This automatic weather station was installed in July 2006 to replace the old one. It measured and recorded: Air temperature and humidity, pressure, wind speed and direction, solar radiation, ground temperature, soil temperature at 30 and 100 cm, and rainfall.
It was installed within the existing Met Enclosure to the south of the main Observatory building. The AWS provided data from 21st July 2006 at 14:52. Initial calibration problems meant that accurate data was collected only from 1st April 2007, at which time a more accurate pressure sensor, or digital barometer, was added. Data was collected until 18th March 2012 at 11:23am. The next AWS was not installed until 2018.
The weather station was manufactured by Casella Instruments and supplied and installed by Alpha Technologies. The instruments were wired to a Sensus data logger which had an RS232 connection to a control computer which communicated with the data logger through a series of Perl scripts written by Benjamin Mueller. Full dataset (climate.armagh.ac.uk)
Solar Radiation Detector
It measures the intensity of total global solar radiation received at the earth’s surface in terms of W/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.
Wind Speed & Direction
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/s. The instrument has an anodised aluminium alloy outer case.
The wind vane was a small lightweight GMR design that offered a rapid response to changes in wind direction with virtually friction free sensing. The analog output changed from 0 to 1.8 V as the wind direction changed from 0 to 359, with no dead-band at north.
Combined Air Temperature and Humidity
The combined air temperature and relative humidity sensor was installed within a single radiation screen. Both transducers were unaffected by condensation and resilient to airborne pollution. The hygrometer was of the capacitative type and largely free from drift. It offered high repeatability over large ranges of humidity for long periods. The temperature sensor was based upon a platinum resistance element to BS1904, DIN 43760, and incorporated linearisation electronics within the sensor. The output from the temperature sensor was used to compensate the hygrometer for temperature effects.
Soil and Grass Temperatures (30 and 100 cm)
These were semiconductor current generator type sensors. They had a range of -40°C to 60°C and a calibrated accuracy of ±0.5°C between -20 to +50°C. There was one at 30 cm depth and one at 100 cm depth in soil.
This was a semiconductor current generator type sensor. It had a range of -40°C to 60°C and a calibrated accuracy of ±0.5°C between -20 to +50°C.
Built-in Pressure Sensor
The barometer was mounted on a circuit board in the data logger, which itself was inside the environmental case. A 1 mm ventilation hole in the case ensured the internal pressure remained at ambient level. A monolithic silicon transducer sensed pressure by means of a silicon diaphragm into which a bridge of piezo resistors had been defused. The silicon crystal, which constituted the diaphragm, was a perfect device and did not suffer from hysteresis. The high signal level obtained was further conditioned to give an exceptional degree of accuracy, repeatability and linearity. The pressure transducer was temperature stabilised at 60°C to minimise temperature effects on the overall accuracy.
It was disconnected in October 2007 and replaced by the Vaisala Digital Barometer.
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).
A more modern variant of this Vaisala sensor continues to be used for pressure measurements to this day, located in the Library building.
Rainfall Sensor
The tipping bucket is a well proven method of monitoring rainfall with a divided bucket assembly mounted on pivots. The bucket assembly was adjusted to tip each time an amount of water proportional to 0.2 mm of rainfall had collected in one or other side of the bucket. Therefore, each time the bucket tipped, a signal proportional to precisely 0.2 mm of rainfall was 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. The bucket, now out of use, can still be seen within the Met Enclosure.
