Campbell Scientific CR7 Measurement and Control System - Manual (Page 74)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-6
PROGRAM
02: P12 RH 207 Probe
01: 3 Reps
02: 1 IN Card
03: 4 IN Chan
04: 1 EX Card
05: 1 EX Chan
06: 1 Meas/Temp
07: 1 Temperature Loc 207 T#1
08: 4 Loc [:RH #1 ]
09: 1 Mult
10: 0 Offset
7.9 ANEMOMETER WITH
PHOTOCHOPPER OUTPUT
An anemometer with a photochopper
transducer produces a pulsed output which is
monitored with the Pulse Count Instruction,
configured for High Frequency Pulses. The
Pulse Count Instruction counts the number of
pulses occurring in each execution interval. An
option in the instruction allows this to be
converted to frequency in Hertz (i.e.,
Pulses/Second). The anemometer used in this
example is the R. M. Young Model No. 12102D
Cup Anemometer, with a 10 window chopper
wheel. The photochopper circuitry is powered
from the CR7 12V supply; AC power or backup
batteries should be used to compensate for the
increased current drain.
Wind speed is desired in meters per second.
There is a pulse each time a window in the
chopper wheel, which revolves with the cups,
allows light to pass from the source to the
photoreceptor. Because there are 10 windows
in the chopper wheel, there are 10 pulses per
revolution. Thus, 1 rpm is equal to 10 pulses
per 60 seconds (1 minute) or 6 rpm = 1 pulse
per second. The manufacturer's calibration for
relating wind speed to rpm is:
Wind speed (m/s) =
0.01632 m/s/rpm x rpm +0.2 m/s
The multiplier and offset to convert pulses per
second to meters per second are:
m/s =
0.01632 m/s/rpm x 6 rpm/(pulse/s)
+ 0.2 m/s = 0.0979 m/s/pulse x
pulses + 0.2 m/s
There are occasionally times when the CR7's
CPU is occupied and does not reset the pulse
counters at the exact time interval programmed.
If the artificially large wind speed that results
from a long interval is used, it causes a false
average or maximum value. To avoid this, the
CR7 is instructed to discard values resulting
from long intervals, and use the previous value
instead.
FIGURE 7.9-1. Wiring Diagram for
Anemometer
PROGRAM
(Execution interval 10 seconds)
01: P3 Pulse
01: 2 Reps
02: 2 IN Card
03: 2 Pulse Input Chan
04: 20 High frequency; Output Hz.
05: 10 Loc [:WS m/s ]
06: .0979 Mult
07: .2 Offset
7.10 TIPPING BUCKET RAINGAGE
WITH LONG LEADS
A tipping bucket raingage is measured with the
Pulse Count Instruction configured for Switch
Closure. Counts from long intervals will be
used, as the final output desired is total rainfall
(obtained with Instruction 72, Totalize). If
counts from long intervals were discarded, less
rainfall would be recorded than was actually
measured by the gage (assuming there were
counts in the long intervals). Output is desired
in millimeters of precipitation; the gage is
calibrated for a 0.01 inch tip so a multiplier of
0.254 is used.