Section 2
Functional Description
13
FT702LT/D Flat Front Wind Sensor Manual
2.2.5
Error Detection
If the FT702LT/D detects that a reading may be invalid this is signalled to the controller or data logger by setting
the current of both loops to a value of 1.4mA by default. The error current level can be configured (see Section 5)
by the user from 1.4mA up to 3.9mA in steps of 0.1mA (only available from build version 22 and above).
The error condition must occur for more than 1 second for the current loops to be set to this condition. If an error
occurs for less than 1 second then the loop outputs will be held at the last valid reading for the duration of the
error.
Figure 9: 4-20mA valid readings range
Always use an average of several readings for any calculations or control decisions because
single readings can accidentally become corrupted.
The data acquisition system needs to not only sample the 4-20mA measurement range, but also should include
logic to process and exclude data if it is outside the measurement range. The data acquisition system should
distinguish between a status flag error sent by the wind sensor and other out of range readings in order to help
diagnose if a fault has occurred with the power supply, cable or wind sensor.
2.3 Heater Setup
The FT702LT/D 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/D using a user
programmable ‘set point’ temperature. The FT702LT/D 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 factory default setting for the heater set point value is 30
°
C. This is the recommended setting for most
applications. To change the heater set point or to disable the heater use the FT702LT/D Set Up and Test
Program (see section 5).
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/D 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 use the FT702LT/D Set Up and Test Program (see section 5).
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.
