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.
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