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It would be fun to be able to directly read the windspeed from my Casella MkII anemometer. Using bicycle computers to obtain a speed reading from its rotating magnet and sensor system is nothing new. The copper rain skirt fitted to the anemometer's cup rotor provides a perfect magnet site. Requiring only a very small hole to hold the spoke magnet safely tucked under the skirt. The spoke clamp, usually attached to the magnet, can be discarded. The wired sensor works as normal and is fixed with zip ties to the rotor shaft housing just below the rotor skirt. I added a simple clamp from a bicycle front gear changer to ensure the sensor was securely fixed. Any movement over time might cause direct contact between magnet and sensor. This would lock up the rotor and might even cause damage. Not least to the bike computer components.
The magnet sensor is completely waterproof by design. So only needs its cable to be brought down the pole to the computer head bracket. (Also referred to as the shoe or docking station) Small contacts in the shoe match those on the back of the computer head or display unit.
A large watch battery is usually housed inside the display head to provide the readings. Six months of service is possible when the bicycle is ridden every day. Bike computer's are usually self-starting when the sensor notices a fresh magnet impulse after going into rest or sleep mode. However, the wind is far more constant than a bicycle is usually ridden so the battery may drain more quickly.
Other than speed a bicycle computer can offer average readings. Though one should be careful to avoid reading fallacies due to low or zero wind speed over given measurement periods. The average will be based on the total time even if no readings were taken during the measuring period.
There are several problems associated with adding a bicycle computer to an existing anemometer. The active circumference of the cup rotor is an unknown. Even if we measure the radius to the centre of each cup from the centre of the rotor shaft this may not help. It will requiring repeated resetting of circumference to obtain an accurate wind speed reading while comparing readings with another anemometer. Bicycle computers use the circumference of the wheel to be able to register speed and distance while counting pulses from the magnet's passage past the sensor. (Wheel or rotor rpm (counted from the pulses) x circumference will give the speed over time) Unfortunately there is slippage of the cups in the wind. It is not a direct drive.
Using another anemometer to calibrate the Casella will require care. Can one trust the reference anemometer? Are they both working at the same height and in the same wind conditions? Are they affecting each other? Is the reference anemometer able to accept wind from all points of the compass like a 3 cup anemometer can? Many digital anemometers have a small propeller or fan in an aperture in the housing. Care is required to ensure the wind is "on axis" with the rotating fan blades.
Climbing a tall ladder to read a bicycle computer is just silly. You want the computer screen to be easily read at ground level. Or even elsewhere. Unfortunately, wireless bicycle computers have a very short range between sensor and head. Were it otherwise the problem of placing the display head in a more accessible place would be almost trivial. Apart from the higher cost of a wireless computer compared with a wired model, of course.
Cutting into and extending the fine wires between the head contact shoe and sensor is possible with care. Though this in itself introduces at least two further problems.Will there be a voltage drop along the greatly extended cable? Will the soldered joints be waterproof? If the extension cable has resistance then there might not be enough voltage from the watch battery in the head to ensure successful operation of the head. Will the extended cable act as a lightning conductor?
How will the voltage be increased if necessary? The head has no room for an extra battery and the batteries have a fixed voltage in each size. Wires could be soldered to the battery contacts inside the head and then brought out through the back sealing cover. Another form of battery could then be connected to ensure extended periods of service. While also providing a higher voltage if it proves essential to operate the bicycle computer despite the much longer cable. Though I doubt any sensible cable would add any resistance. Ordinary, double insulated, lighting flex would probably do.
An update: I have fitted the magnet and tried different circumference settings on the bike computer. A setting of 4000 and miles produced great sensitivity and about a 10mph reading by pushing the cups quickly by hand. At least this offers a ballpark figure to start experimenting. As the head is easily unclipped from its shoe the circumference settings are easily changed. Though it a a fairly slow process because the computer has to be reset completely from scratch each time.
I measured about 30 yards/metres of double insulated, 0.7m^2, twin lighting flex in series (for 60 metres total) at about 0.4 Ohms. So absolutely no need to worry about voltage drop.
Here are the wheel magnet and bike wheel computer sensor. The sensor is fixed to a bicycle, front gear changer, seat tube clamp with zip ties for security.
Well, now I have the system set up with a new bike wheel sensor. This had better wiring which allowed me to solder the ends direct to the twin core lighting flex. The other end I soldered the flex direct to the the computer head contact shoe. On a windy day like today with gusts to 15m/s I have hit 30 mph readings several times using the mph and 4000 circumference computer settings.
Comparisons with my handheld anemometers showed similar maximum readings but the Mk2 was incredibly slow to speed up and slow again during gusts. It had tremendous "inertia" compared with the tiny propellers on the handheld electronic devices.
I have screwed the shoe to the shed at head height below the anemometer so I can read it easily. I was going to run the flex to the house but was short by a few yards.
Trying to build my modest collection of vintage, British, cup anemometers is proving very difficult. Sellers using eBay(UK) are automatically blocking all overseas bids. Which means they sell for absolute peanuts. Fine if you live in the UK. Foolish for the seller if the potential buyers live elsewhere.
An update: I have fitted the magnet and tried different circumference settings on the bike computer. A setting of 4000 and miles produced great sensitivity and about a 10mph reading by pushing the cups quickly by hand. At least this offers a ballpark figure to start experimenting. As the head is easily unclipped from its shoe the circumference settings are easily changed. Though it a a fairly slow process because the computer has to be reset completely from scratch each time.I measured about 30 yards/metres of double insulated, 0.7m^2, twin lighting flex in series (for 60 metres total) at about 0.4 Ohms. So absolutely no need to worry about voltage drop.
Here are the wheel magnet and bike wheel computer sensor. The sensor is fixed to a bicycle, front gear changer, seat tube clamp with zip ties for security.
Well, now I have the system set up with a new bike wheel sensor. This had better wiring which allowed me to solder the ends direct to the twin core lighting flex. The other end I soldered the flex direct to the the computer head contact shoe. On a windy day like today with gusts to 15m/s I have hit 30 mph readings several times using the mph and 4000 circumference computer settings.
Comparisons with my handheld anemometers showed similar maximum readings but the Mk2 was incredibly slow to speed up and slow again during gusts. It had tremendous "inertia" compared with the tiny propellers on the handheld electronic devices.
I have screwed the shoe to the shed at head height below the anemometer so I can read it easily. I was going to run the flex to the house but was short by a few yards.
Trying to build my modest collection of vintage, British, cup anemometers is proving very difficult. Sellers using eBay(UK) are automatically blocking all overseas bids. Which means they sell for absolute peanuts. Fine if you live in the UK. Foolish for the seller if the potential buyers live elsewhere.
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