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SECTION
7.
MEASUREMENT PROGRAMMING EXAMPLES
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.
lf
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
,
A
in
the chopper wheel, there
are
t0
pulses
per
I
'6
revolution. Thus,
1
rpm
is
equalto
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
conveft
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 mls/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.
lf
the artificially large wind
speed
that results from a long interual 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.
7-4
ANEMOMETER
SUPPLY
GROUND
AND
SIGNAT
REFERE
PULSE
OUI
01:
P3
01:
1
02:
1
r03:
20
04:
10
05:
06:
FIGURE
7.7-1. Wiring Diagram
for
Anemometer
PROGRAM
Pulse
Rep
Pulse Input Chan
High frequency;
Output
Hz.
Loc
[:WS
m/s
1
.0979
Mult
.2
Otfset
TIPPING BUCKET RAIN GAUGE
WITH LONG LEADS
A tipping bucket rain
gauge is measured with
the Pulse
Count Instruction configured for
Closure.
Counts
from long intervals will
be
(an option in Parameter 3), as the
final output
desired
is
total rainfall (obtained with lnstruction
T2,Totalize).
lf counts
from long intervals were
discarded, less
rainfallwould 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.
CONNECTIONS
3OO,
LEAD
LENGTH
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
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