In a 0-100 ft tank with a specific gravity of 0.8, what is the highest level measurable?

• A. 100 ft
• B. 80 ft
• C. 60 ft
• D. 120 ft

Answer: (B)

The reading is based on hydrostatic pressure and how instruments calibrated in feet of liquid convert that pressure back to a height. Pressure at depth is P = ρ g h, and ρ is the liquid density. If the level measurement is calibrated assuming water (specific gravity 1.0), the indicated level becomes h times the liquid’s SG: Indicated height = SG × actual height.

Here, the tank is 100 ft tall and the liquid’s SG is 0.8. The maximum indicated level is 100 × 0.8 = 80 ft. So 80 ft is the highest level the instrument would show.

If it were pure water (SG 1.0), you’d read the full 100 ft. A reading of 60 ft would correspond to either a 60 ft actual height or a lower SG, and 120 ft would require a higher SG or a taller tank.