Chapter 5 ___________________________________________Theory of Operation
VAISALA__________________________________________________________33
Sensitivity Check
There is an additional infrared emitting LED that is used for
sensitivity checking to ensure that the background
luminance measurement is working properly during the
night when the background luminance measurement is
saturated into a low value. The light of the LED is reflected
from the back of the lens and measured with the master
detector. Since the optical bandpass filters are outside the
lens, a similar infrared LED can be used for the sensitivity
check as is in use for other diagnostic measurements. Any
signal that is other than the saturated value is acceptable for
the sensitivity check.
Heater Operation
The hood heater operation is controlled by one solid state
relay, which switches the hood heater voltage (28 VAC) on
and off. The dew heater circuit consists of two transistors
which switch power to the external power resistor. The dew
heater operation is monitored by the current measurement
resistor, that generates a voltage proportional to the dew
heater current. The voltage is measured with the
microcontroller's internal analog-to-digital converter.
The hood heater operation is monitored by a temperature
sensor located on the glue sides of the hood heater foil. The
automatic heating control system uses the temperature
sensor output to stabilize the hood temperature. The system
turns the heater on when the hood temperature drops below
15 °C and turns the heater off when the hood temperature
rises above 20 °C.
If the sensor output is invalid, the heater is turned off and a
HOOD HEATER FAULT indication is generated. If the
measured heater temperature has increased less than 2 °C
within the 5 minutes since the automatic heater control
system turned on the heater, an indication is also generated.
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