Pressure instruments with Bourdon tube, bellows, or diaphragm sensing elements may use either force-balance or motion-balance mechanisms to convert the element movement into an output signal. In a motion-balance transmitter (Figure 400-1), the moving tip of the element is connected to an indicator, the flapper of a pneumatic transmitter, or the current-producing section of an electronic transmitter. In a force balance transmitter (Figure 400-2), the force that tends to move the element is opposed by an equal force generated by the electronic or pneumatic output signal. A force-balance transmitter has no moving parts and therefore is not subject to hysteresis or dead band effects.
