Which proximity sensor type is commonly used to detect fluid levels through a tank?

• A. Inductive
• B. Photoelectric
• C. Unshielded capacitive
• D. Shielded capacitive

Answer: (C)

Measuring liquid level through a tank relies on changes in capacitance caused by the liquid’s dielectric constant differing from air. Capacitive sensors are well suited for through-wall sensing because their field couples into the liquid behind a nonmetallic container, so the level directly alters the measured capacitance.

Unshielded capacitive sensors have the sensing surface exposed to the medium, which makes them highly sensitive to the presence or absence of liquid behind the wall. This strong coupling yields a clear change in signal as the level changes. Shielded capacitive sensors, while excellent at minimizing interference from surrounding materials, restrict the electric field to the sensing tip and can dampen the signal when trying to read through a tank wall, making through-wall level changes harder to detect reliably. That’s why, in a typical through-tank level setup, the unshielded design is commonly preferred.

Inductive sensors look for metallic targets and don’t respond to liquids, and photoelectric sensors can be less reliable through opaque walls or with liquids that scatter or absorb the light.