The FDC can be used to measure liquid level in non-conductive containers. Due to very high excitation rate capability, the FDC is able to measure soapy water, ink, soap, and other conductive liquids. Capacitive sensors can be attached to the outside of the container or be located remotely from the container, allowing for contactless measurements.

The working principle is based on a ratiometric measurement; Figure 7-7 shows a possible system implementation which uses three electrodes. The level electrode provides a capacitance value proportional to the liquid level. The reference environmental electrode and the reference liquid electrode are used as references. The reference liquid electrode accounts for the liquid dielectric constant and its variation, while the reference environmental electrode is used to compensate for any other environmental variations that are not due to the liquid itself. Note that the reference environmental electrode and the reference liquid electrode are the same physical size (hREF).

For this application, single-ended measurements on the active channels are appropriate, as the tank is grounded. Use Equation 7 to determine the liquid level from the measured capacitances:

Equation 7.

where

• C_RE is the capacitance of the reference environmental electrode,
• C_RL is the capacitance of the reference liquid electrode,
• C_Lev is the current value of the capacitance measured at the level electrode sensor,
• C_Lev(0) is the capacitance of the Level electrode when the container is empty, and
• h_REF is the height in the desired units of the container or liquid reference electrodes.

The ratio between the capacitance of the level and the reference electrodes allows simple calculation of the liquid level inside the container itself. Very high sensitivity values (that is, many LSB/mm) can be obtained due to the high resolution of the FDC2x1x, even when the sensors are located remotely from the container. Note that this approach assumes that the container has a uniform cross section from top to bottom, so that each incremental increase or decrease in the liquid represents a change in volume that is directly related to the height of the liquid.

Figure 7-7 FDC (Liquid Level Measurement)