DP level sensors determine level by measuring which principle?

• A. Pressure difference within the tank
• B. Temperature distribution
• C. Magnetic field strength
• D. Visual sight gauge

Answer: (A)

This question tests using pressure difference to determine liquid level. A differential pressure level transmitter has one port connected to the liquid at a lower point and the other to a reference point higher up. Because hydrostatic pressure increases with depth, the pressure difference between these two taps equals the liquid density times gravity times the height of the liquid column (ΔP ≈ ρ g h). The instrument converts this differential pressure into a level measurement, so higher ΔP means more liquid height. The other options don’t describe how a DP level sensor works: temperature distribution isn’t how level is inferred in this method (though density changes with temperature can affect accuracy); magnetic field strength isn’t involved in hydrostatic-level measurement; and a visual sight gauge is a direct visual indication, not a differential pressure-based method.