1    Convert between Process Variable Values and Signal Values

In working with process control loops, process control technicians need to be able to convert given process variable values into signal values they expect to obtain.
They also need to be able to convert from a signal value back to what the process variable should be.
Ranges and Spans
An instrument usually has two ranges and two spans; an input range and span and an output range and span.
The input range is the process variable and its units of measurement that the instrument receives.  Some examples of input ranges are shown in Table 1.

Input Range
Process Variable Type
200 to 1000 psi
100 - 300ºC
0 – 100  inches H2O
Inches H2O
3 – 15 psi

Table 1

Range refers to the measurement values or signal values over which an instrument or device operates.
The output range is the type and units of the instrument output.  Output can be a signal value or display of the variable such as a pointer on a scale.
Notice that ranges must have two values – a minimum range value and a maximum range value.  A range of 100ºC to 500ºC will have a minimum range value of 100ºC and a maximum range value of 500ºC.
Span is the difference between the maximum range value and the minimum range value.  If a temperature transmitter has an input range of 100 to 500ºC the span is:
500 – 100 = 400ºC
Such a transmitter has an output range of 3 –15 psi.  Therefore span is:
15 – 3 = 12 psi
With an input of 100ºC the transmitter output will be 3 psi (or 4 mA) and with an input of 500ºC the output will be 15 psi (or 20 mA).
Live Zero
Live zero means using a value other than zero to indicate zero.  A live zero value such as 3 psi or 4 mA lets a technician easily detect signal failure or zero error.  If the zero value were represented by 0 psi or o mA, you would not be able to tell the difference between a genuine zero and an air supply/electrical failure.


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