The resulting data recorded by the rain gauge, based on an OEM version of a Tinytag count logger, accurately reflects the rate at which rain falls, a parameter just as important as the total quantity of rain.
The accurate monitoring of rainfall is of fundamental importance
in fields such as meteorology, agriculture and hydrology, but also
in specialised research fields such as palaeoclimate reconstruction
using cave stalagmites, where the relationships between rainfall
and groundwater flow need to be understood.
Some of the obvious disadvantages of the old bottle and ruler
method of measuring rainfall were overcome in the 17th century by
Sir Christopher Wren who, allegedly, invented the ingenious and
still widely used tipping bucket, a mechanical flip-flop which
records an "event" each time the bucket fills sufficiently to tip
and spill its contents. However, these mechanical devices must be
made to quite high precision, and they are liable to corrode or
suffer from accretions (including insects and spiders' webs) and
consequent seizure. A further problem relates to the resolution of
the instrument, where a fairly typical bucket volume of 5 to 10 mL
equates (with a 5" funnel) to rainfall of 0.4 to 0.8 mm. Any
rainfall event less than this will not be recorded, while remanent
water in the bucket will influence the next recording.
Sensor designer PiTech Research has used Gemini data logging
technology to address these problems in its design of a pluviometer
in which drops of water accumulating on the end of a wettable
nozzle of fixed diameter fall onto a device which records the
impact of the drop on its surface. Since (all other things being
equal) the volume of a water drop is purely a function of nozzle
diameter, then the frequency at which the fixed-volume droplets
fall is directly proportional to the rainfall in mm per unit time.
Using a sensor identical to the "Stalagmate"* (a unique logging
drip counter used for cave monitoring, based on an OEM version of the
Tinytag count logger), these drops are counted at intervals
programmable from minutes to hours (typically 10 minutes), with a
theoretical rainfall resolution of 0.01 mm. The capacity of the
logger is 32k data "buckets" each of 16k capacity, giving a
potential maximum of 229 (537 million) data points The
resulting output is a time series which accurately reflects the
rate at which rain falls, a parameter just as important as the
total quantity of rain.
Monitoring noise levels around wind farms
An unusual application of rainfall measurement is in assessing
the noise impact around wind farms in the UK. The sound of rain
increases background noise and contaminates the signals received by
the noise monitoring equipment, but a measurement of the duration
and intensity of the precipitation allows noise data during periods
of rainfall to be excluded. The sensitivity of the Pluvimate as
compared to a tipping bucket rain gauge is particularly
advantageous for this application.
A study of the relationship between rainfall and
groundwater penetration into caves
In collaboration with Royal Holloway, University of London,
three pluviometers have been deployed since late 2009 on a site
above a cave in South West France, in conjunction with Stalagmate
drip loggers and Tinytag rugged Plus 2
temperature loggers in the cave itself. Plus 2 loggers are also
used to monitor the outside air and soil temperatures. The
pluviometers are mounted at different heights and each uses a
slightly different debris filter and funnel to try to determine
which configuration is best.
Pluviometers are often placed in remote, infrequently visited
locations, so autonomy and reliability is essential, and the 1/2 AA
lithium battery generally lasts for four years or so. The essential
criteria of long battery life, reliability in all weathers and
value for money have been perfectly satisfied by Tinytag loggers.
Their small size and low power consumption make them ideal for use
in rugged field instruments such as the Stalagmate drip logger and
Pluvimate high resolution rain gauge.
* "Using Tinytag temperature and count loggers to monitor caves
and reconstruct past climate change", D. Mattey, Royal Holloway,
University of London (Tinytag information sheet).