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SYSTEM
3.2.3 ELECTRICAL CONDUCTIVITY (EC)
The ATMOS 41W has a small basin that collects a sample of the precipitation stream as the
tipping spoon empties. Two small sensing electrodes measure the electrical conductivity
(EC) of the water in this basin every time the rainfall tipping spoon tips and records the
instantaneous values. At the end of the measurement interval, the ATMOS 41W outputs
the average of the instantaneous values measured during that measurement interval.
Measurement intervals with no spoon tips are represented with a 0 value reported for EC.
There are several factors that affect the EC of the water in the measurement basin. The
most important of these and the quantity that is most desirable to measure is the EC of the
precipitation or irrigation water itself. However, the presence of atmospheric depositions
and especially biological substances (e.g., bird droppings) in the funnel will heavily influence
the EC of the water in the basin until those compounds are completely flushed from the
system. In the case of bird droppings, this may not happen completely even after several
precipitation events. Please see
for instructions on cleaning the funnel.
Residual water and concentrated solutes in the measurement basin will also influence
the EC measurement until the residual water and solutes are completely flushed from the
basin. Additionally, microbial activity in the measurement basin can lead to biofouling of the
electrodes, which can compromise their ability to measure EC. Please see
for
instructions on cleaning the EC electrodes.
If an accurate measure of precipitation and irrigation water EC is desired, it is critical to keep
both the funnel and the EC measurement basin clean between precipitation and irrigation
events. Ignore data from early in the event until the whole system is flushed and the
measurement basin is filled with uncompromised precipitation and irrigation water.
Even if the funnel and measurement basin are not routinely cleaned, the EC of the solution
passing through the measurement basin during precipitation events can be used as an
indicator that the funnel has been polluted by bird droppings and needs to be cleaned for
proper rainfall measurement.
3.2.4 VAPOR PRESSURE AND RELATIVE HUMIDITY
The vapor pressure sensor (
) on the ATMOS 41W is located behind the circular
Teflon
®
screen in the same housing as the sonic transducers. The Teflon screen protects the
sensor from liquid water and dust while allowing water vapor to freely pass to the sensor and
equilibrate with ambient vapor pressure. The sensor measures relative humidity (RH) and
temperature in addition to computing vapor pressure.
