Section 2 Functional Description
11 FT742-DM (RS485) Sensors – User Manual
2.4 Electronic Rotation of the Datum Direction
The datum direction of the sensor can be offset electronically by using the CF command (Section 6.4.2). This
facility can be used to adjust the datum direction in case of any mechanical misalignment within the mounting
arrangement. To offset the datum direction, in either the clockwise or anticlockwise direction, use the CF
command (Section 6.4.2). Once the offset has been set it will be retained in the sensor Flash memory.
2.5 Selective Filter Scheme
In addition to the averaging filter described in Section 2.3, the sensor has a feature called the Selective Filter.
The scheme allows the user to set a “validity period”, during which the sensor will exclude invalid readings from
entering the averaging filter (see Section 6.4.8). The output will freeze on the last previous “good” reading and
only raise an error flag once the number of bad readings exceeds the validity period. This scheme can be
enabled by factory configuration or using the FL command (see Section 6.4.10). The filter is turned off by default
to match legacy behaviour.
2.6 Error Detection
The sensor has a self-checking mechanism which can detect if a reading is invalid. On very rare occasions
where an invalid reading may have been detected, this is signalled to the computer or data logger by setting an
error flag character within the wind velocity output message (see Sections 6.4.27 and 6.4.28).
When an error is detected, the error flag character is set to a value of: 1.
Note: An optional Overspeed Warning Scheme can be enabled (but is disabled by default). See Section 6.4.16
for further details.
It is important that error flags are not ignored. Data associated with an error flag should not be processed as
valid wind data. The system should be capable of riding through rare periods when data may be temporarily
unavailable. If errors are ongoing (more than several seconds), a reset of the sensor should be applied (see
Section 6.4.18 for instructions on sending an RSU command).
It is recommended that errors are monitored and logged. If the frequency of errors has recently increased, then
inspection of the sensor for physical blockages may be required (see Section 4.1).
2.7 Overspeed Warning Scheme
During periods where the sensor detects wind speed beyond the rating of the sensor, the sensor will (by default)
indicate a general error flag status.
An additional overspeed warning scheme can be enabled where in the event of wind speeds beyond the rating
of the sensor, the error flag character will be set to 2 (see Sections 6.4.27 and 6.4.28) unless a general error
condition is also detected and the flag will be set to 1.
To comply with legacy behaviour, this scheme is disabled by default.
The Overspeed Warning Scheme can be enabled by software commands (see Section 6.4.16), using the
Acu-Test Digital kit (see Section 4.4) or enabled in the factory before despatch.
2.8 Heater Setup
The sensor is fitted with an integral three-element distributed heater that can be used to prevent icing-up of the
sensor in freezing temperatures. The heater is controlled automatically by the sensor using a user
programmable ‘set point’ temperature. The sensor uses a control scheme which dynamically changes the
current supplied to each individual heater element in order to maintain the programmed set point temperature.
We recommend selecting a suitable set-point temperature for the sensor environment. Factors to consider
include ambient temperature, relative humidity, wind speed and ice/snow conditions. This setting can be
programmed by the factory, by using the Acu Vis Test software (see Section 4.4) or the heater software
commands (see Section 6.4.11).
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