A differential pressure (DP) flowmeter consists of a differential pressure transmitter integrated with a primary element and has the capability of calculating flowrate based on difference in pressure. DP flowmeters rely on a constriction placed in the flow line that creates reduced pressure in the line after the constriction. A primary element is used to create the constriction in the flowstream. A DP flowmeter also requires a means to detect the difference in upstream vs. downstream pressure in the flow line. While this can be done with a manometer, today’s DP flowmeters use differential pressure transmitters that sense the difference in pressure, and then use this value to compute flowrate.
When most flowmeters are sold, the transmitter and sensor are sold together. This is true for ultrasonic, vortex, Coriolis, turbine, and other types of flowmeters. All these flowmeters operate based on a correlation between flowrate, or mass flow, and some physical phenomenon. For ultrasonic flowmeters, it’s the difference in transit time of sound waves sent across the pipe. For turbine flowmeters, it’s the speed of the rotor. Differential pressure flowmeters also correlate flow with a physical phenomenon; the difference in pressure upstream and downstream from a constriction in the flowstream.
Where differential pressure flowmeters differ from other flowmeter types is that the transmitter is still often sold separately from the primary element that creates the constriction in the flowstream, sometimes from different suppliers. Because a differential pressure flow transmitter cannot make a flow measurement without a primary element, customers who purchase a DP flow transmitter without the primary element are not actually buying a DP ‘flowmeter.’ They don’t have a DP flowmeter until they connect the primary element to the DP flow transmitter. Thus, a DP flowmeter is considered to be a DP flow transmitter that is connected to a primary element for the purpose of making a flow measurement.
In the past, the pressure transmitter companies sold DP transmitters and users ordered their primary elements separately. Now, however, a number of companies are selling DP transmitters already integrated with a primary element, such as an Annubar or an orifice plate. It is tempting to consider these the only true DP flowmeters; however, a better description is that these are DP flowmeters with an integrated primary element. If a customer assembles a DP flowmeter by connecting up a DP flow transmitter to an orifice plate or a Venturi tube from another source, the result is just as much a DP flowmeter as an integrated product.