Basics of industrial flow measurements 6

In this part we will discuss two flow meter types exclusively for a single medium either gas or liquid. For gas flow measurement, 'Optical Flowmeters" or laser meters are used. The gas flow passes through two laser beams. Since industrial gases contain small particles these help in determining the velocity of gas flow. When a particle crosses the first laser beam it disperses the light which causes a pulse generation at the receptor. When the same particle crosses the second beam and light dispersion is detected a second receptor generates another pulse. This time difference is then evaluated to calculate velocity of gas. It is a very accurate way of measuring gases only. The measurement since depend totally on the gas velocity and not dependent on temperature or pressure or change in gas composition, it is a reliable method.

Flow measurement of water in open channel uses a method of level to flow which is suitable for water in open channel. This could be industrial waste water, rain water or simply a flow in canal or river. The level of the water is measured at a designated point behind weirs or in flume using various secondary devices (bubblers, ultrasonic, float, and differential pressure are common methods). This depth is converted to a flow rate according to a formula of the form where flow (Q) is calculated based on level (depth) and an exponent which varies with the device used. It is converted according to empirically derived level/flow data points (a "flow curve"). The flow rate can then be integrated over time into volumetric flow. Level to flow devices are commonly used to measure the flow of surface waters (springs, streams, and rivers), industrial discharges, and sewage.

Another way to measure open channel flow is based on area/velocity where Doppler method or sometimes propellers are used to determine the velocity of flow in a channel. This velocity times cross sectional area of the channel will determine the volume of flow passed. In another variation, non contact method involving laser or radar also are used above the channel to evaluate the velocity of flowing fluid.

As we unfold the methods of measurement of flow many more other industrial flow measuring instruments will be discussed in the series of articles on this newsletter, so next week we will be again coming back to you...Just subscribe to this newsletter INSTRUMENTATION REVISITED and receive updated version every week. Please feel free to share it if you like it. Comments, suggestions, feedback are always welcomed.

(Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance)