Campbell Scientific CR7 Measurement and Control System - Manual (Page 70)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-2
Figure 7.2-1. Since a single ended
measurement is referenced to the CR7 ground,
any voltage difference between the sensor
ground and CR7 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.
FIGURE 7.2-1. Typical Connection for Active
Sensor with External Battery
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 runs to the
CR7 and pH meter are 2 ft. and 10 ft.,
respectively. Typical current drain for the pH
meter is 300 mA. When making
measurements, the CR7 draws about 100 mA.
Since voltage is equal to current times
resistance (V=IR), ground voltages at the pH
meter and the CR7 relative to battery ground
are:
pH meter ground =
0.3A x 10/1000 x 6.5 Ohms = +0.0195V
CR7 ground =
0.1A x 2/1000 x 6.5ohms = +0.0013V
Ground at the pH meter is 0.0182V higher than
ground at the CR7. 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 0.0182V or 0.25 pH units too
high. If this offset remained constant, it could
be corrected in programming the CR7.
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: 7 1500 mV slow Range
03: 1 IN Card
04: 1 IN Chan
05: 1 Loc [:pH ]
06: 0.014 Mult
07: 0 Offset
7.3 THERMOCOUPLE TEMPERATURES
USING 723-T REFERENCE
The use of the 723-T Analog Input Card RTD to
measure the reference temperature is
described in the introductory programming
example (Section OV4).
7.4 THERMOCOUPLE TEMPERATURES
USING AN EXTERNAL REFERENCE
JUNCTION
When a number of thermocouple measurements
are made at some distance from the CR7, it is
often better to use a reference junction box
located at the site rather than using the panel
temperature of the CR7. This reduces the
required length of expensive thermocouple wire
as regular copper wire can be used between the
junction box (J-box) and CR7. In addition, if the
temperature gradient between the J-box and the
thermocouple measurement junction is smaller
than the gradient between the CR7 and the
measurement junction, thermocouple inaccuracy
is reduced. In the following example, an external
reference junction is used on ten thermocouple
measurements. A Campbell Scientific 107
Temperature Probe is used to measure the
reference temperature. The connection scheme
is shown in Figure 7.4-1.
FIGURE 7.4-1. Thermocouples with External
Reference Junction