7.2 Pressure Sensors

7.2.1 Various types of pressure-operated tide gauge systems are in use. They may be classified into open or closed gas systems and electronic systems.

7.2.2 Open gas or "bubbler" systems employ the escape of small bubbles of air or other gas from a pressure line. The pressure of gas in the line varies with the water head and is measured by one of a number of different methods.

7.2.3 Closed gas systems employ a diaphragm or a bladder to detect the changes in water pressure, which are transmitted through a capillary tube to the recorder where they are measured by one of a number of different methods.

7.2.4 In the gas system the sensor is often a mechanical device such as a Bourdon tube or an aneroid capsule but it may also be a crystal transducer or strain gauge.

7.2.5 In the electronic- system the changes in pressure are detected by a sensor immersed in the water and converted directly to an electrical signal. In these systems the sensor may be a crystal transducer, strain gauge or other transducer.

7.2.6 In order to maintain the design performance of these systems the operator should guard against the following:

• marine fouling or siltation of bubble escape, pressure inlet or diaphragm;

• escape of gas from a closed system;

• loss of gas pressure in the bubbler system, whether due to leakage or to failure of gas supply;

• damage to pressure line or electrical cable.

7.2.7 Gas systems require damping of the effects of surface wave action. This is usually effected by a constrictor unit in the pressure line. Electronic systems achieve the same effect by averaging or filtering of the electrical signals.