Many flow technologies are available to accomplish the task. Usually the application will limit the choice; working pressure, temperature, flow range, required accuracy, price, is power available etc.
Variable Area Flowmeters: A simple ‘float in a tube’. Probably the least expensive and least accurate of all gas flowmeters but eminently suitable where a visual indication of flow is all that is required. Care must be taken with regard to not exceeding working pressure and temperature limits … particularly in the case of plastic tube units. V/A flowmeters are available for flows as low as 1 cc/min up to 1500 m3/hr with accuracy 3 to 5 percent at best. It is critical to know the precise working pressure when placing an order so the flowmeter can be scaled correctly as changes in working pressure effect air density and cause measuring errors.
Mechanical Air Meters (TERZ94): Available from 25mm up to 600mm bore to handle flow rates from 2.5 m3/hr up to 2500 m3/hr. Small bore sizes are manufactured in anodised aluminium whilst larger units are available in ductile iron or carbon steel. The flowmeters have a battery powered LCD which can display flow rate and total. High and low speed pulse outputs are also provided with the option of 4-20mA output. Typical measuring errors are 1 to 3%.
Calorimetric Flowmeters: 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 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%. The calorimetric gas flow meter offers excellent accuracy and reliability at a very affordable price and on a short lead time too. ATEX Approved versions are available for Hazardous Area installations.
Vortex Shedding 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 Flowmeters: probably the most expensive but the most accurate gas flowmeter. As density and viscosity of all gases are greatly affected by changes in temperature and pressure the Coriolis flowmeter is a good choice when it comes to gas measurement. A true and accurate mass or volume reading is provided regardless 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 flowmeters 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.
ATEX Approved versions are available for Hazardous Area installations.