Section 2
– Functional Description
13
FT742-DM (4-20mA)
– Sensors
- User Manual
The Overspeed Warning Scheme can be enabled in the factory, or by the user using the Acu Vis test software.
2.3 Heater Operation
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, or by using the Acu-Test software (see Section 4.4).
It is important to consider cable resistance losses and use a suitably rated cable of an appropriate length. FT
recommends cable types in Section 3.2.
Since the heater circuit is thermostatically controlled, the actual power being drawn from the supply will depend
on the programmed set-point and the environmental conditions (i.e. ambient temperature, wind speed,
precipitation etc.). The maximum power that the sensor can consume is by default limited to 99W (4A with the
heater enabled). The power supply must be rated to provide the maximum power that the sensor can consume.
The maximum current limit of the sensor can be adjusted in software from 0.1
– 6A (from the default of 4A and
in increments of 0.1A). The current limit can be programmed at the factory or modified using the Acu-Test PC
software (see Section 4.4). By default the heater requires a minimum of 11V for operation.
2.4 Low Power Operation
The sensor is designed for typical operation at 24VDC, however the sensor can operate below this with reduced
performance between 12-30VDC. By default, the heater will shut down at approximately 11VDC. Below
approximately 8V the sensor may shut down. Lower voltages reduce the overall power consumption and heater
performance.
For further advice on power and heater management strategies, contact FT Technologies.
