Omega Vehicle Security freedom OM-550 - Manual (Page 42)

OM-550 u 37 u
4.14.6 MATCHING AND SCALING A CURRENT TRANSDUCER
Example:
Given a 0 - 150-PSI transducer with a 4-20 mA output.
Program the OM-550 to indicate 0 - 150 PSI with 0.01-PSI resolution.
If the 4-20 mA Current Probe is being used, begin with step 2, below. (This probe is
already matched to the OM-550).
1. First, find the Matching resistor:
R1 = 2 / I
= 2 / 20 mA
= 2 / 0.02
= 100
Ω
When the transducer output is 4 mA, the OM-550 will see 0.4 VDC and when the
transducer output is 20 mA, the OM-550 will see 2 VDC.
2. Next, find m & b:
X1 = 0.4000 VDC ç minimum expected input
= 4000 A/D counts at Transducer minimum output
X2 = 2.0000 VDC ç maximum allowable input
= 20000 A/D counts at Transducer maximum output
Y1 = 0.00 PSI ç minimum transducer output
= 0.00 PSI ç choose decimal point = 2
Y2 = 150.00 PSI
= 15000 display counts
m = (Y1 - Y2) / (X1 - X2)
= (0 - 15000) / (4000 - 20000)
= 0.93750
b = Y2 - (m * X2)
= 15000 - (0.93750 * 20000)
= -003750
Notice the decision to indicate with 0.01-PSI resolution determined a decimal position
of 2.