Tuesday, June 25, 2013

Zero and Span Adjustment - Continuation

Things become more complicated when the input and output axes are represented by units of
measurement other than “percent.” Take for instance a pressure transmitter, a device designed to
sense a fluid pressure and output an electronic signal corresponding to that pressure. Here is a graph
for a pressure transmitter with an input range of 0 to 100 pounds per square inch (PSI) and an
electronic output signal range of 4 to 20 milliamps (mA) electric current:

Although the graph is still linear, zero pressure does not equate to zero current. This is called
a live zero, because the 0% point of measurement (0 PSI fluid pressure) corresponds to a non-zero
(“live”) electronic signal. 0 PSI pressure may be the LRV (Lower Range Value) of the transmitter’s
input, but the LRV of the transmitter’s output is 4 mA, not 0 mA.
Any linear, mathematical function may be expressed in “slope-intercept” equation form:
                                                 y = mx + b
y = Vertical position on graph
x = Horizontal position on graph
m = Slope of line
b = Point of intersection between the line and the vertical (y) axis
This instrument’s calibration is no different. If we let x represent the input pressure in units
of PSI and y represent the output current in units of milliamps, we may write an equation for this
instrument as follows:
                                           y = 0.16x + 4
On the actual instrument (the pressure transmitter), there are two adjustments which let us
match the instrument’s behavior to the ideal equation. One adjustment is called the zero while the 
other is called the span. These two adjustments correspond exactly to the b and m terms of
the linear function, respectively: the “zero” adjustment shifts the instrument’s function vertically
on the graph, while the “span” adjustment changes the slope of the function on the graph. By
adjusting both zero and span, we may set the instrument for any range of measurement within the
manufacturer’s limits.
It should be noted that for most analog instruments, these two adjustments are interactive. That
is, adjusting one has an effect on the other. Specifically, changes made to the span adjustment almost
always alter the instrument’s zero point. An instrument with interactive zero and span adjustments
requires much more effort to accurately calibrate, as one must switch back and forth between the
lower- and upper-range points repeatedly to adjust for accuracy.

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