A short calibration study of the Vantage Pro Plus weatherstation


Francis Massen, meteoLCD 
vantage_calibration.html  version 1.0  16 Feb 2003

 

Abstract

A 20 day long intercomparison between a Vantage Pro Plus weatherstation from Davis and the (professional grade) instruments of meteoLCD shows excellent to good behaviour of the Vantage Pro Plus. Solar and UVB sensors deliver fair results, but one should always keep in mind the more modest accuracy of these inexpensive radiation sensors. 

1.     Operational procedures

A new (never used) Vantage Plus Pro weatherstation ( wireless model, to be used by Prof. A. Kies, Physics Lab, Centre Universitaire, Luxembourg) was mounted at the meteoLCD site and operated for the period from 17 Jan 2003 to 5 Feb 2003 for comparison and calibration factors (sampling interval = 30 minutes). The sensors used as a reference are those of meteoLCD (see the description).

The aim of the study is to check if the rather inexpensive and easy to setup Vantage Pro Plus, manufactured by Davis in the USA, could qualify as a valid weatherstation compared to the professional-grade sensors of meteoLCD.

 Due to an operator glitch, the rain-sensor was operational only from 26 Jan, 16:00 on. The wind direction sensor was not calibrated, so it will not be used in this study. All data were imported into Statistica 6.1. for analysis and graphing; the Excel file test_davis1a.xls corresponds to these data.When calibrating the Vantage Pro Plus, an xy graph with x-axis = Vantage Pro Plus, y-axis = meteoLCD, was used; the linear regression line gives the mathematical function to apply to the Vantage readings to obtain the (assumed) correct meteoLCD values.


2.     Correlation matrix

  The correlation coefficients are all significant at the alpha = 0.05 level; many are 1.



3.     Outside (air-) temperature: (click on small picture to get full size)

Conclusion:

Good behaviour of the Vantage Pro sensor; scaling factor is practically 1. Negligeable offset.



4.  Outside (air) relative humidity

Conclusion:

Acceptable behaviour of the Vantage Pro sensor, humidity values during wet conditions too high; scaling factor practically 0.9; offset about 6 %.

 


5.  Air pressure

Conclusion:

The Vantage sensor was adjusted for real air pressure ( not barometric pressure reduced to sea level) after several hours of working (see start of upper graph). The linear regression is computed on the remaining (comparable) data. Good behaviour of the Vantage Pro sensor: scaling factor practically 0.97; zero-offset not relevant!

 


6. Wind velocity

Conclusion:

Good behaviour of the Vantage Pro sensor; scaling factor practically 0.89; negligeable offset. The sensor was mounted approx. 1 meter lower than the meteoLCD anemometer.

 


  7.  Solar radiation

Conclusion:

1. The meteoLCD sensor is a ventilated CM21 Kipp & Zonen pyranometer.
2. Uncorrected Vantage Pro data are too low of about 7% (scaling factor about  1.07). Small offset, but non-negligeable scattering.
3. Applying the manufacturers  temperature coefficient. of -/+0.12% per C (above/about 25C) betters the scaling factor to 1.04. As measurements were done at a mean outside temperature of 2.4C, this comparison should be repeated at hot sunny summer time.


8. Effective (erythemal) UVB irradiation

The Vantage Pro applies a multilayered filter to approximate the UVB sensors output to the CIE (Diffey) erythemal spectrum; this approximation may be acceptable, but there exist notable deviation in the 310-320 nm region (see Davis info).
Fine reading the specifications shows that Davis applies a (non-European) definition of


1 MED = 220 J/m2


This should be adjusted for European comparisons, where 1 MED = 250 J/m2. The uncorrected UVI-reading of the Vantage Pro should probably be multiplied by 220/250 (Vantage UVI s are probably too high: 1 UVI is 1/40 W/m2 effective UVB; depending on the computation of the UVI (from eff. power? from MED ?)).
There also is some confusion regarding the UVB sensor readings: what Davis calls DOSE ( in the .pdf info-flyer) is a dose-rate in MED/hr. Only if the sampling interval is 1 hour, dose and dose-rate are expressed by the same number. The data column UVB shows readings that the author is incapable to decipher: it is not effective power, as one could think. Thus the comparison is done using the column called DOSE as representing the dose-rate, and comparing to the output of the UVB 501A Biometer of meteoLCD.

Conclusion:

1. The meteoLCD sensor is a thermally stabilized instrument; the Vantage Pro sensor is shielded by a ventilation ring
2. The Vantage Pro data are about 10% low (scaling factor about 0.89); non-negligeable scattering.
3. As measurements were done at a mean temperature of 2.4, this comparison should be repeated at hot sunny summer time.




9. Summary

 This short comparison shows that the Vantage Pro Plus delivers excellent to good quality data, compared to professional-grade sensors. One should not expect too much from the rather simple solar and UVB sensors: certainly their accuracy is way too low to detect minor long-time trends in these parameters. If the solar sensor is used to measure global (monthly or yearly) irradiance (for instance to check the available local yearly solar-energy ), the  accuracy is good enough. This can also be said for the UVB sensor, which should be taken as an inexpensive and sufficient accurate mean to gather UVIs (which as a matter of fact are always rounded to integer numbers).

The Vantage Pro Plus is a good buy for an easy to setup weatherstation to be used in the field (as a temporary station during a research project) or at home.

Diekirch, 16th Feb. 2003

Physics Lab
Lyce Classique Diekirch

meteoLCD  (http://meteo.lcd.lu)

mailto:francis.massen@education.lu

 

 


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