The measurement of low flow is becoming widely used in many industries. However, the smaller the flow, the trickier it is to control and measure, and finding a suitable flow measuring technology at reasonable cost can prove challenging for both users and flow sensor manufacturers.
There is no set definition for ‘low flow’ in terms of measurement limits for fluidics handling. However, low-flow applications encounter amplified flow stability and performance issues not seen in larger flows. The minimal liquid volume being measured in low flows renders them highly sensitive, such that even the slightest disruptions in process or ambient conditions can exert a substantial impact on flow stability. Within the markets Titan Enterprises operates in, we consider low flow rates as those below 50 ml/min, with many customers seeking flow rates of between 2 and 20 ml/min.
Transport of concentrated liquids boosts increased need for measurement solutions
Neil Hannay, Titan’s Senior R&D Engineer observes: “We are certainly seeing an increase in demand for low flow measurement technologies driven by various industries moving towards transporting heavily concentrated liquids, which are then diluted at the point of use. This translates into huge savings on transport and storage costs and also has a positive environmental impact.”
Whether cleaning fluid additives, syrups and flavourings for beer or soda, chemical additives for oil and fuel, paint pigments or administering drugs, low flow flowmeters are required to dose these concentrated fluids at the end process, dispensing the precise amount of liquid to the correct dilution.
As mentioned, measuring low flow is a challenging application to satisfy. The amount of energy available in low liquid flow is unlikely to be sufficient to drive most mechanical flowmeters to give linear results. By comparison, electronic flow meters can be limited by sensitivity, zero drift and slow response times. Here we analyse 5 types of flow meter - Ultrasonic, Turbine, Oval Gear, Thermal and Coriolis - and their suitability for low flow measurement:
Ultrasonic
Ultrasonic flowmeters measure the velocity of flow. Titan's in-line Atrato© models, using patented time of flight technology, are capable of measuring flows down to 2ml/min. Lower flow rates equate to smaller signals to determine flow rate and as such, this lower signal strength can affect the flowmeter’s capability to produce repeatable measurement results.
Design challenge: Straight-forward engineering vs complex electronics.
Advantages: High accuracy; not fluid specific; high signal to noise ratio; no pressure drop requirements; suitable for both turbulent and laminar liquid flow.
Disadvantages: Susceptible to process vibrations/pulsations/noise; sensitive to gas.
Turbine
The energy required to spin the rotor of a turbine flowmeter becomes swamped by the drag from the system at low flow rates. As flow rate reduces and transitions from turbulent to laminar flow, the linearity changes and the measurements become less accurate. Pelton wheel turbines that use low friction, precision bearings can mitigate this effect to some degree and with careful design, are capable of flows down to 1-2ml/min. They are capable of fast response times and operate across wide flow and operating temperature ranges.
Design challenge: Straight-forward electronics vs complex precision engineering.
Advantages: Low cost; can be calibrated in-situ; good accuracy and repeatability with rapid response times.
Disadvantages: Susceptible to changes in fluid properties; requires sufficient pressure to move liquid through the pipeline at a rate that causes the turbine blades to spin.
Oval Gear
Positive displacement flow meters, such as oval gear meters, are particularly effective for measuring low flow viscous fluids, although the resolution can be quite low. To obtain good resolution, the oval gear meters need to be small in low flow applications. Installing an oval gear meter in a horizontal position will reduce rotational friction and improve low-flow measurements. The lower the flow, the smaller the gear size, which are manufactured to tight tolerances with small internal clearances to minimise any fluid leakage around the gears.
Design challenge: Straight-forward electronics vs complex precision engineering.
Advantages: Ideal for viscous liquids, precision chemical dosing; good reliability.
Disadvantages: Not suitable for low flow aqueous solutions as the slippage past the moving element is greater than the volume being measured. Trapped air can prevent small gears from rotating – ensure all gas is purged on initial startup. Low resolution.
Thermal
Thermal flow sensors, primarily used for monitoring gas flow, operate on the principle of monitoring thermal transfer using a reference temperature, a heat injection and a detector. The basic approach is that heat is added to the flowing stream and a temperature imbalance being used to obtain a flow rate. They are fluid-specific as the technology relies on the liquid’s thermal properties and are generally calibrated for the specific fluid properties.
Design challenge: Relatively simple engineering vs complex electronics.
Advantages: Highly sensitive and able to measure flow rates down to nanolitres per minute; suitable for low pressure drop applications; not so reliant on the dynamics of the fluid to make a measurement.
Disadvantages: Fluid-specific. Thermal low-flow liquid flowmeters are non-linear over their temperature range and so require some correction during the process. Not suitable for low boiling point liquids or liquid mixtures with changing composition.
Coriolis
The Coriolis is a mass flowmeter, i.e. measures mass flow directly and independently of the liquid’s properties. The Coriolis provides mass flow and density measurements that are both repeatable and highly accurate, even when the composition of the liquid is unknown or changing. Using the principle of accelerating a moving fluid and detecting the reaction on the vibrating tube with sensors, Coriolis meters are very sensitive and flows lower than 0.2 ml/min are possible.
Design challenge: Complex electronics and engineering.
Advantages: Extensive material compatibility; can be used for either liquid or gas flow measurement; independent of liquid or process variables.
Disadvantages: The primary limitation is the flow must be single-phase and of low viscosity. They are also expensive devices so would not be suitable for low-cost low flow applications.
As flowmeters can be the most limiting component of a low flow fluidic system, it is essential to choose the most suitable high-precision flow sensor for an application.
