Types of flow meters for low-flow liquid

Using flow technology for low-flow liquid applications

There is a wide range of flow measurement technologies available for 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 metering solutions and evaluate use cases of each.

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  1. Coriolis flow meters
  2. Thermal liquid flow meters
  3. Ultrasonic flow meters
  4. Laminar differential pressure flow meters
  5. Turbine flow meters
  6. Technology comparison chart
  7. Alicat’s CODA-Coriolis flow meters

Coriolis flow meters

Coriolis flow meters are the only technology that can measure mass flow directly and independent of fluid properties – with the added bonus of measuring density. The Coriolis 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 meters are excellent for 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 liquid flow meter choice for high accuracy low-flow liquid dosing applications and for flowing DI water in electrolysis.

Best for
  • Dynamic or unknown fluid compositions
  • Aggressive fluids
  • High pressures
  • High accuracy
  • One meter can run liquid or gas
Avoid for
  • Highly viscous fluids
  • Multi-phase flow

Thermal liquid flow meters

Thermal liquid flow meters 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 meters are preloaded with tables containing fluid properties.

These meters are excellent choices when you need a high-accuracy flow rate for a low pressure drop application. The primary limitation of thermal liquid flow meters is their inability 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.

Best for
  • Low pressure drop
  • Micro-scale flow rates
Avoid for
  • Low boiling point liquids
  • Unknown or changing fluid compositions

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.

Best for
  • Non-invasive measurements
  • Low pressure drops
  • Highly corrosive liquids
  • “Dirty” fluids with large particulates
Avoid for
  • High accuracy processes
  • Processes sensitive to vibrations

Laminar differential pressure flow meters (Laminar DP)

Laminar differential pressure liquid flow meters measure mass flow indirectly from differential pressure. These meters 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.

Best for
  • Portable measurement
  • No warm-up time
  • High-accuracy
Avoid for
  • High viscosity liquids
  • High proof pressure
  • Unknown fluid composition

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.

Best for
  • Very rapid response time requirements
  • Extreme temperature range
  • High accuracy
Avoid for
  • “Dirty” fluids with large particulates
  • Corrosive fluids
  • Unknown fluid composition

Technology comparison chart

Coriolis Thermal Ultrasonic Laminar DP Turbine
Accuracy High Low-Medium Low Medium-High High
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

CODA-Series Coriolis instruments

CODA Coriolis mass flow controller for use with low flow liquid applicationsAlicat’s CODA-Series Coriolis instruments have less wetted surface area, many elastomer options, and all-metal seals making them compatible with many aggressive process fluids.  CODA instruments are also more insensitive to external vibrations and environmental disturbances than competing Coriolis products, without requiring a mounting block (check out the demonstration video).

Low-flow Coriolis instruments have a small form factor and are particularly suited to applications which require high precision liquid flow metering and control, such as DI water flow for electrolysis, flavor dosing for food processing, and batch filling for pharmaceuticals. Depending on the specifications required for a given application, there are several low-flow liquid Coriolis model types available. Contact one of our applications engineers to determine the best mass flow solution for your process needs.