FEATURE FLOW, LEVEL & CONTROL Figure 3:


Oval gear positive displacement meters. Target applications include those with high viscosity fluids


For a more in-depth review of low flow measurement techniques, the author has written an informative illustrated guide to help simplify the process of identifying and selecting the best type of small bore flowmeter (< 0.5-inch / 12.5mm) for a range of applications. Visit: www.flowmeters.co.uk/flowmeters-below-half-inch-bore.php


usually relatively low, as the smaller the meter the larger the relative leak path and the lower the operating efficiency. So, employing a positive displacement meter design can be a compromise. Conventional axial turbine flowmeters (propeller in a pipe) are often not suitable for low flows because they work best with turbulent flow. Small axial turbine flowmeters are also sensitive to changes in any part of the system, particularly viscosity and bearing drag. By comparison, Radial flow turbines, often described erroneously as Pelton wheels, are much less affected by system changes and consequently can be designed to operate across the laminar/turbulent boundary. Radial flow turbine meter devices (see Figure 4, below) usually use a lightweight flat


Figure 6: which there are two main types. Thermal dispersion meters


operate by measuring how liquid flow removes heat from an element proportional to the flow rate. This, however, is not very accurate. The second thermal method uses two


or even three elements. One element is used for reference temperature measurement; the second is a heat source; and the third measures the heat dissipation and so the flow rate. These are mass flow devices and are capable of measuring very low flows – although the thermal characteristics of the liquid must be known for precise measurement. Coriolis meters were a second type of


flow device that was developed for measuring low flows. These still use moving parts but only minutely and


Ultrasound, which is injected with the


direction of flow into the fluid by one transmitting sensor, is received by a second sensor further down the tube. The second sensor then sends a ping


Figure 4:


Radial turbine flowmeter. Suitable for low flow, low viscosity liquids


back against the flow to the first sensor. As one pulse is accelerated by the velocity of the fluid and the second retarded, the difference in the flight time is twice the fluid velocity. And, as the flowmeter tubes dimensions are known, the volumetric flow can be calculated. Because the ultrasound is travelling with the fluid in a plain wave it effectively integrates the velocity profile within the pipe. Ultrasonic flowmeters of this design are therefore immune to viscosity effects.


bladed turbine mounted on low friction bearings similar to the ones found in a large watch. With these, a jet of liquid is squirted at the plain face of the blade, causing rotation of the turbine.


ACCURATE MEASUREMENT Drawing on the known limitations of the above metering technologies, engineers realised that the only way that very low flows could be measured accurately was to inject some energy into the system and not disturb the flow too much. This was achieved by the following technological developments: Thermal flowmeters (see Figure 5, right) were the first low flow meters, of


20 JUNE 2014 | INSTRUMENTATION / INSTRUMENTATION


outside the flow tube. Coriolis flowmeters use the fact that fluid accelerated around a bend will cause a reaction at ninety degrees to the fluid’s movement. Such meters are very


Figure 5:


Thermal flowmeter (multi-element). Applications include micro flows and flow control


Titan Enterprises T: 01935 812790 www.flowmeters.co.uk Enter 662


Ultrasonic flowmeter.


Target applications are all liquids that require measurement, but at competitive prices


sensitive and will meter very low mass flows extremely accurately. However, they are typically also relatively expensive, limiting their use. The most recently developed and broadly effective low flow measurement devices are based on ultrasonic technology (see Figure 6, below). Using ultrasonic meters, flows as low as 0.25 millilitres per minute have been measured in trials in Titan’s development lab.


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