Monday, June 10, 2013

Advantages and disadvantages of Pneumatic Instruments

The disadvantages of pneumatic instruments are painfully evident to anyone familiar with both
pneumatic and electronic instruments. Sensitivity to vibration, changes in temperature, mounting
position, and the like affect calibration accuracy to a far greater degree for pneumatic instruments
than electronic instruments. Compressed air is an expensive utility – much more expensive per
equivalent watt-hour than electricity – making the operational cost of pneumatic instruments far
greater than electronic. The installed cost of pneumatic instruments can be quite high as well, given
the need for special (stainless steel, copper, or tough plastic) tubes to carry supply air and pneumatic
signals to distant locations. The volume of air tubes used to convey pneumatic signals over distances
acts as a low-pass filter, naturally damping the instrument’s response and thereby reducing its ability
to respond quickly to changing process conditions. Pneumatic instruments cannot be made “smart”
like electronic instruments, either. However, pneumatic instruments actually enjoy some definite technical advantages which secure their continued use in certain applications even in the 21st century. One decided advantage is the intrinsic safety of pneumatic field instruments. Instruments that do not run on electricity cannot generate electrical sparks. This is of utmost importance in “classified” industrial environments where

explosive gases, liquids, dusts, and powders exist. Pneumatic instruments are also self-purging.
Their continual bleeding of compressed air from vent ports in pneumatic relays and nozzles acts as a
natural clean-air purge for the inside of the instrument, preventing the intrusion of dust and vapor
from the outside with a slight positive pressure inside the instrument case. It is not uncommon to
find a field-mounted pneumatic instrument encrusted with corrosion and filth on the outside, but
factory-clean on the inside due to this continual purge of clean air. Pneumatic instruments mounted
inside larger enclosures with other devices tend to protect them all by providing a positive-pressure
air purge for the entire enclosure.
Some pneumatic instruments can also function in high-temperature and high-radiation
environments that would damage electronic instruments. Although it is often possible to “harden”
electronic field instruments to such harsh conditions, pneumatic instruments are practically immune
by nature.
An interesting feature of pneumatic instruments is that they may operate on compressed gases
other than air. This is an advantage in remote natural gas installations, where the natural gas
itself is sometimes used as a source of pneumatic “power” for instruments. So long as there is
compressed natural gas in the pipeline to measure and to control, the instruments will operate. No
air compressor or electrical power source is needed in these installations.

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