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Xii) The acquisition of accurate operational data permits better performance analysis for current and future maintenance and provides a firm basis for studying future machinery design. A diagram of a typical data logging system is shown in Fig. 2. The sensing/transducing devices, which must provide an electrical output, are connected to a scanning system whose function is to switch each line in turn to a common channel for transmission to the next sub unit. The rate of scanning could be as high as 50 points/second but for marine systems scanning speeds of about 1 point/second are commonly used.

The movement continues until a position is reached where the diaphragm force and the spring force are in equilibrium. As the relation between spring thrust and movement is linear, the valve travel is related linearly to the controller output. The extreme bottom position is reached when 15 psi is applied to the diaphragm. The size of the unit is determined according to : (i) valve stroke, (ii) operating force required. Fortunately, both functions are usually tied to the nominal size of the valve so that the variation is generally compatible.

Solid state devices (see Chapters 8 and 11) are used wherever practicable to ensure reliability and the modules are encapsulated to give protection from the normal marine conditions of vibration, salt, moisture and/or oil-laden atmospheres. This encapsulation does not normally involve "potting" but rather the coating of the module complete with components with a layer of epoxy resin material, polyurethane or equally effective material. ■· ■■■ l l - 1 1 finter] I I ^r ''JOÊÊÊTC R^4*B Modular construct ion I FIG.