Section 2 Functional Description
12 FT722 & FT742 (RS485) – FF & PM 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 6.4.6). 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 6.4.6). 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. 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 by using the FL software command (see Section 6.4.14). 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 6.4.30 & 6.4.31).
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 2.7 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 6.4.21 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: if an overspeed condition is detected (above the
highest speed rating), the error flag character will be set to: 2 (see Sections 6.4.30 and 6.4.31) unless a general
error condition is also detected. Not all high speeds can be detected and may report as the regular error flag 1
instead.
To comply with legacy behaviour, this scheme is disabled by default.
The Overspeed Warning Scheme can be enabled by software commands (see Section 6.4.20), via the Acu Vis
PC software program (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.
It is important to consider the resistive losses in the cable and rate the cable appropriately. In general, power
losses in the cable should be minimised in order to maximise the available heating power to the sensor. A heater
setpoint temperature of >30°C is recommended for most applications. To change the heater set point or to disable
the heater use the Acu-Vis test software or the HT RS485 software command (see Section 6.4.15).
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