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HMRC internal manual

Oils Technical Manual

Measurement: hydrostatic gauges including manometers

Hydrostatic gauges including manometers

The pressure at any given point in a liquid is proportional to the density and height of the liquid above that point. The reading given by a pressure gauge showing the pressure at a fixed point near the bottom of the tank may therefore be used to determine:

  • The depth of liquid in the tank if the density is known: and
  • The weight of the liquid above the fixed point if the horizontal area of the tank is also known.

By assuming average densities or areas, gauges can be calibrated to read in dips or units or weight. These devices are seldom sufficiently accurate for revenue accounting, but may be used for spot checks on quantities.

Examples: Types of pressure gauges.

  • “Contract” manometer. The liquid being measured communicates its pressure to an indicating liquid (usually mercury) and balances the weight of the indicator in a vertical glass tube. The height of the indicator reflects the pressure and the tube can be graduated to read the liquid weight or depth. Because of the weight of the indicator this type can only be used at about the level of the tank bottom.
  • “Air-purge” manometer. A supply of air is used to transfer the pressure in the tank to the indicating liquid in a vertical glass tube. Since the density of air is low, the glass tube is able to be located at levels other than the tank bottom. The air pressure must be maintained by an air pump or a compressed-air reservoir. When the air pressure exceeds the tank liquid pressure, air is allowed to blow off through the tank.
  • “Mechanical hydrostatic gauge”. A pressure-sensitive capsule is exposed to the liquid pressure in the tank and indicates the pressure on a dial or by tracing pen. These mechanical gauges can be connected to electronic equipment for remote indication.