Signal transmission in process instrumentation and control systems is largely done via electrical transmission but there are a few areas where pneumatic transmission of signals is vital especially where electrical signals or sparks can ignite combustible materials for example in petroleum refineries.
In pneumatic transmission systems, the measured quantity (pressure, level, temperature, and so on) is converted to a pneumatic pressure, the standard signal ranges being 3-15 psi (20 to 100 kPa) gauge pressure. The lower limit of pressure provides a live zero for the instrument which can be used to detect system failure for example you can detect line breaks, additionally, the live zero eases instrument calibration and checking and offers an enhanced dynamic response since, when venting to atmospheric pressure, there is still sufficient driving pressure at 20 kPa.
The pneumatic signals can be transmitted over distances up to 300 m in 6.35 mm or 9.5 mm OD plastic or metal tubing to a pneumatic indicator, recorder or controller. Return signals for control purposes are transmitted from the control element. The distance is limited by the speed of response, which quadruples with double the distance. Actually, one of the main downsides of pneumatic signal transmission systems is the long settling time for signals transmitted over long distances for modern processing requirements, when compared to electrical signal transmission systems. Additionally, pneumatic signal lines are bulky, inflexible and expensive compared to electrical signal lines. Pneumatic transmission systems require a dry, regulated air supply. Condensed moisture in the pipework at sub-zero temperatures or small solid contaminants can block the small passages within pneumatic instruments and cause loss of accuracy and failure.
Some advantages offered by pneumatic systems include: Pneumatic instruments are intrinsically safe, and thus can be used in hazardous areas. They provide protection against electrical power failure, since systems utilizing air storage or turbine-driven compressors can continue to provide measurement and control during power failure. Pneumatic signals can also directly interface with control valves which are pneumatically operated and thus do not require conversion to electrical signals for them to work, however, they may be some difficulties in interfacing to data loggers. Pneumatic instruments may operate on compressed gases other than air, which is a key advantage in remote natural gas installations where natural gas itself may be used as a source of pneumatic power for instrument albeit with a good filtering equipment to prevent contaminants in the natural gas from affecting sensitive instrument systems thus there is no need for electrical power source or air compressor.
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