Transmission, pneumatic

For pneumatic transmission systems, the signal range used for the transmission is 3 to 15 psig. In each pneumatic transmission system, there can be only one transmitter, but there can be any number of receivers. When most measurement devices were pneumatic, pneumatic transmission was the logical choice. However, with the displacement of pneumatic measurement devices by electronic devices, pneumatic transmission is becoming less common but is unlikely to totally disappear. 

Pressure recording controller, pneumatic transmission with board niotmted receiver... 

Pressure recording controller (differential), pneumatic transmission with pressure recorder, combined instrument board mounted... 

Flow recorder with pressure recording pen, botli elements pneumatic transmission, Iransmiller local, and receiver board mounted. (Recei er should be written as FR-5 and PR-2, and each transmitter identified by its own element)... 

Flow recorder pneumatic transmission, transmitter local, receiver mounted on board... 

Level recording controller external type, pneumatic transmission... 

Pressure is measured extensively in the chemical processing industries and a wide variety of pressure measuring methods has been developed. Some of these have already been discussed in Volume 1, Section 6.2.2, viz. the manometer (which is an example of a gravity-balance type of meter), the Bourdon gauge (an example of an elastic transducer) and mention is made of the common first element in most pressure signal transmission systems—the differential pressure (DP) cell (Volume 1, Section 6.2.3). The latter also frequently forms part of a pneumatic transmission system and further discussion of this can be found in Section 6.3.4. 

One important application of pneumatic transmission is in the operation of diaphragm actuators. These are the elements generally employed to drive the spindles of control valves (Section 7.22.3) and, if hard-wired transmission systems are employed, require devices which convert electric current into air pressure or air flowrate, i.e. electropneumatic (E/P) converters. The basic construction of a typical E/P converter is illustrated in Fig. 6.77. A coil is suspended in a magnetic field in such a way that when a current is passed through the coil it rotates. This rotation is sensed by a flapper/nozzle system (Section 7.22.1). The nozzle is supplied with air via a restrictor and its back pressure actuates a pneumatic relay. The output from the latter is applied to the feedback bellows and also acts as output from the E/P converter. Electropneumatic valve positioners employ the same principle of operation. 

For pneumatic transmission systems, the signal range used for the transmission is 3 to 15 psig. In each pneumatic transmission system, there can be only one transmitter, but there can be any number of... 

Level recorder, pneumatic transmission, with board mounted receiver, external type transmitter... 

Level recording controller and level recorder, pneumatic transmission combined receiver and board mounted... 

Transmission lines in process control systems rarely make any significant contribution to the overall loop characteristics. Where signals are transmitted electrically, there is no detectable signal attenuation for any frequencies characteristic of the process components, and even for pneumatic transmission lines as long as 200 feet there is little loss of signal. Transmission lines have distributed properties, but according to Bradner (B3) who has studied pneumatic transmission lines extensively, they can be approximated as second-order systems. 

The distributed character of the pneumatic transmission lines dynamics is discussed in Ref. 6 and in ... 

Unless the process changes very fast or the transmission lines are very long, the dynamic behavior of a pneumatic transmission line can be neglected from consideration. When the assumptions above do not hold, it has been found that the following transfer function correlates successfully the pressure at the outlet (P0) to the pressure at the inlet (Pi) of a pneumatic transmission line ... 

Note. In the subsequent chapters, as a rule, we will neglect the dynamics of pneumatic transmission lines. 

TRANSDUCER - The means by which the controller converts the signal from the sensing device into the means necessary to have the appropriate effect on the controlled device. For example, a change in air pressure in the pneumatic transmission piping. 

Here the exit pressure affects a diaphragm and holds a valve in an equilibrium position against the force of a spring. If the pressure rises, this closes the cross-section of the plug against the seat, and if the pressure lowers it opens again. This is also the method of operation of the pressure controllers in pneumatic transmission lines. Supply pressure-reducing valves are controllers without auxiliary power. 

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