Fluid/Liquid Low Flow Meters: How do I meter applications with low flows?
Liquid/fluid, low flow meters, sensors and indicators are a challenging application to satisfy. Mechanical flow meters typically cannot extract enough energy to give linear results. By comparison, electronic flow meters often have problems with sensitivity, zero drift and response times. Below we look at technologies proposed for fluid/liquid low flow measurement applications and their corresponding pros and cons.
Ultrasonic meters – the perfect low flow meters?
Ultrasonic flow meters are probably the newest addition to the low flow metering armoury. The principle of ultrasonic flow meters being ultrasound travelling within the fluid in a small pipe, over a long distance. Over the last 10 years – Titan have developed several generations of ultrasonic low flow meters based upon a patented time-of-flight design able to measure the velocity of the fluid within the pipe. The very high signal to noise ratio from these devices has been widely proven to enable metering of extremely low flows with great precision.
Other Low-Flow Flow Meter Technologies
Coriolis flow meters for use within low flow measurement applications
A few manufacturers have introduced Coriolis flow meters into their product ranges. They are very sensitive and record low mass flows accurately and as such, can be useful as a low flow meter. Unlike the thermal and electromagnetic devices mentioned below, this meter does require the movement of the fluid to react with an energy input from the system. Coriolis flow meters can be an expensive alternative to other technologies.
Positive displacement meters. Useful as a low flow meters?
Positive displacement flow meters can be good alternative low flow meters – particularly for metering of viscous fluids, but their resolution tends to be quite low. For metering low flows of aqueous solutions they are very poor as the “slip” past the various moving elements is greater than the volume being measured. There are some specialist high precision positive displacement devices which are successful but they are typically fluid specific and expensive.
Variable area flow meters and liquid low-flow applications
The ubiquitous ball-in-a-tube flow measurement device commonly referred to as a Rotameter. Variable area flow meters have been the backbone of measuring low flows in laboratories for many years. Unfortunately they are not very accurate at lower flows, typically ±5% for a given set of conditions. This flow sensor type also suffers from changes in liquid properties and will change the height the float attains and so the flow rate indication. Variable area flow meters typically do not offer an electronic output.
Low flow axial turbines with liquid low-flow
Axial flow turbines do not work well at low flow. The energy required to spin the rotor becomes swamped by the drag from the system at low flow rates. Other factors such as changes in fluid density or viscosity will also affect your calibration. These flow measurement devices provide best results for metering turbulent flow which is rarely seen with low flows. Some small polymer axial turbine devices have been quite successful in metering low flows but changes in fluid properties are still an issue.
Pelton wheel turbines. Useful for liquid low-flow applications?
These radial flow devices work better than their axial counterparts as a low flow meter, but still have similar problems. Titan and several other manufacturers try to reduce the drag that occurs with the transition from turbulent to laminar flow but the linearity still changes as the flow reduces. Bearing drag is also an issue, but can be mitigated with Pelton wheel turbines as the system design does permit low friction bearings.
Titan’s first patent for a low flow meter was for a radial flow turbine which had no blades. Two plain discs on low friction bearings were placed 0.8mm apart and a 0.8mm jet of liquid was injected into the gap. The fluid in the gap acted as the resistance to the incoming jet and rotated the discs. Having no blades to “stir” the fluid the overall drag on the system was much reduced. Flows down to 0.5 ml per minute were successfully recorded with this “bladeless” turbine”. We no longer make this product.
Thermal meters and low flow device applications
Thermal flow measurement devices often offer good performance for metering low flows. Unlike the previous technologies, with a thermal flow meter there is energy injected into the system so the meter is not so reliant on the dynamics of the fluid to make the measurement. They are still fluid specific as they rely on the fluid’s thermal transfer properties. Typically these devices have slow response times.
Electromagnetic flow sensors and low-flow measurement applications
Electromagnetic flow sensors are another example of a flow device that does not rely on the moving fluid to do any “work”. As velocity measuring devices – electromagnetic meters rely on an induced voltage in a moving conductor, the fluid. They will not work with non-conducting fluids so a vast number of liquids are excluded from their measurement capabilities.