Campbell CR23X, Operator'S Manual

Page: 91 / 374

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-3
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 CR23X. A typical
connection scheme where AC power is not
available and both the CR23X and sensor are
powered by an external battery is shown in
Figure 7.2-1. Since a single-ended
measurement is referenced to the CR23X
ground, any voltage difference between the
sensor ground and CR23X 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
differential CO
2
measurement using a LI-COR
CO
2
/H
2
O analyzer, model LI-6262.
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 CR23X
and LI-6262 are 2 ft and 10 ft, respectively.
Typical current drain for the LI-6262 is 1000 mA.
When making measurements, the CR23X draws
about 35 mA. Since voltage is equal to current
multiplied by resistance (V=IR), ground voltages
at the LI-6262 and the CR23X relative to battery
ground are:
LI-6262 ground =
1A
∗
6.5 ohms/1000 ft
∗
10 ft = +0.065 V
CR23X ground =
0.035A
∗
6.5 ohms/1000 ft
∗
2 ft = +0.0005 V
Ground at the LI-6262 is 0.065 V higher than ground
at the CR23X. The LI-6262 can be programmed to
output a linear voltage (0 to 100 mV) that is
proportional to differential CO
2
, 100
µ
mol/mol full
scale, or 1
µ
mol/mol/mV. If the output is measured
with a single-ended voltage measurement, it is
0.065 V or 65
µ
mol/mol 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 1 is used to convert
the millivolt output into
µ
mol/mol.
PROGRAM
01: Volt (Diff) (P2)
1:
1 Reps
2: 23
±
200 mV 60 Hz Rejection
Range
3: 1 DIFF Channel
4: 1 Loc [ umol_mol ]
5: 1 Mult
6: 0 Offset
7.3 THERMOCOUPLE TEMPERATURES
USING CR23X REFERENCE
The use of the built in CR23X 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 CR23X, it is
often better to use a reference junction box
located at the site rather than using the CR23X
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 CR23X. In
addition, if the temperature gradient between the
J-box and the thermocouple measurement
junction is smaller than the gradient between the
CR23X 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.
CR23X
FIGURE 7.4-1. Thermocouples with External
Reference Junction