Ever wanted to quickly and cheaply measure the flow rate of a gas flow or water flow, then one of the simplest and most economical flowmeters on the market is the variable area type flowmeter. Often referred to as a rotameter or glass tube flowmeter, this is one of the most economical and reliable instruments for measuring flow. They are available in a wide variety of configurations for different gases, fluids, temperatures and pressures, all with different styles of connections. The flowmeter requires no power and in its smallest form can be carried in your pocket.

It consists of a simple measurement principle of a float moving up and down in a graduated tapered glass tube. As the flow enters the bottom of the meter, which is positioned in a vertical position, the flow pushes the float upwards until a sufficient gap has grown between the tube wall and the float, at this point the float is in equilibrium. Decreasing the flow causes the float to fall and increasing the flow causes the float to rise. With a calibrated graduated scale marked on the tube, the positioning of the float then determines the flow rate through the meter. The float is often a cone shaped slug in the larger meters and a ball in the smaller meters.

The accuracy of the flowmeter varies from manufacturer to manufacturer but typically ranges between 1 and 5% FSD. Typical flowmeters on the market include models from Omega, KDG flowmeter, Brooks, Platon, Fischer Porter, Yokogawa and Krohne, to name a few. You will generally find the variable area flowmeter is specified for use with a specific air or gas, at a standard air density. To gain the best from your flowmeter your operating conditions should be specified at the time of order, as the manufacturer will configure the meter to suit. To improve the in-use accuracy of the meter it will be necessary to measure the temperature and pressure of the flowing media and correct for the difference in tube reference conditions. In applications where the meter is operating close to the tube reference conditions, it is often found that no temperature or pressure measurements are taken, and the tube reading is read directly.

During use the meter will often accumulate foreign material on the tube walls or on the outside of the float, and will cause inaccurate readings, often resulting in the float sticking in the tube. Even worse, if the float has been allowed to hit the end stops of the meter this can permanently damage the meter, and replacement parts will be required. It is essential to maintain a regular calibration of the flow meter when any errors and maintenance issues can be rectified during the calibration process. When organising the calibration of your variable area flowmeter, request the calibration is undertaken on the actual gas/fluid specified for the flowmeter, although it is not always, it is the ideal calibration method. Some calibration laboratories can only calibrate on air or water and then theoretically correct for density changes. This can often incur large calibration errors especially on the smaller size meters. Specialist flow calibration laboratories will often have facilities for flow calibrations on air, helium, argon, propane, nitrox, oxygen, carbon dioxide and with the use a a gas divider can also undertake specific gas mixtures as required.