Measurement Signal Transmission

This chapter covers the techniques used for transmitting measurement signals over what can, in some circumstances, be quite large distances. The discussion begins with electrical transmission. This is the simplest way to convey measurement signals over some distance, but there is a tendency for the measured signal to be corrupted by noise, and there is appropriate discussion about how to deal with such noise problems by amplifying measurement signals and shielding transmission wires. Discussion then continues with current-loop transmission and a.c. carrier transmission (using either amplitude modulation or frequency modulation), which are alternative ways of transmitting signals electrically without noise curruption. Next, pneumatic transmission is explained, which, although much less common than in the past, is still used in some industrial applications because of its intrinsic safety. The discussion then moves on to fiber-optic transmission, which, although more expensive than electrical transmission, offers intrinsically safe transmission without niose corruption of the transmitted signal. Moving on, the chapter covers wireless optical transmission and radiotelemetry. Finally, the strong merits of sigital transmission are considered.