Differential Voltage Measurement; Typical Connection For Active Sensor With External Battery - Campbell Measurement and Control Module CR10 Operator's Manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
FIGURE 7.2-1. Typical Connection for Active Sensor with External Battery
7.2 DIFFERENTIAL VOLTAGE
MEASUREMENT
Some sensors either contain or require active
signal conditioning circuitry to provide an easily
measured analog voltage output. Generally, the
output is referenced to the sensor ground. The
associated current drain usually requires a
power source external to the CR10. A typical
connection scheme where AC power is not
available and both the CR10 and sensor are
powered by an external battery is shown in
Figure 7.2-1. Since a single-ended
measurement is referenced to the CR10 ground,
any voltage difference between the sensor
ground and CR10 ground becomes a
measurement error. A differential measurement
avoids this error by measuring the signal
between the 2 leads without reference to
ground. This example analyzes the potential
error on a water pH measurement using a
Martek Mark V water quality analyzer.
The wire used to supply power from the external
battery is 18 AWG with an average resistance of
6.5 ohms/1000 ft. The power leads to the CR10
and pH meter are 2 ft and 10 ft, respectively.
Typical current drain for the pH meter is 300 mA.
When making measurements, the CR10 draws
about 35 mA. Since voltage is equal to current
multiplied by resistance (V=IR), ground voltages
at the pH meter and the CR10 relative to battery
ground are:
7-2
pH meter ground =
0.3A x 10/1000 x 6.5ohms = +0.0195V
CR10 ground =
0.035A x 2/1000 x 6.5ohms = +0.0005V
Ground at the pH meter is 0.0190 V higher than
ground at the CR10. The meter output is 0-1
volt referenced to meter ground, for the full
range of 14 pH units, or 0.0714 V/pH. Thus, if
the output is measured with a single-ended
voltage measurement, it is 0.0190 V or 0.266
pH units too high. If this offset remained
constant, it could be corrected in programming.
However, it is better to use a differential voltage
measurement which does not rely on the
current drain remaining constant. The program
that follows illustrates the use of Instruction 2 to
make the measurement. A multiplier of 0.014 is
used to convert the millivolt output into pH
units.
PROGRAM
01: P2 Volt (DIFF)
01: 1 Rep
02: 25 2500 mV 60 Hz rejection
03: 1 IN Chan
04: 1 Loc [:pH
05: 0.014 Mult
06: 0 Offset
]