Section 2
Functional Description
11
FT702LT Flat Front Wind Sensor Manual
2.4 Electronic Rotation of the Datum Direction
The datum direction of the FT702LT 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 FT702LT Flash memory.
2.5 Error Detection
The FT702LT 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 6.4.25 & 6.4.26). 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.
2.6 Heater Setup
The FT702LT 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 FT702LT using a user
programmable ‘set point’ temperature. The FT702LT uses a control scheme which dynamically changes the
current supplied to each individual heater 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.
The recommended heater set point temperature for most applications is 30
°
C, however by default the heater is
disabled. To enable the heater or change the heater set point temperature, use the FT702LT Acu-Vis software
(see section 4.2).
Since the heater circuit is thermostatically controlled the actual power being drawn from the supply will depend
on the programmed set point and the prevailing environmental conditions (i.e. ambient temperature, wind
speed, precipitation etc). The maximum power that the FT702LT heater and sensor can consume is by default
limited to 99W. The power supply must be rated to provide the maximum power that the sensor can consume.
From build version 22 and above the maximum current limit of the sensor can be adjusted in software from 0.1
–
6 Amps. To change the current limit of the sensor, please refer to section 6.4.12.
From build version 22 and above the sensor can detect when the supply voltage drops below 9 VDC, when this
occurs the heaters are automatically switched off.
In low temperature environments where a wind sensor may have accumulated ice prior to power being applied,
it is recommended that a User Reset Command (see section 6.4.16) is sent after the sensor has been powered
for a de-icing period of 30mins. This is to ensure that the sensor initialises correctly with the sensing area clear.
