Operating Principles and Setting-up Procedure
This windvane is intended for the control of air sampling devices which are required to operate only when the wind is blowing from a particular sector. When a windvane is observed in normal weather conditions, it will be seen to be oscillating very rapidly about a mean position. If the such a vane were directly connected to an air sampler's pump, there would be a great deal of switching on and off which would damage the pump and give inaccurate results due to the finite time which the pump would take starting up and closing down. It is therefore necessary to build some hysteresis into the system. The simple solution is to introduce mechanical damping by friction or viscosity, but devices of this nature are temperature, time and position dependent and completely unreliable. In this instrument it was decided to allow the vane to oscillate freely in very low-friction acetal bearings and to introduce the hysteresis electronically. A digital Schmitt trigger was therefore devised in order that the performance of the vane would be totally predictable.
The windvane consists basically of a conventional vane/pointer system mounted on a 6mm diameter stainless steel shaft running in a pair of acetal bearings. This shaft drives a precision potentiometer capable of continuous rotation. The electronic system, contained in an IP67 sealed box, consists of a window detector, clock pulse generator and a series of programmable synchronous binary counters with associated logic gates. When the device is powered up, a count of 127 is placed in the registers. If the window detector sees a signal from the windvane within a pre-settable window, the clock pulses are counted upwards. If the count reaches 256, the windvane sets the output to +5 volts. If the window detector sees a signal outside the window, the clock pulses are counted downwards. If the count falls to zero, the windvane sets the output to 0 volts. If the status of the windvane changes before the count reaches 256 or zero, the count direction changes but the output will not change until the count finally reaches 256 or zero. The clock pulse generator is preset such that the counter takes approximately 30 seconds to perform an uninterrupted count from zero to 256. The windvane also delivers an output to drive an optional external low power LED, which indicates the current status of the vane, i.e. inside or outside the window, irrespective of whether the pump control output is 0 or +5 volts. The window, or acceptance angle, is initially set at ±30°, but can be reset by the user within the range ±0° to approximately ±90°. There is no in-built heating system, so the windvane may ice up in very severe conditions. The electronic components, however, will function down to about -40°C.
1. The balance of the arm and tailfin should be checked by holding the windvane so that both its main axis and arm/tailfin are horizontal and then observing whether the arm/tailfin remains horizontal. If a bias is observed, i.e. if either the nose or tailfin dip, then the balance should be restored by slackening only the grub-screw at the very top of the windvane and sliding the arm in the appropriate direction. A 2.5mm Allen key is provided for this purpose. At the same time the verticality of the tailfin should be checked and adjusted if necessary.
2. The window, or acceptance angle should now be set. This procedure is best carried out with the windvane in the relative comfort of ground level. If the preset arc of ±30° is acceptable, or if the user has requested some other figure when ordering the instrument, then no further adjustment will needed. However, should the user need to change the angle, the following procedure should be adopted:
Set the arm/tailfin to the required angle and secure using the Allen Key as a locking pin. If, for example, an arc of ±20° is required, the arm should be locked to the body of the windvane at either +20° or -20° relative to the zero position. A ring of 3mm diameter holes at 10° intervals is provided for this purpose. Open the IP67 box, power up the system and adjust the multi-turn potentiometer using the trimmer tool until the low-power LED (to be provided by the user) is toggling between on and off (clockwise rotation increases the angle). The arc is now set to the desired angle. Reseal the lid.
3. The windvane can now be fitted to the mast (provided by the user) using the bracket and clamp provided. Point the body of the instrument so that the centre of the arc is pointing in the desired direction (there is a short slot milled into the main body of the windvane indicating the arc's midpoint) and check using a spirit level that everything is true. Unlock the arm/tailfin assembly and check that it can rotate freely with hitting the mast or cables.
4. Although the user can alter the timebase speed and hence the switching, time, this is not advisable unless an oscilloscope is available. Please note that some of the electronic components are static-sensitive and can be destroyed by careless touching.