Posted by Richard Campbell In Vallejo, Calif. on January 30, 2003 at 12:46:29:
In Reply to: Re: Heathkit/Vantage Pro comparisons posted by anon on January 29, 2003 at 15:03:37:
: I was just making a point :)
: (and that there are instaneous gusts that last less than a second)
: The old MUNRO stations with its voltage system (but large heavy cups) are very good at capturing instantenous gusts.
: The ultimeter samples every second, as does the heathkit.
: The other thing richard is in the design I use, its a 4 pole magnet, and a hall effect sensor in a maximum instruments anenometer. at 75 knots, the RPM and pulses generated means you do need that a high sampling rate (much higher than 2.25 seconds) to capture all the changes in speed.
: by measuring the time between pulses, that can then be extrapolated to the whole second, to get the instaneous gust. This method does actualy work
: just my thoughts.
Hi Anon,
You might find a simple experiment to be of value in estimating the actual value of a high sampling rate for a cup type anemometer. No matter what type of counter is used (reed switch, multiple diodes. etc.) there is always the problem of inertia with a rotating mechanism. Just measure the time that elapses before the cups ramp up to full speed from a dead halt using your vacuum cleaner outlet as a moving air source. Then measure how long it takes for the cups to return to zero after the artificial wind source has been removed. Conservation of angular momentum dictates that a heavier cup assembly must take a longer time to accelerate. I think that you will find from the empirical evidence derived from this little test that sampling rates of less than one second with a cup-type unit have little meaning because of the inertia of the rotating mechanism. In actual practice for a weather instrument, a sample rate of 5 seconds would be reasonably accurate because of the larger volumes of air that are involved. Often, in the storms here on the Pacific Coast, the wind pulses have very long durations, long enough to set up harmonic swings on something as massive as the Golden Gate Bridge. Even in swirling, squall-like conditions, wind direction and speed take a few seconds to stabilize. There is little value in a very high sample rate for wind direction. What good is any directional value when a dust devil causes the directional vane to start spinning wildly?
The fastest wind speed that I have ever recorded at this location on my Heathkit station was 52 mph over a 20 year period. That is precisely the highest reading that I have recorded on the Vantage Pro station during the last year. Again, the empirical data tends to indicate that the two instruments are similar in their ability to capture high wind-gust velocities. Maintenance wise, I found the Heathkit multiple photo diode sytem of measurement to be very reliable; however, the Heathkit bearing system required semi-annual cleaning and oiling to maintain accuracy, and the bearing assembly needed replacing every few years. The VP anemometer appears to be maintenance free so far, although some users have reported failures caused by cracks in the reed switch glass envelope.
To effectively apply a faster sampling rate if you really insist on such a thing, you would have to use a mass flow sensor similar to the type used in automobile intake manifolds. That type of sensor can furnish data very quickly for adjustment of the fuel/air mixture because it doesn't have any movable parts. I have no idea how accurate the automobile type mass flow sensors are. They would not have to be extremely accurate because the final fuel/air ratio is fine tuned through the use of oxygen sensors in the exhaust manifolds. I also have no idea how much electrical power those auto sensors use, but perhaps you could design the perfect, high-sample rate anemometer using such a sensor.
Richard
Subject:
Comments:
Optional Link URL:
E-Mail:
Link Title:
Optional Image URL: