Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-2
Figure 7.2-1. Since a single- ended
measurement is referenced to the 21X ground,
any voltage difference between the sensor
ground and 21X 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 analyzes the ground potential
error of a single-ended measurement of pH
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 feet. The power runs to the 21X
and pH meter are 2 feet and 10 feet,
respectively. Typical current drain for the pH
meter is 300mA. When making measurements,
the 21X draws about 60mA. Since voltage is
equal to current times resistance (V=IR), ground
voltages at the pH meter and the 21X relative to
battery ground are:
pH meter ground =
0.3A x 10/1000 x 6.5ohms = +0.0195V
21X ground =
0.060A x 2/1000 x 6.5ohms = +0.0008V
Ground at the pH meter is 0.0187V higher than
ground at the 21X. 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 0.0187V or 0.26 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: 5 5000 mV slow Range
03: 1 IN Chan
04: 1 Loc [:pH ]
05: .014 Mult
06: 0 Offset
7.3 THERMOCOUPLE TEMPERATURES
USING 21X REFERENCE
The use of the built in 21X thermocouple
reference thermistor 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 21X, it is
often better to use a reference junction box
located at the site rather than using the 21X
panel for the reference junction. This reduces
the required length of expensive thermocouple
wire as regular copper wire can be used
between the junction box (J-box) and 21X. In
addition, if the temperature gradient between the
J-box and the thermocouple measurement
junction is smaller than the gradient between the
21X and the measurement junction,
thermocouple accuracy is improved. In the
following example, an external reference
junction is used on 5 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.