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HMRC internal manual

Oils Technical Manual

Flow meters - other information: What are the common types of meter and what affects them?

All meters need to be full of oil at all times, and sited in such a way that there is noturbulence in the flow ahead of the meter (this usually means there has to be a straightpipe into the meter) and there is nothing in the immediate vicinity of the meter thatcould interfere with the way the meter works – such as vibration.

| Type of flow meter and description | What the meter measures | Main usage and strengths | Weaknesses | || | Positive displacement – there are various types involving gears, ovals and screws – effectively they are like a pump working in reverse – a fixed volume is trapped in a chamber of the meter whilst it rotates. | Volume – this is in fact the only meter that is purely volumetric – its action is solely dependent upon the volume of liquid that passes through the meter. | Accurate at low flow rates – used for tanker loading gantries. | Needs clean fluids. If fluid (such as white spirit) has no lubricating properties the meter will wear out quickly.
Viscous fluids will find it difficult to push through the meter.
Compensations for temperature have to be made. | | Turbine – the faster the flow the faster it spins. | Speed of flow – rotations of the turbine are counted – usually by electronic sensor. | Little resistance to flow and can be fitted in large diameter pipes – used in pipelines. | Real time compensations have to be made for density, pressure and temperature. These calculations are made by a flow computer. Turbine meters often have a pipe running in parallel with them, in which a ‘proving meter’ can be run. | | Coriolis – usually a U tube which is made to vibrate. There is nothing in the tube. Passing fluid through the tube causes it to bend. | Mass – which is commonly understood as weight – the more the tube bends the greater the mass flowing through it. (Mass has to be converted to volume ). | They can be very accurate and are not affected by density changes in the fluid. They offer little resistance to flow. They may become common for revenue accounting. | Since vibration is a factor in their operation, external vibration can make them inaccurate. They are not accurate at low flow rates. They are not accurate for high viscosity fluids. |