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This new automatic weather station was installed in July 2006 to replace the old one. It measures and records; air temperature and humidity, pressure, wind speed and direction, solar radiation, ground temperature, soil temperature at 30 and 100 cm, and rainfall. It is installed within the existing met enclosure to the south of the main observatory building.
Initial calibration problems meant that accurate data was collected only from 2007 April 1st at which time a more accurate pressure sensor, or digital barometer, was added.


Met Enclosure

The weather station was manufactured by Casella Instruments and supplied and installed by Alpha Technologies. The instruments are wired to a Sensus data logger which has an RS232 connection to a control computer which communicates with the data logger through a series of Perl scripts written by Benjamin Mueller.



Solar Radiation Detector

It measures the intensity of total global solar radiation received at the earth's surface in terms of Watts/m2 and responds to both direct and diffused sunlight received from the whole hemisphere. Diffused sunlight continues to reach the earth even when the sun is obscured behind cloud, and in the twilight period before sunrise and after sun set. The sensor is based upon a silicon photo detector which is housed within a weather proof, anodised aluminium body designed for reliable long term operation in outdoor conditions.

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Wind Speed

Based on the classic cup anemometer design invented at Armagh Observatory by TRR Robinson. The rotational speed is monitored using an infra-red light source, interrupter and optical sensor that provides 20 pulses from 0 to 5 V per revolution. This non-contacting design can measure wind speeds up to 75 m/sec. The instrument has an anodised aluminium alloy outer case.

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Wind Direction

The wind vane is a small lightweight GMR design that offers a rapid response to changes in wind direction with virtually friction free sensing. The analog output changes from 0 to 1.8 V as the wind direction changes from 0 to 359, with no dead-band at north.

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Combined Air Temperature and Humidity

The combined air temperature and relative humidity sensor is installed within a single radiation screen. Both transducers are unaffected by condensation and are resilient to airborne pollution. The hygrometer is of the capacitative type and is largely free from drift. It offers high repeatability over large ranges of humidity for long periods. The temperature sensor is based upon a platinum resistance element to BS1904, DIN 43760, and incorporates linearisation electronics within the sensor. The output from the temperature sensor is used to compensate the hygrometer for temperature effects.

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Soil (30 and 100cm) Temperatures

These are semi conductor current generator type sensors. They have a range of -40oC to 60oC and a calibrated accuracy of ±0.5oC between -20 to +50oC. There is one at 30cm depth and one at 100cm depth in soil.

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Grass Temperature

This is a semi conductor current generator type sensors. It has a range of -40oC to 60oC and a calibrated accuracy of±0.5oC between -20 to +50oC.

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Built-in Pressure Sensor

Disconnected 2007 October.
The barometer is mounted on a circuit board in the data logger, which itself is inside the environmental case. A 1 mm ventilation hole in the case ensures the internal pressure remains at ambient. A monolithic silicon transducer senses pressure by means of a silicon diaphragm into which a bridge of piezo resistors have been defused. The silicon crystal, which constitutes the diaphragm, being a perfect device does not suffer from hysteresis. The high signal level obtained is further conditioned to give an exceptional degree of accuracy, repeatability and linearity. The pressure transducer is temperature stabilised at 60C to minimise temperature effects on the overall accuracy.

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Digital Barometer

The Vaisala BAROCAP Digital Barometer PTB220 is designed for measurements in a wide environmental pressure and temperature range. It uses a silicon capacitive absolute pressure sensor with excellent hysteresis and repeatability characteristics. It has a measurement range of 50 - 1100 hPa and an operating temperature range of -40 to +60 C. The total accuracy is ±0.1 hPa/yr (1 hPa = 1 mbar).

  Pressure Sensor


Rainfall Sensor

The tipping bucket is a well proven method of monitoring rainfall with a divided bucket assembly mounted on pivots. The bucket assembly is adjusted to tip each time an amount of water proportional to 0.2 mm of rainfall has collected in one or other side of the bucket. Therefore, each time the bucket tips, a signal proportional to precisely 0.2 mm of rainfall is sent to the logger. The gauge has a body and funnel constructed of aluminium alloy, with an accurately machined Septum Ring. The aluminium base plate is equipped with levelling screws and a spirit level for precise adjustment.

  Automatic Weather Station

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