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HMRC internal manual

Oils Technical Manual

Measuring the Content of Tanks: Factors affecting measurement in tanks

Water – from condensation forms a layer (known as a “waterbottom”) underneath the oil which then floats above. Only the oil is to be measured,so the presence of water needs to be detected, measured and deducted from the dipping orullaging measurement. This can be done with a dipping tape and special water detectingpaste.

Deadwood – the term for anything that sticks out into the insideof a tank, such as bracing, that changes the volume of the tank – this is hidden fromview and might only occur at a particular height. This should be accounted for by having atank calibration table that converts the height of any measurement of liquid into thevolume left in the tank.

Floating roofs – the weight of the roof displaces the oil onwhich it floats, but below a certain level there will be stops to stop the roof sinkingfurther. Consequently, at a certain point the floating roof stops displacing the oil onwhich it is floating and affecting the volume calculation.

Irregular construction - and the fact that tank floors can sink inuse. Whilst a tank calibration table will account for irregular construction, sinking tankfloors may easily be missed and will affect both the measurement and accuracy of Automatic(tank) Level Gauges (ALGs).

Temperature – because density decreases with temperature and thesame amount of oil will take up less space (volume) the colder it is, then the temperatureof the oil needs to be known. ALGs usually measure temperature as well as the height ofany liquid in a tank.