IV.1.4 Transmission Lines

Measurement and/or control signals are carried through various transmission lines. Various process piping, connection and transmission lines, as per the standard set by International Society of Automation (ISA), are listed in the following figure.

Fig. IV.4: Representation of process piping, connection and transmission lines

A heavy solid line represents piping,a thin solid line represents process connections to instruments, a dashed line represents electrical signals (e.g., 4–20 mA connections), a slashed line represents pneumatic signal tubes, a line with circles on it represents data links. Other connection symbols include capillary tubing for filled systems, (e.g., remote diaphragm seals), hydraulic signal lines, and guided/unguided electromagnetic or sonic signals. Electric/electromagnetic signals are instantaneous. Unless the process changes very fast and/or the transmission lines are too long, the dynamic behaviors of electric/electromagnetic transmissions are also usually ignored.