The under-range error is reported when a sensor reports a value below its operating range.
An under-range error for the generic 4-20mA sensor means that the sensor is reporting a value below 4mA. The Under-range error is usually caused if:
- there is a wiring issue.
- the physical reading is out of the operating range of the sensor.
- the sensor has failed.
Resolving the Error
Make sure that the cables or wires are securely attached to both the device and the field unit, and not loose. You can try unplugging the device and plugging it back in again properly.
Inspect the cable for any damage. If wiring is damaged, it will need to be replaced.
Reading is below operating range
The 4-20mA sensor has an operating range of 4mA to 20mA. So, the output you're trying to measure will also have a range, provided in the technical documentation for the sensor itself.
If the physical reading is below the minimum that the sensor can measure, an under-range will be reported.
If you have checked everything else, it is likely that the sensor has failed electrically and will need to be replaced.
Understanding the Error
Generic 4-20mA sensors (Loop Powered) can be used with a field unit to monitor common phenomena such as pressure, water level, length, etc. Basically, the field unit detects a current reading (in mA or milliampere) from the sensor, and translates it into your pressure reading, or level reading, or whatever it is that you're trying to measure.
As the name suggests, the 4-20mA sensor has an operating range of 4mA to 20mA. So, the output you're trying to measure will also have a range, provided in the technical documentation for the sensor itself.
For example, a pressure sensor might have a reading range of 0—100 PSI. So, a current reading of 4mA will mean your pressure is 0 PSI. Similarly, a reading of 12 mA will mean the pressure is 50 PSI.
An under-range error means that the sensor is drawing current below 4mA. This indicates that something is wrong, because under normal operation, the current should not be below 4mA.