Choosing A Flowmeter - How To Buy Smart

Today, buying smart is more critical than ever.  Flowmeter buying is no exception.  Purchasing a flowmeter that does not meets its published specifications (or meet your needs) or paying too much for what you do need is bad business.  With major decisions being made based upon flowmeter data, (i.e., billing, sizing, and capacity levels, as well as regulatory mandates/fines,) one cannot afford to take chances with the purchase and use of flow instrumentation products.  Today, many flowmeter users prefer flow instruments based on the velocity/area concept.  These instruments measure both velocity and level so as to complete the continuity equation of flow Q = V x A where Q = Flow, V = Velocity, and A = Area at the location where the velocity measurement is made.  

Flow accuracy is dependent upon the accurate measurement of both the average velocity and the depth. 

Depth Measurement
Most velocity/area flowmeters have the depth transducer submerged in the flow, typically at the pipe bottom.  The depth transducer is generally a submerged piezo pressure device that essentially measures the weight of the water above the sensor.  These devices can have two major error sources: 

1. Errors caused by pressure effects of the water flowing around the sensor and 

2. Temperature effects that cause signal drifts. 

Both of these potential error sources can be large.  For instance, independent tests have shown that pressure effects of flowing water around some commercial sensors cause depth errors as great as two inches under typical velocity conditions seen at most measurement sites.  Sensors utilizing temperature compensation by 'passiveä techniques still can exhibit depth errors as great as ±0.7 inches/10¼F.  In contrast, active temperature compensation techniques utilizing microprocessors tend to have five to seven times less temperature drift than those that are 'passivelyä compensated.  

Velocity Measurement
Two popular velocity measurement techniques are electromagnetic (EM) and acoustic Doppler.  The EM technique measures a local velocity at a defined distance from the sensor. Utilizing equations empirically derived from years of flow data, this local velocity is converted to average velocity over a broad range of pipe sizes.  Independent flow lab tests and extensive field evaluations have shown the conversion accuracy to be typically better than ±5%.

Most Doppler meters send an ultrasonic signal beam outward and upward into the flow stream.  Signals reflected from particles in the flow are processed to obtain a direct estimate of average velocity.  It is reported that the major difficulty with a continuous wave (CW) Doppler velocity system is that many of the processing electronics have no knowledge of the location of the particles in the flow from which the sound waves reflect.  Thus, if there are slower moving particles on the channel bottom, the sensor will yield an entirely different 'average velocityä than from particles near the surface.  It should also be noted that many manufacturers of Doppler meters, while purporting to measure average velocity directly, inexplicitly provide only a 'velocityä specification - not an 'average velocityä specification.  

Critics of EM flowmeters described the EM sensor as being prone to 'foulingä by grease and oil.ä  However, new non-fouling electrode designs, as well as enhanced signal processing has significantly improved the accuracy of electromagnetic sensors.