Hersey Measurement Company

Target Flowmeter

Measurable fluids range from heavily sedimented or hot, tarry liquids including Bunker C, hydrocarbons, acid or caustic solutions, water, air, natural gas, ammonia, chlorine, other chemical gases and supersaturated steam.

Output consist of 4-20 mA

For complete specifications please contact factory.

THE FLOW TRANSDUCER:

Principal of Operation

The transducer is actually an electronic "inside-out" orifice plate. Figure 1 shows the pressure differential between the front and the rear surfaces of a cirular disc produced by the flow stream from left to right. The force of this pressure drag is transmitted via a cantilever arm to a flexure tube of unique design which permits
Fig. 1	the strain gage elements to be mounted external to the flowing medium. The four active arm strain gage bridge provides infinite resolution, high linearity, excellent sensitivity, wide range temerature compensation and zero hysteresis. It is evident that flow may be from either direction. This bi-directional flow characteristic is of particular importance in many gas and steam applications.
Unlike an orifice plate, which requires separately placed pressure ports that are difficult to locate at the precisely correct position, the pressure differential is sensed electronically and directly by the force imbalance across the disc. In this sense, the transducer may also be likened to a dynamic "weigh beam" in that it responds to pounds of force produced by the flow dynamics. In fact, the user may re-verify its calibration in the field by removing the transducer element from the flow pipe and suspending from it an accurately known weight. The dynamic force is proportionally related to the product of the velocity head, the fluid density (p), the drag coefficient of the disc and its surface area. The descriptive equation is as follows: F=(Vsqr/2g) pCd A This relationship holds true for Reynolds numbers of 2,000 or higher, in which range the output is unaffected by variations in fluid viscosity. For Reynolds numbers in the region between 1000 and 2000, the output for that part of the flow range may be vicosity dependent. Flow velocity, however, can be sensed all the way down to zero since computer control renders the transducer to be essentially drift-free. At extremely low flow rates special calibration is required as the above equation is valid for Reynolds numbers greater than 2000. In actual practice, Reynolds numbers higher than normally calculated values are produced in the region of the bluff body due to its presence in the stream. Straight piping runs of 10 pipe diameters upstream and 5 downstream are required for the highest accuracy, as with all conventional dP systems.