Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-4
7.7 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 anemometer used
in this example is the R.M. Young Model No.
12102D Cup Anemometer which has a 10
window chopper wheel. The photochopper
circuitry is powered from the 21X or 21XL 12V
supply. Supplemental charging, AC or solar,
should be used with the 21XL. If a charging
source is not practical, back-up 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
Pulse count instruction has the option of
converting counts to frequency in Hz
(counts/second). The multiplier and offset to
convert Hz to meters per second are:
m/s =
0.01632 m/s/rpm x 6 rpm/Hz x XHz + 0.2 m/s
= 0.0979 m/s/Hz x XHz + 0.2 m/s
There are times when the 21X is not able to
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 21X is instructed to discard
values resulting from long intervals and use the
previous value instead.
FIGURE 7.7-1. Wiring Diagram for
Anemometer
PROGRAM
01: P3 Pulse
01: 1 Rep
02: 1 Pulse Input Chan
03: 20 High frequency; Output Hz.
04: 10 Loc [:WS m/s ]
05: .0979 Mult
06: .2 Offset
7.8 TIPPING BUCKET RAIN GAUGE
WITH LONG LEADS
A tipping bucket rain gauge is measured with
the Pulse Count Instruction configured for Switch
Closure. Counts from long intervals will be used
(an option in Parameter 3), 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 gauge (assuming
there were counts in the long intervals). Output
is desired in millimeters of precipitation. The
gauge is calibrated for a 0.01 inch tip; a
multiplier of 0.254 is used.
FIGURE 7.8-1. Wiring Diagram for Rain
Gauge with Long Leads
In a long cable there is appreciable capacitance
between the lines, which is discharged across
the switch when it closes. In addition to