Siemens ET 77E Series Spezifikationen PDF herunterladen (Seite 29)

SD77

3.0 OPERATION

The input coil and the float are attached to a common center shaft and make up what is referred to as the “moving

coil assembly.” This assembly is free to move vertically. The float is submerged in Silicone fluid and is sized so that

the resultant buoyant force equals the weight of the moving coil assembly. This puts the moving coil assembly into

a state of neutral buoyancy, which, together with the viscous damping of the Silicone fluid, makes the transducer

insensitive to shock and vibration.

FIGURE 3-1 Schematic

A permanent magnet provides a magnetic field that passes through the input coil. Current flowing through the coil

reacts with the magnetic field to force the moving coil assembly closer to the nozzle. The top end of the common

center shaft serves as a nozzle seat that restricts the flow of air exhausting from the nozzle.

Air is supplied to the nozzle through a restriction. The restriction and the nozzle form a pressure-divided circuit.

The pressure in the nozzle (back-pressure) varies according to the restrictive effect imposed by the nozzle seat. The

nozzle backpressure is the output of the transducer.

The nozzle forms a column of air that has a diameter equal to the nozzle diameter. This column of air acts on the

nozzle seat to oppose and equal the force produced by the coil. The pressure of this column of air (nozzle

backpressure) is determined by the upward force of the moving coil assembly divided by the area of the column of

air (i.e. the area of the nozzle). Thus, the force produced by the coil is continuously balanced by the nozzle

backpressure so that the transducer output pressure is, at all times, directly proportional to the coil current.

Zero adjustment is accomplished by varying a spring force on the moving coil assembly.

Range adjustment is accomplished by changing the gap between the permanent magnet and the end of the range

adjustment screw. This screw shunts a portion of the magnetic field. Varying the gap changes the amount of

shunting which, in turn, changes the flux density through the coil.

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Siemens ET 77E Series Spezifikationen Seite 29