Why 4-point measurements?Latest updated: February 25, 2020
A potentiostat / galvanostat is an instrument that can control both the current and the voltage imposed to a cell. For this purpose, Bio-Logic instruments require at least 4 cables:
- 2 power cables (P+, P-)
- 2 (or 3) senses (S+, S- with or without reference)
For current control mode (galvanostatic), the instrument directly delivers the current specified in the settings through the power cables. In this case, the senses are only used to measure the voltage of the cell and they are not a part of the regulation process. The current is modulated by an adjustment of the voltage applied on a shunt inside the instrument. This is the reason why the user has to select a specific current range in galvanostatic mode.
Note: as a demonstration, it is possible to connect a resistor directly to the power cables, and to short circuit the senses together (to avoid voltage fluctuations).
For voltage control mode (potentiostatic), the instrument applies a determined voltage between both power cables according to the cell-desired voltage. In this case, the senses are a part of the regulation: the system adapts the voltage applied between the power cables to obtain the desired voltage between the senses.
We’ve seen the senses position is decisive for the measurement or for the control of the cell under test. It is possible to use a 2-point connection to the cell, but to guarantee qualitative experiments, a 4-point measurement is recommended.
A 4-point connection to an electrochemical device separates the current-carrying leads from the voltage-sensing leads. This ensures that there is no current passing through the voltage-sensing leads, or through connectors, cables, or connection interfaces (extension cables, battery holders…).
Let us consider the following example. In this case, connection cables are considered as resistors R1 and R3, and the cell as a resistor R2 (Fig. 1).
Figure 1: Difference between 2 and 4-point measurements
If the instrument is in galvanostatic mode, using a 2-point measurement connection, R1 and R3 resistors induce an additional ohmic drop: the voltage measured is not the one of the cell, but the one resulting from the the addition of the 3 resistors. If the instrument is in potentiostatic mode, the specified voltage is applied to the system (R1 + R2 + R3), and not to the cell.
On the other hand, 4-points measurements allow users to avoid these issues. The measurement or regulation is directly done on the cell itself. Result differences between both connection mode can be seen on Fig. 2.
Figure 2: Nyquist diagram showing the impact of the ohmic drop induced by a 2-point measurement.
It can be seen that the connection interface add an additional resistance of around 2 mΩ, that induces a shift of the blue curve to the right on the Nyquist diagram.