2-wire connection technology

If the two-wire technique is used to connect the EL3204-0162, the terminal is connected via a two-core cable, whose internal resistance R_L is connected in series with the resistance sensor R_t (fig. Two-wire connection technique with internal compensation in the terminal). The addition of these resistances causes a systematically increased temperature display, making compensation necessary. The compensation is performed internally in the terminal by the object 0x8000:1B / 0x8010:1B. Here the line resistance can be set with a resolution of 1/32 ohm.

Example:

The cross-section of the 50 meter long connecting cable made of stranded copper wire is 0.5 mm².
The specific resistance of copper corresponds to 0.0175 Ω mm² per meter

Determination of the total resistance of the supply line:

R_Ltot = 0.0175 Ω mm² / m x (2 x 50 m / 0,5 mm²) = 3.5 Ω

For a resistance change of the Pt100 of ≈ 0.39 Ω/K , the resulting temperature deviation is:

3.5 Ω / (0.39 Ω/°C) = 8.97 °C

Compensation

In order to compensate this error, the EL3204-0162 internal circuit generally assumes a cable resistance of 10 Ω, which is set via the object 0x8000:1B / 0x8010:1B.

For the adjustment the resistance value R_Ltot (in the example 3.5 ohm) is entered into the index 0x8000:1B / 0x8010:1B. With a resolution of 1/32 ohm this results in a value to be entered of:

3.5 Ω / (1/32) Ω = 112_dec

Further calculations for the compensation take place internally in the terminal.