How to connect a sensor with 4-20 milliamps output to a Datalogger/PLC

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How to connect a sensor with 4-20mA output to a Datalogger

4-20mA output sensors are widespread in the world. This is because the output in ‘current loop’ is the most robust and reliable kind of analogical output.

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When signal precision is needed, or datalogger-to-sensor distances are over 18m, or in case of electromagnetic disturbances, it’s necessary to use a sensor with output in current loop. On the other side a datalogger/PLC with input channel in 4.20mA is required. Two wires to supply and retrieve sensor data are sufficient.

Frequently the sensor is not of the same manufacturer as the datalogger, and you can experience complications when trying to wire the sensor with the datalogger. You may also have trouble setting up the range of values.

Let’s solve these problems.

Possible wiring setups

Sensor and datalogger/PLC input channel have the same number of wires (2 or 3)

Just cable the sensor to the datalogger/PLC as per instructions.

2-cable sensor, 1-cable datalogger input channel

Refer to the diagram below.

2-cable sensor, 1-cable datalogger input channel

2-cable sensor, 3-cable datalogger/PLC input channel

Refer to the diagram below.

2-cable sensor, 3-cable datalogger/PLC input channel

3-cable sensor, 2-cable datalogger/PLC input channel

3-cable sensor, 2-cable datalogger/PLC input channel

Possible case setups

Once the sensor is connected to the datalogger with an input in electrical current, it is necessary to enter the calibration data. The purpose of this page is to help the operator enter the correct data required in any possible situation.

The datalogger requires OFFSET and GAIN

The range values are provided by the sensor data sheet. The application will calculate the required Slope Gain and Offset factors. OFFSET is the value of slip with respect to the reference value indicated by the manufacturer and GAIN (also called SLOPE) is the ratio between the range scale of the actual parameter measured by the sensor and the parameter of electrical current indicated by the manufacturer.

Example:

The datalogger requires OFFSET and GAIN

Datalogger configuration requires BEGINNING OF SCALE and FULL SCALE values

Enter the values from the sensor data sheet operation ranges. BEGINNING SCALE=enter the values at the lower end of the range indicated by the manufacturer. FULL SCALE=enter the values at the upper end of the range indicated by the manufacturer.

Example:

Example 2

Datalogger configuration doesn’t allow current values in (mA) because they are out of limit but indicated in sensor data sheet

You need to obtain the value corresponding to the 4mA and 20mA starting from the sensor data sheet.

Example:

Example 3

The system requires a calibration with different values of the range the scale

You need to find the value in (mA) corresponding to the new parameter.

Example:

Example 4

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