Campbell Scientific CS725 Snow Water Equivalency Sensor - Manual (Page 37)

CS725 Snow Water Equivalency Sensor
12. System Power Requirements and
Recommendations
12.1 Non Solar-Powered Sites
It is always recommended that the CS725 be operated from an uninterruptable
power supply. Normally an AC charger is used to charge +12 Vdc batteries.
For sites that operate this way, it is recommended that the batteries have the
capacity to supply the system for a minimum of 48 hours. This should be
increased for sites where frequent power outages are possible.
12.2 Solar-Powered Sites
The maximum power draw from the CS725 is 180 mA or 4.32 Ah per day. It
is recommended that solar-powered systems be designed with a minimum
reserve capacity of 60 days. This results in a minimum reserve capacity of 260
Ah for the CS725. It is also important to account for the power draw from
other equipment that may operate from the power supply.
The following are some reasons for the large reserve capacity.
• Solar panels can become covered in ice and snow and charging may not be
present for long periods of time.
• Battery capacity decreases with cold temperatures.
• Cold temperatures also significantly reduce a battery’s capacity to charge.
Generally very little charging occurs at temperatures below -30°C.
Solar Panel Size
The size of solar panel required for charging the CS725 system should be sized
according to the geographic location of the installation and total maximum
system power draw.
13. Detailed Measurement Theory
The CS725 sensor is, in fact, a gamma radiation spectrometer that has been
specifically designed for the purpose of measuring SWE. The CS725 uses a
thallium doped sodium iodide crystal NaI(Tl) for detecting the gamma
radiation.
The natural gamma radiation from the ground is produced by the traces of
long-lived radioactive elements that it contains. The most abundant of those
elements is Potassium_40 (
40 K) and Thallium_208 ( 208 Tl). Potassium_40 emits
gammas at 1.460 MeV (megaelectronvolts), and Thallium_208 at 2.613 MeV.
Gamma rays have a well defined “probability of interaction” per unit of length
as they go through a material according to the energy of the gamma and the
nature of the material. The net effect when the natural gamma rays from the
ground go through the snow cover is that a fraction of the gammas do not
interact at all with the snow, and that another fraction is either degraded in
energy, or totally absorbed in the snow. The precise measurement of the
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