Upgrading Davis Vantage Pro with a helical MeteoShield Pro for improved response and temperature accuracy

QUESTION: How to mount a Davis temperature humidity sensor inside a MeteoShield Professional?

Davis weather station with a helical MeteoShield Professional installed on the right and connected to the davis wireless console.

Davis weather station with a helical MeteoShield Professional installed on the right and connected to the davis wireless console.

ANSWER: I looked at the Davis sensor and checked with the Davis community about fitting it into the MeteoShield Pro. Installation instructions and advice follow:

The old Vantage Pro, before 2016 uses a Sensirion SHT11 sensor, the new version use the SHT31-LSS, in any case the sensor is not the one that we can buy online but a specific version for Davis. The Davis PCB (green) where the sensor is installed is large and will fit into the MeteoShield Pro only vertically and only if the cable strain relief is removed and the cable is bent to follow the long side of the PCB and out the bottom of the MeteoShield Pro.

It's also possible to cut the Strain Relief on the original cable to be able to pass it through the screen plates. After this, you can attach a new connector, which is not a big problem but some users will want to make this step or will be scared. The length of the original sensor cable is also very short, about 50 cm (20 inches), so it may require an extension.

A big thank you goes out to the Italian and USA amateur meteorological community with special thanks to Mauro Serenello who was kind enough to provide us with instructions and pictures for this post.

What is the lifetime of a humidity sensor and service/replacement interval.

QUESTION:

  • What is the lifetime of the sensor and when do we need to replace them.

  • How often to clean and replace the sensor filter cap.

Humidity sensor contamination comparison in real world applications (LEFT = Helical shield, standard solar shield on the right)

Humidity sensor contamination comparison in real world applications (LEFT = Helical shield, standard solar shield on the right)

ANSWER: Cleaning and replacement is very dependent on application and contamination.  Please let us know how you plan on using the sensor and we can be more specific.  If you use it in clean indoor conditions, you will most likely never have to replace the filter but a calibration check once a year is recommended since indoor VOC's may affect long-term accuracy.

The best way to determine the recalibration interval is to perform a calibration check after a year on one sensor and decide whether it is still within specifications in your environment. This will guide you in your decision when to recalibrate your other sensors which are in a similar environmental.

A practical approach like this is the only sure way to determine recalibration/replacement intervals of all humidity sensors.  Many temperature sensors do not have this problem but their long-term drift should be validated using the above approach.

Differences in recording minimum and maximum temperatures with the MeteoHelix and Davis weather stations

QUESTION: Will the temperature peak between two data samples be recorded? Davis, sends data every 5 minutes and shows the minimum, maximum and average temperature with sampling every 2.5 seconds, then on a total of 120 samples.

ANSWER: The sampling time of the newest MeteoHelix weather stations is 15 seconds. MeteoHelix sends the following data every 10 minutes. Output description follows along with the open source JavaScript decoder code link: https://www.baranidesign.com/meteohelix-message-decoder

Battery, 10 min Temperature, Temperature_min, Temperature_max, 10 min Humidity, 10 min Pressure, 10 min Sun Irradiation, Solar_max, 10 min Rain amount. (All measurements are taken every 15 seconds.)

SENSIRION SENSOR RESPONSE TIME WITHOUT A FILTER CAP AND WITH 1 METER/SECOND AIR FLOW

As you can see, the output includes: 10 minute average temperature and minimum and maximum temperature values over the 10 minutes as measured every 15 seconds. 10 minute solar irradiation average and Max irradiation value along with humidity, atmospheric pressure and rain.

Yes, the Davis weather stations sample every 2.5 seconds, yet the τ63% reaction time of their Sensirion SHT sensor and passive radiation shield is much slower than 2.5 seconds (we do not have detailed data on the 7714 shield, but know what the Sensirion time constant is, as shown in this picture from their datasheet). Additionally, the 7714 shield has slower reaction time than MeteoShield Pro to temperature changes. What we do know so far from independent studies is that the helical MeteoShield Pro has the fastest reaction time of any passive radiation shield. After taking the aforementioned information together, one can conclude that the 15sec measurement interval for temperature and humidity is more than fast enough as these atmospheric physical properties change much more slowly than wind speed for example, due to the above factors.

One must also take into account the time constant of the filter cap used on the sensor, the sensor itself and its thermal mass and the radiation shield time constant. They all add up to a reaction time greater than 30 seconds for 99% of all outdoor meteorological sensors that have decent long-term stability and protection. In many situations, the effects of morning dew, rain, drizzle and snow have a significantly higher impact on accuracy as can be seen in fan-ventilated (aspirated) active solar shields which tend to provide unrealistically low temperature readings and unrealistically high humidity readings in these conditions by drawing in moisture and saturating the temperature and humidity sensors.

* τ63% response time of sensor with a filter cap will vary based on cap porosity, material and fluid (air) flow. In applications where sensors are used in wet, dirty and dusty environments, we recommend regular inspection of filter cap cleanliness to maintain long term accuracy. Inspection interval should be determined by the application and user experience in their application environment.