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PNEUMATIC RELAY (Booster relay)
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- 5.4 Differential Pressure Transmitter
| PNEUMATIC RELAY (Booster relay) A relay is a pneumatic amplifier. The function of the relay is to convert a small change in the input signal to a large change in the output signal. Typically a 1 psi change in the input will produce approximately a 12 psi change in output. 5.4 Differential Pressure Transmitter Pneumatic DP transmitter, or DP cell, receives a supply of air at 20 psig (the metric equivalent is 138 kPa) and reduces it to an output of 3 to 15 psig (20 to 100 kPa) that is directly proportional to the measured DP. The electronic transmitter has output (normally 4 to 20 mA) that is proportional to the differential pressure. In the DP transmitter, process fluid (if not harmful to the instrument) is piped to each side of a capsule containing twin diaphragms. Any pressure increase in the high-pressure chamber causes the capsule to move toward the low- pressure side and, in turn, to impart this motion to a laminated force bar. In the pneumatic transmitter, the upper end of the bar reacts by moving the flapper toward the nozzle, which is the source of supply air. The nozzle feed line is connected to one side of a diaphragm in the air relay. 97 Field Instrumentation GEN Rev) As the flapper covers the nozzle, a pressure buildup acts on the air relay diaphragm. This opens the relay and delivers more air pressure to both the feedback bellows and the output. This action continues until the pressure exerted against the range rod by the bellows counterbalances the original pressure of the force bar. The zero adjustment spring establishes a starting value (0 DP = 20 kPa). The adjustable range wheel is the pivot point for the range rod’s motion. The first step in calibrating this kind of transmitter is to vent the low side of the transmitter to atmosphere. Then apply a known pressure to the high side. If the transmitter range is 0 to 205 in of water, apply a pressure of 205 in of water and expect the transmitter output to be 15 psi or 20 mA, depending on the kind of transmitter. Position the adjustable range wheel to obtain the correct output. With no pressure applied to the high side of the transmitter, the output should be 3 psi or 4 mA. Now adjust the zero screw to obtain the correct output. |