1. Bubbler System: In this system, air is supplied under pressure through a flow indicator such as a rotameter and an orifice. A dip tube extends into the tank to a point approximately 75 mm above the sludge level. As the liquid level rises, the hydrostatic pressure at the tip of the tube increases. This pressure is directly proportional to the liquid head and is used to operate indicators, alarms, and control circuits. The pressure required to just overcome the liquid head and allow air bubbles to escape is measured and used to determine the tank level.

2. Pneumercator Gauge: This system is commonly used for measuring liquid levels in deep tanks and double bottom tanks. It operates on the principle of a well-type mercury manometer. Air pressure is transmitted to the well of the manometer, where the pressure is balanced against the force exerted by the liquid head in the tank. The resulting mercury level difference provides a direct measure of the hydrostatic pressure and hence the liquid level in the tank.

The readings from manometric gauges typically indicate the volume of liquid in the tank. However, for accurate gauging of mass content, the following corrections must be applied:

• Temperature Correction: The density of the liquid varies with temperature, so it must be corrected to standard conditions.
• Density Correction: The actual relative density at tank temperature (T °C) is calculated as:

$$R_{T}=R_{15}\:\times\left(1-.00065\left(T-15\right)\right)$$

• where R15​ is the relative density at 15°C.

Mass Calculation: Once the corrected density is known, the mass is calculated as:

$$Mass=Volume\times Relative\:density\:at\:T^{o}C$$

When no records of the main engine’s daily fuel consumption are available, bunkering requirements can be estimated by determining the engine's power output using alternative methods such as:

• Torsion Meter: Measures the torque transmitted through the shaft to calculate power.
• Draw Cards (Indicator Diagrams): Provide engine pressure-volume data to calculate indicated power.

Using the power output and known specific fuel oil consumption (SFOC) values for the engine, the approximate daily fuel consumption can be estimated, and thus, the bunkering requirement can be reasonably calculated.