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Updated : 16/10/2016

GHowSAW Weather System Setup

General

To assist other weather amateurs to install home/amateur weather stations, the various recommendations regarding the proper methods for installing the various sensors have been extracted from: FMC-S4-1994, "Federal Standards for Siting Meteorological Sensors at Airports", and meet both FAA and NWT guidelines.

A pdf version of the full document

In following these standards you will ensure your weather station is providing the most accurate readings possible. Obviously, you will need to adapt these guidelines to your particular installation and may not be able to obtain the exact results listed. These standards, at minimum, should provide guidance in the mechanics involved in obtaining accurate sensor readings.

The sensors most commonly found in a home unit are addressed:

surface wind speed and direction
ambient air temperature
dew point temperature
atmospheric pressure
precipitation accumulation
lightning detection

(this does not address manufacturer specific installation issues)

Atmospheric pressure:

Should usually be installed in a weatherproof building or enclosure (most home units include this sensor in the main display). Avoid areas that are affected by pressure changes caused by compression due to closing doors in small rooms, avoid jarring, vibration and rapid temperature changes. Avoid direct sunlight, drafts from open windows, and air currents from heating or cooling systems. Air flowing over a sensor can cause artificially low readings. It should be at least 3 feet above ground level, but less than 100 feet above average terrain. If installed in a closed room, venting to the outside may be necessary.

Wind speed and direction:

Should be oriented to TRUE NORTH, not magnetic north! BE AWARE that in some areas true north may vary by 15 degrees or more from a raw compass reading. Call the nearest Coast Guard unit for your local magnetic variation. The site should be relatively level but small gradual slopes are OK. This sensor should mounted 30-33 feet above average terrain, EXCEPT it should be at least 15 ft above any obstruction within 500 feet. If installed on a tower, it should be at the top, but a side bracket can be used and should be at least 3 feet away from the tower side. Towers should be of an open design to permit free air flow. This will be the hardest sensor to install at a home location to meet the intended guidelines. Don't cut down your neighbors trees to reduce obstructions! Achieve the best compromise possible. You should also check this sensor at least monthly to ensure free movement of the wind cups and vane, as well as proper orientation to true north.

Temperature and dew point:

Should be at 5 feet (+/- 1ft) above ground level, or 2 feet above average maximum snow depth. It must be adequately ventilated but needs protection from direct sunlight, earth radiated heat, etc. Also it should not be influenced by artificial conditions like concrete or blacktop, heat radiating from buildings, heating/cooling exhausts, etc. Avoid placing above house roofs or near windows, doors or roof eaves. Vegetation near a sensor should be clipped to 10 inch height or less.

Lightning detector:

Install according to manufacturers directions. Metal obstructions should be no closer than 2 times their height.

Precipitation accumulation:

Must be level (measure the upper rim or orifice). Surrounding terrain should be as flat as possible. It should be as close as possible to ground level, and free from surround obstructions (no closer than twice their height). Avoid hard surfaces which may allow splashing into sensor. To avoid losses due to wind, install an "alter-type" wind shield. In areas with rain or sub-freezing temperatures, it must be heated to measure snow or freezing rain. In home units, this is usually the least accurate sensor. Check periodically to ensure sensor is free of debris.

Consult the manufacturer for guidelines in extending cables. You should also keep RF sources as far away as possible from sensor cables and the processor unit. Cable shielding may be necessary. You will also need to set your corrected sea level pressure. Call the nearest airport and ask the control tower for CORRECTED SEA LEVEL PRESSURE, not airfield local altimeter pressure.

Radiation detector:

The positioning requirements of a radiation detector are self evident in that it must be in a location that is exposed to full sun throughout every day of the year and not be shaded at any time by anything. The detector surface must also be horizontal.

My Station

Specific recommendations regarding the siting of the various components of a weather station are given in the sections on individual devices whereas this page described the general setup of my weather station.

The station comprises the following components:-

  1. Modified 'HobbyBoards' Anemometer Controller Board
    (see below for the modification to add radiation sensing).
    1. 'Inspeed' anemometer.
    2. 'Inspeed' wind vane.
    3. Radiation sensor.
  2. 'HobbyBoards' Lightning detector.
  3. 'RainWise' tipping bucket rain gauge.
  4. 'HobbyBoards' Humidity and Temperature sensor.

Items 1&2 are attached to 1.5m tall 50mm diameter thick-walled PVC pipe (do not use 50mm diameter PVC waste pipe which is too flexible for this application) with a 200mm parallel pipe attached to the main mast using T-pieces. Both the mast and the parallel pipe are closed with pvc-welded end-caps. The short parallel pipe houses the lightning detector(4) and a radiation sensor(3) which is press-fitted and sealed into the top end-cap.

The T-pieces used to connect the two pipes are not welded but fitted with rubber 'O' rings for sealing. This allows the short pipe to be rotated to ensure that the radiation sensor(3) is perfectly horizontal. Similarly, the main mast is attached to the parapet of the garage roof using 'U'-bolts which allow adjustments to ensure the anemometer(1) and wind-vane(2) are horizontal. The final position of the anemometer is 5m above ground level.

The T-pieces are also adjusted such that the radiation sensor(3) is due North of the main mast and the horizontal conection piece sized to to provide sufficient seperation to avoid any shadows from the anemometer(1) and the wind-vane(2). North because this system is in the southern hemisphere.

The 'Rainwise' rain gauge is fastened to the garage roof.

To avoid the expense of building a Stevenson Screen the 'Hobbyboards' temperature and humidity sensor(4) is enclosed in a 50mm diameter PVC vent-valve and closed off with an end-cap which has been drilled with several holes to provide through ventilation. The whole unit is mounted under the eaves of a South facing wall to ensure that it receives no direct sunlight.

Wiring

The 'Rainwise' rain guage was supplied with 20m of exterior grade 3-core wiring and this was use to connect the system to the GHowSA one-wire system. The wiring is run from the office interface to the temperature humidity sensor and then to the anemometer wind vane interface card without any joints. The lightning detector unit is connected to the anemometer interface using a short (150mm long) CAT5 RJ45 connector cable. All the wiring is run in 20mm diameter electrical conduit even though the 'Rainwise' cable is exterior grade.

The only other wiring required is an earth connection for the lightning detector for which a 60 amp earth cable was used to connect the detector directly to a specifically installed earthing rod.

Further information on the wiring of the one-wire network is available here.

Anemometer Controller Board Modification

The original 'HobbyBoards' Anemometer Controller Board uses a DS 2438 to sense the wind direction but the current sensor attribute (Vsens) of this slave is not used. By adding a CLD240 diode and a 390 ohm resistor across the Vsens pins as shown in the image below the Anemometer Controller Board can also detect radiation

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