Guard Ring Extension
In the Novocontrol sample cells BDS 1200 and ZGS (Alpha-A Active Sample Cell), the generator voltage is applied to the bottom electrode, while the current signal is measured at the top electrode. The latter is connected to the virtual ground of the current measurement electronics.Measurements without guard electrodes may suffer from two effects:
- Influence of electric fringe fields developing at the outer rim of the electrodes. These add to the measured capacity and thus tend to lead to increased values for the measured capacity (real part, Cp’) and permittivity ε’.
- Currents flowing over the sample surface. This tends to increase the measured conduction and may lead to increased tan δ values. Both effects can be reduced or even suppressed using a grounded extra electrode in a concentric configuration around the top (current collecting) electrode, see Figure 1. Ideally, both edge fields and surface currents are blocked by the grounded guard electrode.