Pressure Measuring Devices (Hydrostatic Systems)
Measuring using hydrostatic systems
The hydrostatic pressure due to a liquid
is directly proportional to its depth and hence to the level of its
surface. Several instruments which use this principle for measuring
liquid level are available and are widely used in many industries,
particularly in harsh chemical environments.
In the case of open-topped vessels (or
covered ones which are vented to the atmosphere), the level can be
measured by inserting an appropriate pressure transducer at the bottom
of the vessel. One source of error in this method can be imprecise
knowledge of the liquid density. This can be a particular problem in the
case of liquid solutions and mixtures (especially hydrocarbons), and in
some cases only an estimate of density is available. Even with single
liquids, the density is subject to variation with temperature, and
therefore temperature measurement may be required if very accurate level
measurements are needed.
Where liquid-containing vessels are
totally sealed, the liquid level can be calculated by measuring the
differential pressure between the top and bottom of the tank. The
differential pressure transducer used is normally a standard diaphragm
type, although silicon-based microsensors are being used in increasing
numbers.
An additional problem which can occur is
an accumulation of liquid on the side of the differential pressure
transducer which is measuring the pressure at the top of the vessel.
This can arise because of temperature fluctuations, which allow liquid
alternately to vaporize from the liquid surface and then condense in the
pressure tapping at the top of the vessel. The effect of this on the
accuracy of the differential pressure measurement is severe, but the
problem is easily avoided by placing a drain pot in the system. This
should of course be drained regularly.
A final pressure-related system of level
measurement is the bubbler unit which uses a dip pipe which reaches to
the bottom of the tank and is purged free of liquid by a steady flow of
gas through it. The rate of flow is adjusted until gas bubbles are just
seen to emerge from the end of the tube. The pressure in the tube,
measured by a pressure transducer, is then equal to the liquid pressure
at the bottom of the tank. It is important that the gas used is inert
with respect to the liquid in the vessel. Nitrogen or sometimes just air
is suitable in most cases. Gas consumption is low, and a cylinder of
nitrogen may typically last for 6 months. The method is suitable for
measuring the liquid pressure at the bottom of both open and sealed
tanks. It is particularly advantageous in avoiding the large maintenance
problem associated with leaks at the bottom of tanks at the site of the
pressure tappings required by alternative methods.

More Level Measurements
|