| Wind Speed and Direction | |
| Temperature and Relative Humidity | |
| Water Temperature | |
| Shortwave Radiation | |
| Longwave Radiation | |
| Precipitation |
| Lake Tahoe Site Sensors | |
Wind speed and
direction
The Met One cup and vane and the RM Young propeller and vane units operate
on similar principles. For speed, the cups or the propeller rotate a shaft.
The data logger counts the number of shaft rotations over a period of
time and calculates the wind speed. For direction, the vane is mounted
on a vertical shaft which turns a potentiometer (a variable resistor).
The resistance of the potentiometer is proportional to direction. The
data logger measures the resistance by applying a fixed voltage, and then
converts the result to wind direction.
The Vaisala/Handar sonic wind unit uses the speed of sound to measure wind speed and direction. There are three sensor arms, each with a sound transmitter and a microphone. The three sensors form an equilateral triangle, with two of the sensors aligned north-south, while the third is to the east. The unit transmits a sound signal both ways between each pair of sensors, and measures the time it takes for the sound to travel from one to the other. Since the sound will travel faster downwind than upwind, the unit uses the measured travel times to calculate both wind speed and direction.
Temperature and relative
humidity
The Campbell Scientific (Vaisala) HMP45C unit uses a platinum resistance
thermometer (PRT) and Vaisala’s Humicap® 180 capacitive relative
humidity sensor.
The RM Young 42372VC uses a platinum resistance temperature detector (RTD) for temperature and a Vaisala Intercap® interchangeable capacitive humidity sensor.
Water temperature
The Campbell Scientific 107 temperature sensor uses a thermistor with
a half-bridge circuit to measure temperature. A thermistor is a resistor
whose resistance varies with temperature. The datalogger applies a fixed
voltage and measures the output voltage, which is converted to temperature.
Shortwave radiation
The Kipp and Zonen CM21, and the two CM3 shortwave sensors which are part
of the K+Z CNR1 net radiometer, measure solar radiation with wavelengths
between 0.3 and 2.8 microns (micrometers), which includes the visible
light spectrum.
Longwave radiation
The Eppley PIR, and the two CG3 longwave sensors which are part of the
K+Z CNR1 net radiometer, measure infrared (thermal) radiation, with wavelengths
between approximately 5 and 50 microns.
Precipitation
The TE525 rain gauge uses a tipping bucket to measure precipitation. A
funnel collects rain or snow over a fixed area and directs it to the center
of the unit. Inside the unit is an arm with a small container on each
end. One or the other of the containers is always underneath the funnel.
When the container fills with a calibrated amount of water, its weight
causes the arm to tip, spilling the water and moving the opposite container
into position. The data logger counts the number of tips and calculates
the amount of rainfall. In the TE525, each 0.254 millimeters (0.01) inch
of rain causes the arm to tip.
