Section 2
– Functional Description
12
FT742-DM (4-20mA)
– Sensors
- User Manual
2.2.6
Error Conditions
If the sensor detects that a reading may be invalid this is signalled to the 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 4.4) by the user
from 1.4mA up to 3.9mA in steps of 0.1mA.
Note: An optional Overspeed Warning Scheme can be enabled (disabled by default). See below for further
details.
The error condition must occur for more than 0.5 seconds for the current loops to be set to this condition. If an
error occurs for less than 0.5 seconds then the loop outputs will be held at the last valid reading for the duration
of the error.
Figure 6: 4-20mA valid readings range
Always use an average of many 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 of this range. The data acquisition system should distinguish
between a status flag error sent by the wind sensor and other out of range readings (see Figure 6) in order to
help diagnose if a fault has occurred with the power supply, cable or wind sensor.
It is important that error flags are not ignored. The datalogger should be able to recognise and interpret error
codes within the data. If errors are ongoing (more than several seconds), a power reset of the sensor should be
performed.
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).
Overspeed Warning Scheme
During periods where the sensor detects wind speed beyond the rating of the sensor, the sensor will (by default
settings) indicate an error by setting both current loops to the error flag current level, the same as it does for
other invalid readings. This is to match the legacy behaviour of the FT702LT/D sensor.
An additional feature is included in the FT742 sensor called the Overspeed Warning Scheme (disabled by
default). This scheme can be used to distinguish between overspeed events and other invalid readings. When
enabled and during overspeed events, the scheme works in the following way:
1. The direction current loop will be set to the error flag current level (1.4mA by default).
2. If the wind speed scaling range is equal or above the maximum speed; the speed current loop is set
to maximum (FT742 example: default 0-75m/s range, will read 20mA in overspeed conditions).
3. For scaling ranges below the maximum speed; the speed current loop is set to a value of
20.48mA. (FT742 example, set to 0-40m/s range, set to 20.48mA).
