Know How
Conductivity Systems
An electrode (metallic rod or tube) is placed in a conductive liquid (e.g. water, acid, sewage & other aqueous solutions) at the point where switching is required. When the liquid touches or leaves the tip of the electrode, an electrical circuit between it and earth is completed. This causes a switch to operate in the control unit. This is a very simple, accurate and economical method of controlling levels in liquids and covers up to 70% of applications.
Capacitive Systems
When the liquid (or free flowing powders) is non-conducting (e.g. oil products, viscous & granular materials) the conductivity method does not work, and a different type of electrode containing electronic circuitry is required. This relies on the capacitive change as the liquid (powder) leaves or touches the tip of the electrode. The same principle is used in the "Flexicap" system which gives continuous level indication.
Hydrostatic Systems
These employ a Pressure Transducer or Transmitter which when placed in a liquid, measures the pressure. Changes in the pressure reflect changes in depth so that a continuous indication of level is obtained.
Float Systems
A small magnetic float slides on a tubular shaft in which reed switches are mounted at intervals. The float follows the rise and fall of the liquid and actuates the switches to produce a continuous level indication. A useful system where a vessel is used for liquids of varying S.G.s and/or is pressurised.
Bubbler (Hydrostatic) Systems
Because of the aggressive nature of some liquids,it is impossible to use normal hydrostatic or ultrasonic methods. However by mounting a pressure transducer remote from the liquid and measuring the pressure by an interposed bubbler system, a continuous level indication can be obtained.
Ultrasonic Systems
A non-contact system which comprises an Ultrasonic Transducer which contains a crystal and associated electronic circuitry. The crystal is activated electrically to produce an ultrasonic sound wave or pulse, which is focused on the liquid whose depth is to be measured. The ultrasonic pulse is reflected back to the transducer, and the time for transmission and return, corrected for temperature variation, is a function of depth. Bursts of pulses are transmitted at frequent intervals. Spurious pulses outside a "Dynamic Window" based on the transducer frequency, are rejected and the average taken of the remainder. This is converted into a 4/20mA signal, proportional to the depth or distance from the transducer, whichever is selected. The 4/20mA signal may be passed to a PLC, Digital/Analogue meter, or to the "Flexilevel" controller.
