Air is probably the easiest of all gases to meter and, consequently, many flow meters are available to accomplish the task. Usually the application will affect the choice working pressure, temperature, flow range, required accuracy, price and whether power is available.
Variable Area flowmeters are probably the least expensive option and particularly choice if high accuracy is not a priority – they are eminently suitable where a visual indication of flow is all that is required. Care must be taken to not exceed working pressure and temperature limits, particularly in the case of plastic tube units. Variable Area flow meters are available for flows as low as 5 cc/min up to 1500 m3/hr with accuracy 3 to 5% at best. It is critical to know the precise working pressure when placing an order so the flow meter can be scaled correctly. Change in working pressure effect air density and can cause measuring errors.
Thermal Mass flow meters are available from ¼” to 2” bore, fitted into a stainless steel measuring section (spool piece) with threaded process connections. The flow sensor is calibrated whilst being installed in the measuring section to ensure optimum accuracy and performance. An integral display shows flow rate and total in various volumetric or mass engineering units, and also provides a scaled pulse and 4-20mA outputs. For pipe installations greater than 2”, a hot-tap insertion version is available with flanged spool piece options. This flowmeter provides a very wide operating flow range with a measuring error of 1.5 to 2%. Thermal Mass flow meters offer excellent accuracy and reliability at a very affordable price and are available on a short lead time. Flowquip’s VA520 Air Flow Sensor provides an affordable option and offers high accuracy due to the defined measuring section.
Vortex Flowmeters offer the advantage of a wide flow range with the added benefit of no moving parts. An integral, or remote mounted, LCD indicates flow rate and total and provides options of a pulse and/or 4-20mA output. Typical flow turndown is 30:1 with measuring errors of 1 to 2%.
Coriolis Flow meters are a more expensive option, but the most accurate measuring solution. As density and viscosity of all gases are greatly affected by changes in temperature and pressure, the Coriolis flow meter is a good choice when it comes to air measurement. A true and accurate mass or volume reading is provided regardless of changes in gas characteristics. This type of flowmeter offers no obstruction to flow, although the shape of the measuring tubes may create a small pressure loss. Straight tube Coriolis flow meters can be employed where pressure drop is an issue. Measuring accuracies of better than 0.5% of reading can be obtained over a wide operating flow range.