WATER / WASTEWATER Ultra-Low Flow Ultrasonic Flow Meter
The ultra-low flowmeter target was to investigate the feasibility of developing an ultrasonic device operating at lower flow rates than any other ultrasonic devices currently known to us. The target was 0.2 to 20 ml/minute which is ten times lower than existing Titan devices. The production aim was to incorporate this range into our next generation Atrato®
range.
Fig 7. Ultra Low Flow Meter Prototype with 1/16” tube connections
Fig 5. Low flow calibration rig design with liquid on piston side of cylinder to improve the resolution at very low flows: 0.0054 to 10 ml/min (0.324-600ml/h)
Fig 6. Main calculations for 0.0054 to 10.00ml/min low flow calibration rig
Maximum Flow Rate (input) Piston-Shaft Area (input) Piston Velocity max Flow
10 ml/min 0.377 cm2 4.42mm/s
Screw Lead on Precision Threaded Rods (input) 0.3048mm Motor Revolutions required max Flow Stepper Motor micro-steps (input) Drive frequency for max Flow
870RPM 8
Min Frequency Stepper motor (input) Min Flow Attainable
Microsteps per revolution Volume dispensed per revolution
K factor (microstep pulses per litre volume) Dispensed Volume ml per pulse Calculated Uncertainty of Rig
3. Challenges
Once the principle of design for the calibration equipment was decided upon, the practicalities of implementation presented several challenges. Off-the-shelf equipment is not designed for this type of application, so a significant amount of work had to be done to compensate for the differences. For example, the decision to use stepper motors gave the advantage of precise operation, but the drivers and associated software are designed for positioning rather than velocity management. The single steps of the motors would also offer pulsation into the system, potentially causing aliasing of measurement; so microstepping drivers implemented in both software and hardware terms with the dual drive rods effectively smoothed the delivery.
Once the main operational software and hardware had been developed and tested to deliver the level of accuracy required, the process of ensuring safe, reliable operation came into play. This included piston positioning and safety cutoffs in case of over pressure, misalignment or other failure modes. All this then linked to an easy-to-use operating software interface which offers standard traceable calibrations and adaptable, multiple flow scenario testing with the ability to expand such testing with ease. All components are designed to operate automatically, often for extended periods and sometimes for many hours at a time.
The final design addition was to implement a constant back pressure system, which could be maintained at constant value at both maximum and minimum flow rates. This is especially important for ultrasonic flow system design, as without adequate back pressure, microbubble formation attenuate the signal, preventing reliable flow measurement. The production calibration systems rely on a pressurised airline and sealed buffer vessel system to do this, but this option was not readily accessible for ultra-low flow designs. Instead, a hydraulic lock system was developed and implemented to give a minimum required back pressure.
Results: Ultra-Low Flow Development Using the New Calibration Rigs The two types of meter Titan are developing have differing operational requirements.
The ultra-low flow device needs to have good linearity over the whole flow range with excellent repeatability, without compromising too much on the response time and ultimately packaged in a reasonable dimensioned housing for both end user and OEM installation.
The clamp-on device is primarily for medical applications and therefore must have good stability over a few hours or days (the typical maximum period for medical tubing systems). It must be highly reactive with relatively fast response times and be able to measure bolus deliveries over a wide range of flow rates and volumes to mimic syringe sizes and dispense speeds.
23.2kHz 12.5
0.00538ml/min 1600
0.011491ml 134.24million 7.1817E-6 ±0.1%
The design principle has a compromise of velocity in the flow measurement tube and length of the tube. Increasing both, increases the time-of-flight difference of the ultrasonic wave travelling in both directions. Increasing velocity requires small bore for low flows which suffer from attenuation of signal and interference of background noise; whilst lengthening the tube makes the response of the meter much lower and suffer from loss of flow measurement signal strength. Prototyping of a variety of configurations, together with additional novel design aspects, becomes a crucial part of development in these advanced technical breakthroughs.
Fig 8. Example calibration 0.05 to 20ml/min flow range
The graph in Fig 8. above displays the results obtained from the fifth evolution of the ultra-low flowmeter. The 0.2ml/min target was easily achieved, and flow readings were possible down to 50 microlitres per minute under laboratory conditions. The result provided confidence to meet the 200 microlitres per minute target for the final device when set for production.
Development work continues to optimise the flowmeter design based on the best accuracy available at reasonable response times. Novel engineering of the flow sensor element has enabled subsequent designs to improve the flow response time by a factor of 50, whilst maintaining accurate readings.
The chart in Fig 9. illustrates the results of two measurement tube flow range designs, meeting ±1.0% + 0.04ml/min accuracy with raw measurement. Calibration linear correction is expected to improve on these results for later designs.
Fig 9. Two prototypes of ultra-low flow ultrasonic meters. Accuracy of ±1% + 0.04ml/min without linear correction implemented. Measuring reliably down to 0.15ml/min.
6 | AET NOVEMBER 2025 |
ENVIROTECH-ONLINE.COM
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