Campbell Scientific CR10X Measurement and Control System - Manual (Page 87)

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 CR10X. A typical
connection scheme where AC power is not
available and both the CR10X and sensor are
powered by an external battery is shown in
Figure 7.2-1. Since a single-ended
measurement is referenced to the CR10X
ground, any voltage difference between the
sensor ground and CR10X 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 CR10X
and LI-6262 are 2 ft and 10 ft, respectively.
Typical current drain for the LI-6262 is 1000 mA.
When making measurements, the CR10X draws
about 35 mA. Since voltage is equal to current
multiplied by resistance (V=IR), ground voltages
at the LI-6262 and the CR10X relative to battery
ground are:
LI-6262 ground =
1A ∗ 6.5 ohms/1000 ft ∗ 10 ft = +0.065 V
CR10X 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 CR10X. The LI-6262 can be programmed to
output a linear voltage (0 to 100 mV) that is
proportional to differential CO
2
, 100
µmol/mol full
scale, or 1
µmol/mol/mV. If the output is measured
with a single-ended voltage measurement, it is
0.065 V or 65
µmol/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
µmol/mol.
PROGRAM
01: Volt (Diff) (P2)
1:
1 Reps
2: 25
±2500 mV 60 Hz Rejection
Range
3: 1 DIFF Channel
4: 1 Loc [ umol_mol ]
5: 1 Mult
6: 0 Offset
CAMPBELL
SCIENTIFIC
INC.
CR10
MADE IN USA
WIRING PANEL NO.
EARTH
DIFF
SE
AG H L AG H L AG H L AG G G E3 AG
4 5 6
7 8 9 10 11 12
G G G G 12V 12V
SWITCHED
12V
G 12V
POWER
IN
SERIAL I/O
SE
DIFF AG H L AG H L AG H L AG G G E1 E2
1 2 3
1 2 3 4 5 6
G 5V 5V P1 P2 C8 C7 C6 C5 C4 C3 C2 C1
CONTROL
12V
SWITCHED
CR10TCR
LT#343403
FIGURE 7.3-1. CR10TCR Mounted on the CR10X Wiring Panel