FIGURE 7-1. Angle θ
a
is defined as the angle between the vector of oncoming wind (U) and
the wind component along the a-sonic path, (U
a
).
7.2 Theory of heater controller operation
The CSAT3BH and CSAT3H heater controller box use a heater algorithm to variably control the
heaters on the sonic head. There are two zones of heating. Zone 1 is the arms and strut, while
zone 2 is located on each transducer finger.
The controller is programmed to operate in two modes. The first mode controls the heater
voltage to maintain a temperature on the transducers above dew point when the ambient
temperature is less than 2 °C, as determined by the temperature/relative humidity probe.
The second mode controls the heater voltage to clear a blockage of ice or snow, based on sonic
diagnostic flags. If, within 60 seconds, the average number of diagnostic flags coming from the
CSAT3BH is greater than 25 percent, the controller will go into the second mode. In the second
mode, the heater voltage will increase every 15 minutes until the blockage is clear. Once the
blockage is clear, indicated by the average number of diagnostic flags dropping below 25
percent, the controller will go back into the first mode and control the heaters to maintain the
temperature setpoints in zone 1 and zone 2 above dew point.
The heaters will not allow the temperature on the transducers or arms to go above 38 °C.
Additionally, if any of the temperature readings on the sonic head are above 45 °C, the heaters
will shut off. Finally, in the situation of a heater failure, indicated by a delta temperature between
the maximum and minimum temperature recorded on the arms and transducers greater than
15 °C, the heaters will shut off and the controller will stop running.
CSAT3BH Three-Dimensional Heated Sonic Anemometer
64
