Campbell Scientific CR10 CR10 Measurement and Control - Page 86

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES CRlO 1H '11 +12V G It- SI GN AL + PH METER ll - -*+12Y POWER 2FT, 18AWG POWER 1OFT, .18 AWG TXTERNAL 12 VOLT BATTERY 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 equalto current multiplied by resistance (V=lR), ground voltages at the pH meter and the CR10 relative to battery ground are: PH meter ground = 0.3A x 10/1000 x 6.5ohms = +0.0195V CR10 ground = 0.035A x2l1OO0 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. lf 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 lnstruction 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 02: 03: 215 Rep 2500 mV 60 Hz rejection lN Chan 04: 05: 1 0.014 Loc [:pH Mult 1 06: 0 Offset 7-2