[wxqc] Sensor drift

[dr.]

The following are the specs direct from the Davis web site:

4, Wireless Vantage Pro® & Vantage Pro Plus™ Stations
VANTAGE PRO
Barometric Pressure (sensor located in console)
Resolution and Units . . . . . . . . . . . . . . . . . . . . . 0.01" Hg, 0.1 mm Hg, 0.1 hPa/mb (user-selectable)
Corrected Range . . . . . . . . . . . . . . . . . . . . . . . 26.00" to 32.00" Hg, 660.0 to 810.0 mm Hg, 880.0 to 1080.0 hPa/mb
Uncorrected Range. . . . . . . . . . . . . . . . . . . . . . 18.00" to 33.50" Hg, 457.0 to 850.0 mm Hg, 592.0 to 1130.0 hPa/mb
Elevation Range . . . . . . . . . . . . . . . . . . . . . . . . -999’ to +12,500’ (-305 m to 3810 m)
Uncorrected Reading Accuracy . . . . . . . . . . . . . ±0.03" Hg (±0.8 mm Hg, ±1.0 hPa/mb) (at room temperature)
Sea-Level Reduction Equation Used . . . . . . . . . United States Method employed prior to use of current "R Factor" method
Equation Source . . . . . . . . . . . . . . . . . . . . . . . . Smithsonian Meteorological Tables
Equation Accuracy . . . . . . . . . . . . . . . . . . . . . . ±0.01" Hg (±0.3 mm Hg, ±0.3 hPa/mb)
Elevation Accuracy Required . . . . . . . . . . . . . . ±10’ (3m) to meet equation accuracy specification
Overall Accuracy. . . . . . . . . . . . . . . . . . . . . . . . ±0.04" Hg (±1.0 mm Hg, ±1.4 hPa/mb)
Trend (change in 3 hours) . . . . . . . . . . . . . . . . . Change Š0.6" (2 hPa/mb, 1.5 mm Hg) = Rapidly
Change Š0.2" (.7hPa/mb, .5 mm Hg)= Slowly
Trend Indication . . . . . . . . . . . . . . . . . . . . . . . . 5 position arrow: Rising (rapidly or slowly), Steady, or Falling (rapidly or
slowly)
Update Interval . . . . . . . . . . . . . . . . . . . . . . . . . 15 minutes or when console BAR key is pressed twice
Current Data . . . . . . . . . . . . . . . . . . . . . . . . . . . Instant, 15-min., and Hourly Reading; Daily, Monthly, High and Low
Historical Data . . . . . . . . . . . . . . . . . . . . . . . . . 15-min. and Hourly Reading; Daily, Monthly Highs and Lows
Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Threshold from Current Trend for Storm Clearing (Rising Trend
Low Threshold from Current Trend for Storm Warning (Falling Trend)
Range for Rising and Falling Trend Alarms . . . . 0.01 to 0.25" Hg (0.1 to 6.4 mm Hg, 0.1 to 8.5 hPa/mb )

I just sent in a 30 day old console from a Vantage II Plus for the barometer moving over 5 to 10MB in a 24 hr time period after being set from a local standard known to be accurate. It is an automated station set up at a local airport by the FAA and NWS Ckeched regurly by them to be accurate to use for interment flying.  If I relied on the Davis it would have me screwed in the ground the way it goes all over the range.  This unit was not stable at all and did not even meet the specs!!!!

N5IHE




On 5/22/2006 11:40:48 AM, spamfree (spamfree at pensom.org) wrote:
> Hi,
> 
> It being a WM-II does make a difference.
> 
> Two thoughts come to mind. First, Davis kind of implies that the 
> barometer is not temperature compensated when they state in the manual 
> "The barometer may be less accurate at extreme temperatures [i.e., 
> less than 50°F (10°C) or greater than 90°F (32°C)].  Also, given the 
> way the barometer is set, I'm assuming they use a fixed calibration 
> offset to adjust for elevation. However, the algorithm for calculating 
> a true altimeter value does not produce a static delta between sensor 
> pressure and altimeter over all pressures. For example, at an 
> elevation of 460 ft., if the sensor pressure is 960.00 mb, the 
> altimeter would be 975.94 mb (a delta of 15.94), and if the sensor 
> pressure is 1020.00 mb, the altimeter would be 1036.75 mb (a delta of 
> 16.75). So, it's possible you could see see a drift of 1 mb