Measurement resistance
Note on measuring resistances or resistance ratios
With 2‑wire measurement, the line resistance of the sensor supply lines influences the measured value. If a reduction of this systematic error component is desirable for 2‑wire measurements, the resistance of the supply line to the measuring resistance should be taken into account, in which case the resistance of the supply line has to be determined first.
Taking into account the uncertainty associated with this supply line resistance, it can then be included statically in the calculation, in the EL3751 via 0x8000:13 and in the ELM350x/ ELM370x via 0x80n0:13.
Any change in resistance of the supply line due to ageing, for example, is not taken into account automatically. Just the temperature dependency of copper lines with approx. 4000 ppm/K (corresponds to 0.4%/K!) is not insignificant during 24/7 operation.
A 3‑wire measurement enables the systematic component to be eliminated, assuming that the two supply lines are identical. With this type of measurement, the lead resistance of a supply line is measured continuously. The value determined in this way is then deducted twice from the measurement result, thereby eliminating the line resistance. Technically, this leads to a significantly more reliable measurement. However, taking into account the measurement uncertainty, the gain from the 3‑wire connection is less significant, since this assumption is subject to high uncertainty, in view of the fact that the individual line that was not measured may be damaged, or a varying resistance may have gone unnoticed.
Therefore, although technically the 3‑wire connection is a tried and tested approach, for measurements that are methodological assessed based on measurement uncertainty, we strongly recommend fully‑compensated 4‑wire connection.
With both 2‑wire and 3‑wire connection, the contact resistances of the terminal contacts influence the measuring process. The measuring accuracy can be further increased by a user‑side adjustment with the signal connection plugged in.
Notice | |
Measurement of small resistances Especially for measurements in the range < 10 Ω, the 4‑wire connection is absolutely necessary due to the relatively high supply and contact resistances. It should also be considered that with such low resistances the relative measurement error in relation to the full scale value (FSV) can become high ‑ for such measurements resistance measurement terminals with small measuring ranges such as EL3692 in 4‑wire measurement should be used if necessary. |
Corresponding considerations also lead to the common connection methods in bridge operation:
- Full bridge: 4‑wire connection without line compensation, 6‑wire connection with full line compensation
- Half bridge: 3‑wire connection without line compensation, 5‑wire connection with full line compensation
- Quarter bridge: 2‑wire connection without line compensation, 3‑wire connection with theoretical line compensation and 4‑wire connection with full line compensation