Going with the flow

Written by: Tom Shelley | Published:

A standard electric motor and a pair of pressure sensors form the basis of an improved flowmeter. Lou Reade reports

A new design of flowmeter could be half the cost of existing models, because it uses a different principle to measure fluid flow rate.
Instead being able to rotate freely, the fanblade is driven by a small motor until it reaches the speed at which it takes no energy from the surrounding fluid. At the same time, a huge increase in the range of flow rates that can be accurately measured – the so-called 'turndown ratio' – means the new device could take the place of several conventional flow meters in a single application.
So far, the device – developed by Cambridge-based engineering consultancy PA Consulting – has only been tested at laboratory scale. However, the company has filed an outline patent, and has begun talking to potential customers.
“We can see potential in precision flow measurement for gas, water, chemical dosing and even medical instrumentation,” says Martin Hughes, managing consultant in the company's product and process engineering group.
Flowmeters work by allowing a fluid to turn a freely rotating fanblade. Measuring its rotational speed then gives flow rate. However, friction in the bearings causes 'slip' between the blades and the flow, which affects accuracy at lower flow rates. At higher flow rates, a pressure drop develops between the 'upstream' and 'downstream' sections of the blade – further reducing accuracy.
“The less sophisticated the bearings, the more the slip,” says Hughes.
Slip occurs because the blade takes energy from the fluid. PA's answer is to use a motor to power the fanblade to the point where there is no energy transfer between the fluid and the blades. The speed of the motor is adjusted until the upstream and downstream pressures – measured by a pair of pressure sensors – are the same. At this point, there is no slip, and the speed of the blade accurately reflects the flow rate. PA says this could increase the turndown ratio from 20:1 up to around 100:1.
“Because there is no slip, there is very little potential for damaging the fluid,” says Hughes. “This makes it potentially useful for measuring blood flow. You want high accuracy, but you have a very delicate fluid.”
Despite the fact that this design uses more components, Hughes says that it will cost less.
“I think it will cost 50-70% of a conventional flowmeter,” he says. “The motor and the electronics are not very sophisticated. You need good pressure sensors, but they are not really very expensive.”
The turndown ratio of a conventional flowmeter could be boosted with smoother bearings or by redesigning the fan blades – but these would add cost, says Hughes.
He points out that the device is not appropriate for two-phase measurement, so could only be used for a liquid or a gas – not both at the same time.
PA has also begun work on a cheaper, more elegant solution. Hughes says that the 'zero slip point' coincides with the minimum amount of energy going into the system – which could be determined simply by monitoring the current in the motor.
“In theory it should work – though we've not tested it yet,” he says.

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