Operator`s manual

7-1
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
This section gives some examples of Input Programming for common sensors used with the 21X. 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 programs. In general, the
examples are written with the measurements made by the lowest numbered channels, the
instructions at the beginning of the program table, and low number Input Storage locations used to
store the data. It is unlikely that an application and 21X configuration exactly duplicates that assumed
in an example. THESE EXAMPLES ARE NOT MEANT TO BE USED VERBATIM; 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 DIFFERENTIAL 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/m
2
. 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 76.9µA/kW/m
2
, which
when multiplied by 100 ohms equals
7.69mV/kW/m
2
. The multiplier used to convert
the voltage reading to kW/m
2
is
1/7.69mV/kW/m
2
= 0.13004.
Most LI-COR calibrations run between 60 and
90µA/kW/m
2
, which correspond to calibrations
of 6.0 to 9.0mV/kW/m
2
. Above the earth's
atmosphere, the flux density through a surface
normal to the solar beam is 1.36kW/m
2
;
radiation on earth will be less than this. Thus,
the 15mV scale provides an adequate range
(9.0mV/kW/m
2
x 1.36 kW/m
2
< 15mV).
CONNECTIONS
The pyranometer output is measured with a
differential voltage measurement on channel 3.
The low side of signal is jumpered to datalogger
ground to hold the signal within common mode
range (Section 14.3-2).
FIGURE 7.1-1. Wiring Diagram for LI200S
PROGRAM
01: P2 Volt (DIFF)
01: 1 Rep
02: 2 15 mV slow Range
03: 3 IN Chan
04: 1 Loc [:RAD kW/m2]
05: .13004 Mult
06: 0 Offset
7.2 DATALOGGER AND SENSOR WITH
A COMMON EXTERNAL POWER
SUPPLY
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 21X. A typical
connection scheme where AC power is not
available and both the 21X and sensor are
powered by an external battery is shown in