Unit installation

12-23

109: Configure Wind Speed Reading

This command can be used to tell the controller to count the pulses coming from an anemometer

(those whirly things with three cups that spin in the wind) to determine the wind speed. It counts

the number of pulses in a default of 2.26 seconds and treats that number like a voltage that could be

read by an analog line. All of the normal analog commands will work with the resulting number:

command 102 will control the scaling, command 100 will provide the current reading, the analog

alarms can be used to notify you if the wind exceeds a certain speed, etc.

<109> 00 Don’t calculate wind speed

<109> XX Use contact closure input line ‘XX’

for wind speed

Parameters:

XX - the contact closure input line to use for wind speed. 1..8 for I/O board #1, 9..16 for

I/O board #2.

Details:

To read wind speed, hook up the anemometer so that as it rotates, the reed switch intermittently

connects a contact closure input line on the controller to ground. Using a voltmeter or logic probe,

make sure that the line pulses every time the anemometer goes around. The line can be connected

directly to any of the contact closure input lines. Then use command 109 to tell the controller

which input line it is connected to. From that point on, reading the analog line of that number will

give you a number related to the wind speed. Note that although the anemometer is connected to

contact closure input line, the wind speed is recalled using the commands for the analog line. The

actual voltage read by the analog line can’t be read while that line is being used for wind speed.

To make the controller read the wind speed in miles per hour, you will need to find out how fast

your anemometer pulses relative to the wind speed. One common anemometer (Davis) is rated at

26.6 pulses per minute per mile per hour. 60/26.6 = 2.2556 seconds to get one pulse per mile per

hour. The wind speed timer is set to count the pulses in 2.26 seconds. If the analog conversion

ratio is left at the default 1:1 (it can be changed with command 102), the analog line will read the

speed in miles per hour. The wind speed measurement can be adjusted by changing the timer or by

changing the conversion ratio with command 102. Using the calibrate command for wind speed is

not recommended, as any calibration offset will keep it from reading zero when it is calm. You can

use command 100 or the analog special words to make the controller speak the wind speed.