Re: Heathkit/Vantage Pro comparisons

Posted by Richard Campbell In Vallejo, Calif. on January 31, 2003 at 18:31:42:

In Reply to: Re: Heathkit/Vantage Pro comparisons posted by anon on January 31, 2003 at 00:35:17:

: Hi
: well, if you watch a generator/volt meter anenometer (like the munro), then you will see what i am gettting at (in winds of 50 to 65 knots)
: i.e, the needle and drum recording do change (ramp up) very fast, in a spli second in fact.
: I have seen this
: and thats with a large and heavy cup suystem like the munro!
: a samll light weight ciup assebmly found on home weather stations has extremly good respnse times, becuase they are light weigth:
: i.e wind force is a square function, and sudden change from 50 kts to 60 kts is a large change in force applies, and so a light plastic low resistance to turning cup anenometer has no trouble chaging its acceleration to match the windspeed...i.e the stronger the wind, the more responsive it will be because it becomes more of a goliath against a david.
: Just my observations!

Hi Anon,

Actually, wind force is not a square function. Newton's first law is Force = Mass x Acceleration.
If you are referring to momentum, that equals mass times velocity. Momentum is a measure of inertial force, and that is the key factor that limits the acceleration rate. In this case, the angular momentum of the rotating cup assembly acts to limit acceleration. The squaring effect that you refer to comes into play if you are talking about energy. Energy equals mass times velocity squared, but energy is not momentum.

In any case, a large heavy cup assembly, especially one with a significant magnetic friction component, cannot possibly be as accurate as a light-weight instrument in measuring rapid changes in wind speed. I have a large hedgerow of Eucalyptus Robustus on my property. This particular type of tree has a large sail area and a very flexible trunk and branch network. In stormy weather, I can watch them lean way over when assailed by heavy gusts. The tops of the trees which are about 40 feet tall may move 15 feet or more in a heavy wind. The Weatherlink display from my Vantage Pro mirrors the observable conditions outside in real time for all practical purposes. I cannot physically see any lag time between the wind speed shown on my computer, and the effects of the wind on the trees. Of course, a tree cannot instantly bend over just as the wind cups cannot
instantly change speed.

In short. the 2-1/2 second update interval used by Davis is perfectly sufficient for measuring even quickly changing variables such as wind speed and direction. I certainly would not rate a shorter update interval as very high on my priority list. In fact, if Davis extended the update interval to five seconds in order to use a more complex statistical error correction scheme on the radio transmissions, I would consider that a more important factor in judging the quality of a weather station.

Just my personal opinion based on actual observation,

Richard

Follow Ups:

• Re: Heathkit/Vantage Pro comparisons anon 12:57:31 02/02/03 (0)
• Re: Heathkit/Vantage Pro comparisons Steve/ Prescott Valley 07:10:58 02/01/03 (0)

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Comments:
: : Hi : : well, if you watch a generator/volt meter anenometer (like the munro), then you will see what i am gettting at (in winds of 50 to 65 knots) : : i.e, the needle and drum recording do change (ramp up) very fast, in a spli second in fact. : : I have seen this : : and thats with a large and heavy cup suystem like the munro! : : a samll light weight ciup assebmly found on home weather stations has extremly good respnse times, becuase they are light weigth: : : i.e wind force is a square function, and sudden change from 50 kts to 60 kts is a large change in force applies, and so a light plastic low resistance to turning cup anenometer has no trouble chaging its acceleration to match the windspeed...i.e the stronger the wind, the more responsive it will be because it becomes more of a goliath against a david. : : Just my observations! : Hi Anon, : Actually, wind force is not a square function. Newton's first law is Force = Mass x Acceleration. : If you are referring to momentum, that equals mass times velocity. Momentum is a measure of inertial force, and that is the key factor that limits the acceleration rate. In this case, the angular momentum of the rotating cup assembly acts to limit acceleration. The squaring effect that you refer to comes into play if you are talking about energy. Energy equals mass times velocity squared, but energy is not momentum. : In any case, a large heavy cup assembly, especially one with a significant magnetic friction component, cannot possibly be as accurate as a light-weight instrument in measuring rapid changes in wind speed. I have a large hedgerow of Eucalyptus Robustus on my property. This particular type of tree has a large sail area and a very flexible trunk and branch network. In stormy weather, I can watch them lean way over when assailed by heavy gusts. The tops of the trees which are about 40 feet tall may move 15 feet or more in a heavy wind. The Weatherlink display from my Vantage Pro mirrors the observable conditions outside in real time for all practical purposes. I cannot physically see any lag time between the wind speed shown on my computer, and the effects of the wind on the trees. Of course, a tree cannot instantly bend over just as the wind cups cannot : instantly change speed. : In short. the 2-1/2 second update interval used by Davis is perfectly sufficient for measuring even quickly changing variables such as wind speed and direction. I certainly would not rate a shorter update interval as very high on my priority list. In fact, if Davis extended the update interval to five seconds in order to use a more complex statistical error correction scheme on the radio transmissions, I would consider that a more important factor in judging the quality of a weather station. : Just my personal opinion based on actual observation, : Richard

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