Instead of analog current signal, air pressure may be used as an alternative signaling medium to electricity. Imagine a pressure transmitter designed to output a variable air pressure according to its calibration rather than avariable electric current. Such a transmitter would have to be supplied with a source of constant pressure compressed air instead of an electric voltage, and the resulting output signal would be conveyed to the indicator via tubing instead of wires refer to figure 1.
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| Figure 1 And the most common range of air pressure for industrial pneumatic instruments is 3 to 15 PSI. An output pressure of 3 PSI represents the low end of the process measurement scale and an output pressure of 15 PSI represents the high end of the measurement scale. In each case, the transmitter and controller are both supplied clean compressed air at some nominal pressure (20 to 25 PSI, usually) and the instrument signals travel via tubing. One of the most common applications for pneumatic control system components is control valve actuation, where pneumatic technology still dominates. This pneumatic signal originates from a device called an I/P transducer, or current-to-pressure converter, taking a 4-20 mA control signal from the output of an electronic controller and translating that information as a pneumatic 3-15 PSI signal to the control valve’s positioner or actuator. |
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05 - Pneumatic Instrumentation
