Campbell Scientific CR7 Measurement and Control System - Manual (Page 69)

7-1
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
This section gives some examples of Input Programming for common sensors used with the CR7. These
examples detail only the connections, Input, Program Control and Processing Instructions necessary to
perform measurements and store the data in engineering units in Input Storage. Output Processing
Instructions are omitted, it is left for the user to program the necessary instructions to obtain the final
data in the form desired. NO OUTPUT TO FINAL STORAGE WILL TAKE PLACE WITHOUT
ADDITIONAL PROGRAMMING.
The examples given in this section would likely be only fragments of larger program tables. In general,
the examples are written with the measurements made by the first channels on the first cards in the I/O
Module, the instructions at the beginning of a program table, and low number Input Storage locations
used to Store the data. Because it is unlikely that an application and CR7 configuration exactly
duplicates that assumed in an example, THESE EXAMPLES ARE NOT MEANT TO BE USED
VERBATIM; CARDS AND CHANNELS REFERENCED, SENSOR CALIBRATION AND INPUT
LOCATIONS SELECTED MUST BE ADJUSTED FOR THE ACTUAL CIRCUMSTANCES. UNLESS
OTHERWISE NOTED, ALL EXCITATION CHANNELS ARE SWITCHED ANALOG OUTPUT.
7.1 SINGLE ENDED VOLTAGE - LI200S
SILICON PYRANOMETER
The silicon pyranometer puts out a current
which is dependent upon the solar radiation
incident upon the sensor. The current is
measured as the voltage drop across a fixed
resistor. The Campbell Scientific LI200S uses a
100 ohm resistor. The calibration supplied by
LI-COR, the manufacturers of the pyranometer,
is given in uA/kW/m2. The calibration in terms
of volts is determined by multiplying the µA
calibration by the resistance of the fixed
resistor.
The calibration of the pyranometer used in this
example is assumed to be 76.9 µA/kW/m2,
which when multiplied by 100 ohms equals 7.69
mV/kW/m2. The multiplier used to convert the
voltage reading to kW/m2 is 1 / 7.69 mV/kW/m2
= 0.13004.
Most LI-COR calibrations run between 60 and
90 µA/kW/m2, which correspond to calibrations
of 6.0 to 9.0 mV/kW/m2. The flux density
through a surface normal to the solar beam
above the earth's atmosphere is 1.36 kW/m2;
radiation on earth will be less than this. Thus,
the 15 mV scale provides an adequate range
(9.0 mV/kW/m2 x 1.36 kW/m2 < 15 mV).
CONNECTIONS
The pyranometer output is measured with a
single ended voltage measurement on channel
5. There are twice as many single ended
channels as differential channels and they are
numbered accordingly: single ended channel 5
is the high side of differential channel 3, and the
low side is single ended channel 6.
FIGURE 7.1-1. Wiring Diagram for LI200S
PROGRAM
01: P1 Volt (SE)
01: 1 Rep
02: 3 15 mV slow Range
03: 1 IN Card
04: 5 IN Chan
05: 1 Loc [:R kW/m^2 ]
06: .13004 Mult
07: 0 Offset
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 CR7. A typical
connection scheme where AC power is not
available and both the CR7 and sensor are
powered by an external battery is shown in