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HMRC internal manual

Oils Technical Manual

Flow meters - other information: Terms relating to meters

  • Linearity – if a meter’s results are linear, it means the meter would produce a straight line on a graph that plots volume against flow. In other words, the meter would give the same degree of accuracy at both low medium and high flow rates. (In theory only a positive displacement meter will achieve this – as many types of meter will not register much at low flow and will underestimate at high flows). To maintain accuracy, with non-linear meters adjustments have to be made to the readings according to flow rates.
  • Inferential – a meter that gives a volumetric reading but is actually measuring a different factor, from which it is able to infer or to produce an equivalent volume, e.g. an inferential meter may make an assumption that because a turbine has done 10 revolutions this must mean 3 litres of gas oil has flowed through it.
  • Meter proving – testing the accuracy of a meter by comparing the flow through the meter to be used, against a reference meter that is both known to be and has been proved as accurate. Some sites have their own reference meters but ‘meter proving’ is often done by contractors. Meters for ‘revenue accounting’ have to ‘proved’ when installed, then again at 3 months and 6 months. If these results are satisfactory routine proving may then be carried out at six-monthly intervals.
  • MPE – Maximum Permitted Error, the maximum error a measuring system deviates from the proving meter and can still be used. (MPE is not an allowable tolerance: it does not allow a trader to understate by the amount of the MPE).
  • Flow computers – this a term for a particular device, often the size of a pocket calculator, mounted on or near the flow meter which can make calculations that adjust the meter reading for factors that can alter readings in meters such as pressure, temperature, density, time over which readings take place, low flow rates, high flow rates.

(These are particularly important for turbine meters). Not all these factors occur orupset all meters (See the Note* below).

  • Automatic temperature compensating (ATC) meters – as previously mentioned, we account for oil by volume, but volume varies with temperature. Any measurement system has to adjust for what the oil temperature is at the time the oil is measured, and calculate what the volume would be if the oil temperature would have been 15ºC. We prefer meters that do this automatically.
  • Meter creep – this describes the tendency for some meters to move slightly forward between consecutive loadings. It is more noticeable when the smallest amount the meter can read is greater than one litre, i.e. the meter cannot accurately account for small amounts or fractions, which then have to be read or recorded as a higher amount.
  • Repeatability – this is measured during meter proving and indicates the range of results between successive proving tests under the same conditions.
  • Meter factor – this is established during meter proving tests and is the volume actually passed through the flow meter divided by the meters indicated volume.

Note* The trade recommends that flow computers should only be used where youcannot depend upon the accuracy of the meter alone.

This is because of the extra complexity flow computers add to the calculations involved inmeasurements, and their potential for unintended and undetected errors which can affectthe accuracy of the measurement.