Titan Enterprises analyses solutions for low flow liquid measurement
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 .
Neil Hannay , Titan ’ s Senior R & D Engineer observes : “ The increase in demand for low flow measurement technologies is being driven by various industries moving towards transporting heavily concentrated liquids , which are then diluted at the point of use .”
Whether cleaning fluid additives , syrups and flavourings for beer or soda , chemical additives for oil and fuel , 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 . Here we analyse 5 types of flow meter - Ultrasonic , Turbine , Oval Gear , Thermal and Coriolis - and their suitability for low flow measurement .
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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 entrainment
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 fluidspecific 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 highprecision flow sensor for your application .
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