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.
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
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.
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.
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.
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.
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.
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.
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.
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).
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.