Achieving accurate gas flowmeter calibration

The calibration of gas flowmeters and/or flow controllers is one of those important, inescapable tasks that can be something of a nuisance to the engineer responsible for getting the job done. Sending flowmeters back to the manufacturer for recalibration can be expensive and cause production delays. For this reason, most big users of flowmeters opt for in-house calibration. This approach is often best—that is, as long as the user makes the required investment in the equipment needed to do the job the right way. Make-do calibration setups can waste a lot of time, only to produce inaccurate results. Such practices often lead to a degradation of process quality, excessive downtime, and increased labor costs coupled with reduced productivity.

This article describes gas calibration methods that have the highest accuracy potential —specifically, those based on the positive displacement principle. In each approach, the calibration gas displaces a measured volume over a measured elapsed time, 12 - ty. The displaced volume V is divided by the elapsed time to obtain Q, the volumetric flow rate —i.e., Q = V/(t2-tx). The gas mass flow rate, m, is obtained by simply multiplying the volumetric flow rate by the density p of the calibration gas —i.e., m = pQ.

Usually the mass flow rate of gases is expressed as the volumetric flow rate Qe, referred to standard conditions (typically, 0°C and 1 atmosphere). In this article, we`ll express the mass flow rate as Q8 = (p/pB) [V/(*2 —`1)]. where the subscript s refers to standard conditions.