Technical Note - Generic 4–20 mA Sensor

Sumaiya

Created: ; updated: .

Description

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.

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.

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

 

Applications

Typical applications include:

  • Pressure monitoring
  • Water level monitoring

Any sensor can be calibrated and set to display using any standard engineering unit such as PSI, kPA or metres.

 

Connection to Field Unit Ports

Field Unit Port  Interface cables and Connectors
 C3 Output port N/A 
Solo Sensor port Solo sensor accessory pack Item#10124 

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Configuration

The Generic 4-20mA device can be found under the 'Generic Devices' category in Manage Equipment.

The device's port configuration will be different for:

Adding to a C3 unit

For C3 units, change the POWER to the Output port that the sensor is connected to. So if it's connected to the Output 2, choose Output2. 

Adding to a Solo

For Solo, change the POWER to Default. 

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You will also need to configure how the reading range (4 mA – 20 mA) scale is interpreted as output units. 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.

Output Units

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.

Value Range

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.

Threshold Settings

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. 

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