for temperature and pressure effects on the laminar element dimensions, lack of basic stability of the element itself, and failure or inability to apply the full flow equation using the actual conditions of the flowing gas. Studying the limitations and finding solutions for them has allowed the development of a mass flow calibration standard that meet today's requirements for accuracy, speed and low flow performance.
InnoCal, the primary provider of calibration services to , recognized that the traditional methods of performing mass flow measurement are less than ideal, and chose instead DH Instruments molbloc/molbox flow measurement standard as its primary standard for day-to-day calibrations. Our selection of this standard was based on providing accurate, repeatable, NIST-traceable measurements quickly and cost effectively. The molbloc/molbox flow measurement standard follows established laminar flow theory. In accordance with well-known laws of gas behavior, the flow of a known gas in the laminar flow regime can be calculated from the flow path geometry and the gas pressure and temperature. This standard achieves unprecedented levels of stability and precision from the laminar flow principle by applying today's modern sensor, mathematical modeling and data processing technologies to a flow element design so novel it has been granted US and international patent protection. This combination improves knowledge of gas pressure and temperature, provides a better definition of flow path and allows comprehensive, real time flow calculation using the thermodynamic properties of the gas under actual flowing conditions.
With the addition of this new flow standard InnoCal is capable of performing flow measurements up to 80 lpm with uncertainties of ±0.2% of Reading. Correlations to a wide variety of gases are available.
Martin Girard, "A high accuracy, portable calibration standard for low mass flow", Proceedings of the XIII IMEKO World Congress of Metrology, September, 1994, Torino, Italy
Pierre Delajoud and Martin Girard, 1996, The need for evolution in standards and calibration to improve process measurement and control of low mass flow.