Flow instruments for low-flow liquid applications
There is a wide range of flow technologies available for measurement and control in low-flow liquid applications, from mass flow and differential pressure to velocity flow and positive displacement. Here we analyze some of the most popular low-flow liquid flow solutions and evaluate use cases of each.
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Coriolis flow instruments
Coriolis flow meters and controllers measure mass flow directly and independently of fluid properties – with the added bonus of measuring density. The Coriolis instrument does this by calculating mass from flow-induced vibrational changes and then converting that into a flow rate and fluid density. For a thorough explanation of the Coriolis operating principle, read here.
These devices provide repeatable, high accuracy mass flow and density measurements even when the percent composition of your liquid is unknown or changing. Another noteworthy benefit of Coriolis technology is extensive material compatibility. A single Coriolis unit can be used for either liquid or gas (often including corrosive/aggressive fluids) – and most don’t even require an annual calibration. The primary limitation is that flow must be single-phase and cannot have high viscosity. These features make Coriolis an excellent choice for high accuracy low-flow liquid applications.
Ultra-low flow demonstration with CODA Coriolis controller
Thermal liquid flow instruments
Thermal liquid flow meters and controllers typically calculate flow by measuring resistance changes across two heaters in an electrical bridge arrangement. The differential is measured by sensors inside the flow body, and the flow calculation also relies on temperature-dependent fluid properties. Because of this, the devices are preloaded with tables containing fluid properties.
These instruments are excellent choices when you need a high-accuracy flow rate for a low pressure drop application. The primary limitation of thermal liquid flow devices is that they rely on K-factor corrections when flowing fluids other than that which they were calibrated for. Additionally, they are unable to accurately measure liquid mixtures or fluids with changing composition. This can be especially problematic when using liquids with very low boiling points, which have the potential to phase change when heated up within the flow body. Thermal technology can be a good choice for applications with extremely low mg/hour flow rates.
Ultrasonic flow meters
Ultrasonic liquid flow meters are composed of a pair of transducers clamped onto a pipe and use the Doppler Effect to determine fluid velocity. An ultrasonic wave is then sent through the fluid in the pipe, and the transducers calculate the liquid flow rate from changes in the signal frequency.
Ultrasonic flow meters are ideal for applications that demand a non-invasive flow measurement method like wastewater treatment. Since the meter is not in-line with fluid flow, these meters can measure highly corrosive liquids, there are no pressure drop requirements, and maintenance tends to be cheaper. The primary disadvantages are that the measurements tend to be lower accuracy and they are especially susceptible to process vibrations and external environmental interferences.
Laminar differential pressure (laminar DP) liquid flow meters and controllers calculate standardized mass flow rates using differential pressure measurements. These devices use sensors to measure the pressure drop of flow across laminar flow elements. This value is then used to calculate a volumetric flow rate which is converted into a mass flow rate using information from preloaded tables with fluid properties.
These units provide high accuracy readings with no warm-up times. Laminar DP meters are also available as portable, battery-powered units, which can be used for quick and mobile process validation and flow calibration. The major downside is that they cannot be used for liquid mixtures of unknown composition or properties and must be calibrated for use with the exact process liquid.
Turbine flow meters
Turbine flow meters are a low profile solution for measuring low-flow liquids. They work by measuring the speed of rotation of a multi-bladed rotor mounted in the flow stream. The rotational speed is proportional to the volumetric mass flow rate, which can be used with known fluid properties to calculate a mass flow rate.
With precision ball bearings, turbine meters are highly accurate for low flow rates and capable of very fast response times (even down to 3 ms). They also operate across wide flow and operating temperature ranges. These features make turbine liquid flow meters a good choice for measurement of fuel and coolant flow rates in motor and engine development. It is not recommended to use turbine meters for flowing dirty or corrosive liquids, as the bearings inside can be damaged and the calibration disrupted.
Technology comparison chart
|Response time||Medium||Medium||Medium||Fast||Very Fast|
|No warm-up required||✓||✓|
|Aggressive fluid compatible||✓||✓||✓|
|Unknown or changing fluid composition||✓|
|Mixed-phase fluid compatible||✓|
|Does not affect fluid temperature||✓||✓||✓||✓|
|Large fluid particulates||✓|
|Same unit works for gas||✓|