Watlow 12LS Controller - Manual (Page 55)

4-22
The low input reading is referenced to 60mV and is calculated using this equation: (see section 3.3.6)
(Sensor Output @ Low PV)
LOW RDG = (1000) * ------------------------
( 60mV )
Press YES to select the menu or NO to advance to the next menu.
Press UP and DOWN to set the low reading.
Press ENTER to store the value or BACK to return to the SETUP LOOP INPUT main menu prompt.
Linear Input Scaling Example:
PROBLEM: We want to connect a pressure sensor to enable us to read pressure directly in PSI. The sensor has an
output of 4-20mA over a range of 100-400PSI. The 4-20mA is converted to 12-60mV (see section 3.3.6 and 3.3.7)d.
How do we set the input scaling?
SOLUTION: 1. Set the HIGH PV to 400 and enter the units as P, S,
and I.
2. Calculate the HIGH RDG and enter it:
(60mV) <--Sensor output @ High PV
HIGH RDG = 1000 * ------
(60Mv) <--Full scale 8LS input
= 1000
3. Set the LOW PV to 100 and enter the units as P, S,
and I.
4. Calculate the LOW RDG and enter it:
(12mV) <--Sensor output @ Low PV
LOW RDG = 1000 * ------
(60mV) <--Full scale 8LS input
= 200
4.5.2.4 SOFTWARE CALIBRATION OF INPUTS
The scaling function may be used to calibrate for the inaccuracies of the input signal at a given point. For example, the
type J T/C may be up to 4o F high or low from true temperature, when the temperature is below 400o F. When
adjusting to correct for linear offset in the input calibration, use only the PV scaling function. Using both the PV and
RDG function will not shift the calibration. Adjusting the RDG should never be done as this will effect the overall T/C
range.