The measurement of low liquid flow rates is becoming increasingly important across many industries. However, as flow rates decrease, it becomes more challenging to control and measure them accurately. Finding an appropriate measurement method at a reasonable cost is a significant challenge.
There is no fixed definition for “low flow” in fluid handling, but low flow rates are generally characterized by limitations in stability and accuracy, which are not present at higher flow rates. The small liquid volumes involved make flow meters extremely sensitive, so even minor disturbances in process or environmental conditions can have a considerable impact on measurement stability.
In the markets where DDM Sensors and Titan Enterprises operate, flow rates below 50 ml/min are considered “low.” Many customers, however, require solutions for flow rates between 2 and 20 ml/min.
Neil Hannay, Senior R&D Engineer at Titan, explains: “We are seeing increasing demand for technologies that measure low flow rates. Various industries are looking to transport highly concentrated liquids, which are diluted at the point of use. This leads to significant savings in transportation and storage costs and has a positive environmental impact.”
From cleaning concentrates, syrups, and flavorings in beverages to chemical additives in oil or fuel, accurate flow meters are necessary to ensure the precise dosing of liquids for proper dilution at the end of the process.
Challenges in Measuring Low Flow Rates
Measuring low liquid flow rates presents a significant challenge. The energy available in the liquid flow is often too low for conventional mechanical flow meters to achieve accurate results. Electronic flow meters, on the other hand, may be limited by sensitivity, zero point drift, and slow response times.
Below, we analyze five common flow measurement technologies in terms of their suitability for low flow rates:
Ultrasonic Flow Meter
Ultrasonic flow meters measure the flow velocity of liquids. Titan’s Atrato models, which use patented time-of-flight technology, can measure flow rates as low as 2 ml/min. However, low flow rates result in smaller time differences and weaker signal strength, which can affect the repeatability of measurements.
Advantages: High accuracy, no significant pressure drop, suitable for both laminar and turbulent flows.
Disadvantages: Susceptible to vibrations, noise, and gas bubbles in the liquid.
excl. 19% VAT plus shipping costs
Connection: 3/8” John Guest, 1/2” BSP, 1/2” NPT
Measurement range: 2 … 500ml/min to 0,1 … 20l/min
Repeatability: ±0.1% (25 to 100% v.MB); ±0.5% (< 25 v.MB)
Material of wetted parts: PEEK / 316 ; PEEK / Borosilikatglas
Output: Impuls (NPN or PNP); 4 – 20mA; 0 – 10V; 0 -5V
excl. 19% VAT plus shipping costs
Data sheet
pdf | 1 MB
Select options
excl. 19% VAT plus shipping costs
Connection: 1/4″ BSPF, 1/4″ NPTF
Measurement range: 0,05… 0,5 l/min to 0,50 … 15,0 l/min
Repeatability: 0,1%
Material of wetted parts: PVDF or stainless steel
Output: Impuls (NPN or PNP)
excl. 19% VAT plus shipping costs
Data sheet
pdf | 410 KB
Select optionsPelton wheel flowmeter
The energy required to rotate the rotor in a Pelton wheel flow meter is affected by system resistance at low flow rates. Precision bearings in Pelton wheel turbines can partially offset this effect, allowing for the measurement of flow rates as low as 1 to 2 ml/min.
Advantages: Low cost, high response speed, good repeatability.
Disadvantages: Sensitive to changes in fluid properties, requires sufficient pressure to move the liquid through the turbine housing.
Oval Gear Flow Meter
Positive displacement meters, such as oval gear meters, are particularly effective for measuring low flow rates of viscous liquids. To achieve high resolution, the gears need to be small and precisely manufactured.
Advantages: Ideal for viscous liquids, precise dosing.
Disadvantages: Not suitable for water-based liquids, low resolution.
excl. 19% VAT plus shipping costs
Connection: 1/4″ BSP F or NPT F
Measurement range: 0,01 … 1,0l/min at 30cst oil; 0,1 … 1,0l/min with water
Repeatability: ±0,1%
Output: Impuls NPN; voltage
Pressure Ranges: <10bar, <50bar, <400bar, <700bar
excl. 19% VAT plus shipping costs
Data sheet
pdf | 253 KB
Select optionsThermal Flow Meter
Thermal flow meters measure flow by monitoring heat transfer. They are highly sensitive and can measure flow rates down to the nanoliter range.
Advantages: Very sensitive, suitable for applications with low pressure drop.
Disadvantages: Fluid-specific and non-linear across the entire temperature range.
Coriolis Flow Meter
Coriolis flow meters directly measure mass flow, independent of liquid properties. They provide highly precise and repeatable results, even with changing fluid compositions.
Advantages: High precision, independent of process variables.
Disadvantages: High initial cost, not economical for OEM applications.
Selecting the right flow sensor is critical for ensuring stable and accurate measurements in low flow liquid applications. Flow meters are a key component in many processes where precision is essential. “We see significant potential in the market for low flow meters and are actively working with OEMs to develop innovative solutions,” says Neil Hannay.