SECTION 7. MEASUREMENT PROGRAMMING EXAMPLESFIGURE 7.2-1. Typical Connection for Active Sensor with External Battery
7.2DIFFERENTIAL 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

6.5ohms/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:

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

 

7-2