Mass flow devices, developed in the 1960’s, are critical components is systems that produce the specialty gas standards required to calibrate a wide variety of instruments that analyze gases.
Mass flow devices generate a signal that is proportional to the mass flow of a gas. Since the specific heat of a gas is unique to a particular gas, mass flow devices are unaffected by pressure and temperature changes. Thus, the signal they generate is very accurate and stable.
If the signal voltage from a mass flow device is used to indicate flow, it is classified as a mass flowmeter (MFM). If the signal from such a device is used in conjunction with a reference signal and a controlling valve, it is classified as a mass flow controller (MFC).
When several MFC’s are operated in parallel, and are electronically controlled within tight limits, the gas mixtures produced are extremely accurate. This is the principle behind a commonly used gas mixing technique called “Dynamic Blending.”
When a certified gas mixture is ordered from a producer of calibration gases, lead time and price are important considerations. These factors can affect whether your analytical instrument is properly calibrated.
Where volume and urgency are major considerations in meeting calibration requirements, the purchase of a dynamic gas blender may be more cost-effective than purchasing the calibration gas. Dynamic gas blending is preferred to other types of systems that produce calibration standards since they can, in some cases, produce the end product more quickly and at a lower cost, and still ensure high accuracy.
Some dynamic gas blenders that are used to produce mixtures for calibrating ambient air pollutant monitors, also produce precision levels of ozone. This ozone can then be used to calibrate ambient ozone monitors and oxides of nitrogen (NOx) monitors that use gas phase titration with ozone.
Many factors can affect the overall accuracy of an MFC-based dynamic gas blender. These can range from the selection of the MFC to the care taken by the end-user in operating and maintaining the equipment.
To read more about selecting the right MFC, stay tuned for Part II next week!
(an excerpt from "Using mass flow controller-based dynamic gas blenders to produce accurate calibration gas," written by Director of Engineering Tom Bamford for Specialty Gas Report)