Section 7. Measurement Programming Examples
7-3
;Turn 50Y off.
;
05:
Do (P86)
1:51
Set Port 1 Low
INPUT LOCATION LABELS
1: Temp_C
2: RH_pct
7.2 Differential Voltage Measurement
Some sensors either contain or require active signal conditioning circuitry to
provide an easily measured analogue voltage output. Generally, the output is
referenced to the sensor ground. The associated current drain usually requires a
power source external to the CR10X. A typical connection scheme where AC
power is not available and both the CR10X and sensor are powered by an external
battery is shown in Figure 7-2. Since a single-ended measurement is referenced to
the CR10X ground, any voltage difference between the sensor ground and CR10X
ground becomes a measurement error. A differential measurement avoids this
error by measuring the signal between the two leads without reference to ground.
This example analyses the potential error on a water pH measurement using a
Martek Mark V water quality analyser.
CR10X
PH Meter
+12V
G
External 12V Battery
Signal
+12V
1L
1H
3m, 1mm Diameter
+
1m, 1mm Diameter
Power
Power
+
–
Figure 7-2 Typical Connection for Active Sensor with External Battery
The wire used to supply power from the external battery is 1mm diameter with an
average resistance of 2.2 ohms/100m. The power leads to the CR10X and pH
meter are 1m and 3m, respectively. Typical current drain for the pH meter is
300mA. When making measurements, the CR10X draws about 35mA. Since
voltage is equal to current multiplied by resistance (V=IR), ground voltages at the
pH meter and the CR10X relative to battery ground are:
pH meter ground = 0.3 x 2.2/100 x 3 = +0.0198V
CR10X ground = 0.035 x 2.2/100 x 1 = +0.0008V
Ground at the pH meter is 0.0190V higher than ground at the CR10X. The meter
output is 0-1 volt referenced to meter ground, for the full range of 14 pH units, or
0.0714V/pH. Thus, if the output is measured with a single-ended voltage
measurement, it is 0.0190V or 0.266pH 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 re-
maining constant. The program that follows illustrates the use of Instruction 2 to