Answer / carl_ellis

Because it's a standard, widely used conventional pneumatic
control transmission range.

The 3-15 psi signal is a standard (ISA SP7) that came from
widespread use even before that standard came into being.
As early as the 1920's pneumatic regulators, pressure and
temperature controllers were using 15 psi at the high end.
Various low end 'live' or elevated zero values were used
before standardization.

Any pneumatic control or transmission signal starts with an
'elevated' or 'live' zero' at 3 psi because there is
internal friction and a fair amount of hysteresis in the
movement of pneumatic components, the flapper, nozzle,
diaphragms, spring preloads, pivots and bellows that make up
the transmitters and receivers. This friction and
hysteresis manifests as non-linearity below 3 psi. A 3 psi
minimum pressure puts the transmission signal into a linear
response range. Obviously, control system measurements,
signal transmission and the receivers need a linear response
to effect accurate and precise control.

Not all pneumatic control signals are in the 3-15 psi range.
The ISA SP7 standard (1996) that addresses instrument air
quality standards also lists 5-25 psi, 6-30 psi, 3-27 psi as
standard pneumatic transmission ranges, along with the
minimum supply pressure associated with each range. These
ranges all include an elevated zero.

Carl Ellis
Measure First

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