Campbell Scientific CURS100 100 Ohm Current Shunt Terminal Input Module - Manual (Page 14)

CURS100 100 Ohm Current Shunt Terminal Input Module
5. Sensor and Programming Example
In this example, the input voltage range, and the multiplier and offset values
are calculated for a 4 to 20 mA output pressure transducer. Examples showing
the differential measurement made on Channel 1 are then given for the
CR1000, CR9000(X), CR10(X), and 21X dataloggers; programming for the
CR800, CR850, CR3000, and CR5000 is virtually identical to the CR1000.
5.1 Voltage Range
The voltage range on which to make the measurement should be the smallest
range that will accommodate the maximum signal the sensor will output.
Using the smallest possible range will give 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
transducer 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, CR1000, and CR10(X), or on the ±5000 mV range on the 21X,
CR3000, CR5000, or CR9000(X).
5.2 Calculating Multiplier and Offset—An Example
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 transducer that detects pressure where the
sensor specifications are as follows:
Transducer range — 200 to 700 psi
Transducer output range — 4 to 20 mA
The transducer 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
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