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5.
Sensor and Programming Example
5.1
Voltage Range
5.2
Calculating Multiplier and Offset—An Example
CURS100 100 Ohm Current Shunt Terminal Input Module
In this example, the input voltage range, and the multiplier and offset values
are calculated for a 4 to 20 mA output pressure transmitter. Examples showing
the differential measurement made on Terminal 1 are then given for the
CR1000X and CR9000(X) dataloggers; programming for the CR6, CR300,
CR800, CR850, CR1000, CR3000, and CR5000 is virtually identical to the
CR1000X.
Select the smallest voltage range that encompasses the maximum output signal
from the sensor. Using the smallest possible range will provide the best
resolution.
The voltage across the resistor, V, is equal to the resistance (100 ohms)
multiplied by the current, I.
V = 100 I
The maximum voltage occurs at the maximum current. Thus, a 4 to 20 mA
transmitter will output its maximum voltage at 20 mA.
V = 100 ohms • 0.02 A = 2 V
An output of 2 volts is measured on the ±2500 mV range on the CR800,
CR850, and CR1000 or on the ±5000 mV range on the CR6, CR1000X,
CR3000, CR5000, or CR9000(X). The 2 volt output is measured on the –100 to
+2500 mV range of the CR300.
The multiplier and the offset are the slope and y-intercept of a line and are
computed with Ohm's law and a linear fit.
For example, measure a current loop transmitter that detects pressure where the
sensor specifications are as follows:
Transmitter range –
Transmitter output range –
The transmitter will output 4 mA at 200 psi and 20 mA at 700 psi. Using
Ohm's law, the voltage across the resistor at 200 psi is:
V = I • R
V = 0.004 • 100
V = 0.4 V or 400 mV
and at 700 psi is:
V = 0.020 • 100
200 to 700 psi
4 to 20 mA
8
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