To provide access to a wide range of sensors, Observant hardware and software support the ability to connect any loop-powered 4–20 mA sensor.
Limitations of Observant Support for 4–20 mA Sensors
Please take note that the Channel Partner will be entirely responsible for the correct operation and all support of generic 4–20 mA sensors in hardware terms. Selection, troubleshooting, and warranty must all be directly handled by the Channel Partner. The extent of the support offered by Observant will be to provide guidance about the electric specifications for 4–20 mA Sensor connection and to display information recorded from the sensor using the current measurement and scaling settings provided by the Channel Partner in Equipment Manager.
4-20mA (and 0-20mA) sensors can be monitored by an Observant system, but it is important to determine how the sensor operates before installing it.
Current loop sensors come in two distinct varieties: Loop Powered (2-wire) and Self Powered (3 or 4 wire). Observant only supports Loop Powered sensors which are connected to an output port which both provides power to the sensor and measures its response.
Self Powered sensors are not supported due to the inherent difficulty in establishing the correct sense resistor for a given sensor and accurately converting the current reading to the desired value.
Any sensor can be calibrated and set to display using any standard engineering unit such as PSI, kPA or metres.
Loop Powered Sensors:
- Require an external power supply on the signal wires (ie. the Observant unit powers the sensor)
- Limit an externally provided current to 4-20mA
- Only have 2 wires
- Connect to an Observant OUTPUT PORT
- Must be rated to operate at 20V (Observant C3, Solo or Pico), or down to 11V or lower.
- In 4-20 mA current-sensor mode, readings have +/- 1% accuracy over full-scale range
The main configuration settings relate to how the reading range (4 mA – 20 mA) scale is interpreted as output units and hence a final measurement. The sensor readings in mA will be transformed to the specified units and a current value and historical values will be based on the relative.
The instantaneous reading is compared to the input scale to produce a percent of full scale. i.e. a reading of 12 mA is half way between 4mA and 20mA and is thus 50% of scale. If the output scale is 0-100 PSI then 12mA would equate to 50 PSI.
It is necessary to specify the reference units that readings will be displayed in. This will typically be a unit of measure for common phenomena such as temperature, humidity, length, pressure etc.
The second set of information needed is the extremities of the value range in the target units. This information will be provided in the technical documentation for the sensor itself. For example, a pressure sensor might have a reading range of 0—100 PSI, a temperature sensor may have a range of -40°C — +70°C.
The Generic 4–20 mA Sensor support two alarm settings: low value and high value. These are set in the same units as the output range.
Diagnostics and Trouble Shooting
The most important thing to take note of will be the current reading in equipment manager