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HMRC internal manual

Oils Technical Manual

Flow meters - other information: What can make flow meters inaccurate?

The type of flow meter used is a function not just of the accuracy of the meter itself,but also of its suitability to cope in the conditions in which it is used.

The factors that affect meters are:

  • Turbulence caused by bends in pipe work before the meter – this can affect all types of meter – indeed, some types of meter work by measuring turbulence caused by a fixed obstruction – so that any additional turbulence will make the meter inaccurate.
  • Bubbles – anything that allows air or gas into the pipe could again cause most meters to be inaccurate, by changing the volume.
  • Viscosity of the fluid – may be illustrated by the difference in viscosities between lubricating oil and white spirit. Thicker lubricating oil has to be heated to thin it and then pumped and stored in heated or insulated pipes and tanks. Thinner white spirit has 2 adverse effects: it has no lubricating properties to protect the meter through which it is flowing, and its ‘thinness’ means that the meter has to maintain a very tight seal where there are moving parts – accordingly they tend to wear out quickly.
  • Temperature – the density of the oil reduces the hotter it becomes – plus some flow meters work by measuring temperature variation.
  • Vibration – ‘Coriolis’ meters in particular are affected by vibration because they use vibration in order to produce a measurement, so mounting them on a vibrating jetty as has happened, will render the meter inaccurate.
  • Small amounts – a meter should be able to measure less than 2 litres for revenue accounting but some find it difficult to measure flows that stop and start – (also see ‘linearity)
  • Changing flow rates – flow meters used for loading have to be accurate from a standing start, and initially the loading of tankers is at a low flow rate. If the loading were done at a high flow rate, the fuel sloshing round an empty tank could generate enough static electricity to cause a spark. After sufficient fuel is in the tank, the flow rate is turned up to finish the load as quickly as is practical and safe.
  • Electromagnetic interference – (possibly generated by electrical equipment, transformers, microwave aerials such as mobile phone masts) can affect meters that work by measuring magnetic interference and might affect computers and transmission equipment built into the meter. They could cause extra current to be induced into a wire or flaws/pulses to be generated. They may also interfere with microwave transmission, where meter readings are wireless, and are transmitted instead to a control room.
  • Non oil products – Meter systems should include suitable filters upstream so that any particles or solids do not enter the meter and cause interference with the measurement device.