ABSTRACT
These tapes are usually made of steel or brass or aluminium coiled over a wheel and the end of the tape is attached to a metallic ball or bob made of brass or aluminium. For low viscous and non-sticky liquids, a dip tape is lowered into the liquid until it touches the bottom of the tank or the datum plate in the tank. To ensure the correct vertical distance from the top to the bottom, it is essential to check the tape reading against a reference height marked on the tank. The height of the liquid is then read as the difference between the reading of the wetted portion and the bob end. Colour-sensitive pastes are used to mark the beginning of the wetted portion. For example, oil fi nding and water fi nding pastes (two different colour-sensitive dyes that change colour as they contact oil and water, respectively) are used. This type of measurement is known as the innage method of dipping. For sticky and viscous liquids or semi-solid or solid materials, outage dipping is done. In this method, the height of the space above the stock of materials is measured by a tape as lowering of the same will be diffi cult through the liquid. The typical measuring methods for both innage and outage dipping are presented in Figure 7.1. Level measuring instruments, such as fl oat tape, a differential pressure gauge, a displacer level gauge, and electrical level gauges, are also used for measurement and recording in a distant control room. Since measurement of level by these instruments may not be accurate, it is therefore manual dipping, as discussed earlier, that is done for more accuracy over a long period of time, e.g., every day, once in a month or once in a year, where commercial transactions are involved. However, instrument reading is essential for continuous evaluation and recording.
