Q: Long wires used to connect analog sensors to the data logger can suffer from lead resistance and electric interference. What are the possibilities to cope with these problems?
A: 2, 3 and 4-wire configuration explained.
T: When great distance between the sensor and data logger has to be covered, choose for a digital readble sensor.
2-wire configuration
The simple, two wire connection, is only used where high accuracy is not essential and /or short, low resistance leads are used. (1..2 Ohm, 10 meters).
The resistance of the connecting wires is always included with the sensor resistance, leading to errors in the signal. (Resistance of element and the two wires). The values of the lead resistance can be compensated by a two point calibration, available in the sensor-setup menu data logger.
3-wire configuration
A more accurate configuration is using 3 wires, one of the sensor leads has an extra (parallel) wire) to compensate the lead resistance. Although there is lead resistance in each leg of the bridge is assumed, the two lead resistances are equal. Advice is to use high quality connection cables. (1..10 Ohm, 50 meters).
The PT1000 3 wire configuration is very popular for general industrial applications and widely used with 4 to 20mA current sensor types. Three wire sensor support is available on the Ydoc ML-OI-AD-PT1000 option board.
4-wire configuration
The most accurate wiring arrangement is the four wire configuration. This provides full cancellation of spurious effects with the bridge type measuring technique. (1..15 Ohm, 100 meters). The wire resistance is compensated through the extra (current less) wires, connected to the sensor.
Note: A 4-wire sensor configuration is not available on Ydoc analog option board. When very high accuracy is needed with long distance between sensor and data logger has to be covered, advice is to choose for a digital readable sensor (ASCII, SDI-12 or MODBUS)., skipping long wires in the sensor measurement circuit.