WATER / WASTEWATER


In addition, the prototype designs for a clamp-on meter for use with silicone tubing on blood type liquids, were also proven to be reliable for flows of 5 to 400ml/min with linearity of ±2%.


With the versatile nature of the new calibration designs, meters were tested for linearity, repeatability, batch dispensing at multiple volumes and flow rates, as well as long-term calibration drift.


Problem: Limiting Factors


Commercial low-flow calibrators with the required accuracy and versatility were not available, and bespoke systems proved prohibitively expensive. Building on prior experience with motorised piston systems, Titan therefore committed to developing next-generation low-flow calibration rigs in-house.


Based on previous work with ball screw driven piston systems, one of the main issues encountered was the method to ensure accurate measurement of the piston movement, coupled with smooth, reliable delivery. The commercial piston provers used controlled pneumatic or hydraulic feed and relied on linear encoder to measure the progress of the piston. Complex and expensive as this method is, it inherently provided a good, measurable liquid flow. By moving to a mechanical threaded driver, reliance on the thread rotation and piston connection to be accurate and consistent, becomes crucial; as does fine accurate control and measurement of the motor rotation.


Fig 2. Improved precision dual screw driven piston prover


Investigative Journey to Expanding Calibration Capability 1.Technical Considerations in Calibration Rig Design


Designing these rigs required careful attention to both hydraulic and mechanical factors. Key considerations included: • Achieving stable and smooth linear motion at ultra-low displacement rates.


• Optimising valve synchronisation and timing to eliminate disturbances. • Ensuring sufficient mechanical stiffness in the drive system for accuracy. • Accurate temperature measurement throughout the process. • Selecting suitable, rigid liquid piping to maintain repeatability. • Implementing an automated calibration routine for prover validation. • A reliable system bleeding mechanism. • Integrating safety systems, including end-of-stroke detection and overpressure protection.


On the software side, control algorithms had to be developed to ensure accurate, stable flow delivery and precise monitoring from the piston driving the flow and the meter’s pulse output whilst under test. Full automation was to be enabled to allow the rigs to run a wide variety of test protocols, from steady-state operation to complex transient flow scenarios.


Existing software for production systems was manual and designed for operation with manually operated equipment. Previous research tools provided the beginnings of the software for both control and measurement but would require development for the final equipment design and to be expanded for all expected scenarios of testing.


2. System Set-Up & Reference Tests


The first designs (Fig 1.) used stepper motors due to their precision of rotation and ball screw systems available off the shelf. They were found to be reliable, but the coarseness of the single ball screw meant that flow rates cycled ±1% over a few seconds. Volume delivery calibrations were accurate enough for assessing new flowmeter designs, but the variation of flow meant it was very difficult to accurately measure scatter with a flow meter in stable operation.


Altering the drive rod to fine precision threaded lead screws gave significantly increased levels of granularity of dispensing rate and also provided a very steady and smooth flow pattern along the entire length of the piston operation. The final designs (Fig 2.) incorporated two drive rods to give greater force. This also allowed the stepper control to be staggered between the two drives, doubling the effective granulation of control at low Hz of operation.


Fig 3. Chart showing Flow Cycling of original design versus new design as measured by Atrato® flowmeter


Calibration Report: 45mlpm flow rate calibrator: Flow Range 0.02525 to 48.4ml/min (1.515-2,904ml/h)


Rate (ml/ min)


Temp (o


C)


Relative Density (g/ml)


45.00 26.0 0.99678 45.00 25.75 0.99685 45.00 25.5 0.99691 45.00 25.5 0.99691 45.00 24.25 0.99723 45.00 25.0 0.99704 45.00 25.0 0.99704 45.00 25.0 0.99704


Reference Mass (g)


16.237 16.244 16.246 16.247 16.247 16.247 16.251 16.248


Reference Volume (ml)


Rig


Pulse Count


K Factor (pulse per litre)


Error %


16.289 481651 29568364.2 -0.04 16.295 481648 29557395.2 -0.00 16.296 481650 29555806.2 0.00 16.297 481673 29555398.4 0.01 16.292 481672 29564809.2 -0.03 16.295 481644 29557472.8 -0.00 16.299 481681 29552467.6 0.02 16.296 481654 29556267.3 0.00


Mean K Factor: 29558497.6 Mean Error: -0.01%


Fig 4. Example Mass Calibration report on the new Piston Prover Calibrators Fig 1. Original single ball screw driven piston prover


Moving on to develop a calibrator rated for flows well below 2ml/min, the design was miniaturised and the piston operated in reverse to further reduce the volume delivered per mm of piston movement (Fig 5. and Fig 6.). The resultant calibration unit operated to the same uncertainty but had a range of 0.0054 to 10 ml/min. Of course, accurately calibrating this unit at the lowest flows offers its own challenges, as the lowest flow rate would mean that to deliver the full 7.5ml of piston volume would take over 23 hours. When looking at small dispense volumes using mass reference calibration, fine accurate scales must be used and careful control of measurement must be considered, as even evaporation could give significant errors.


ENVIROTECH-ONLINE.COM | AET NOVEMBER 2025 | 5


The graph below (Fig 3.) shows the improvement of smooth delivery for the original ball screw versus the precision machined dual screw operation. The stepper motor can be controlled to deliver fine rotation at constant rate, but for the initial design the actual liquid delivery cycled with the variation on the large uneven thread on the ball screw driving the piston. Moving to a fine precision threaded rod meant the motor operating rate was directly related to the liquid being dispensed, giving accurate and very smooth delivery. The effective drive frequency outputs for 25Hz to 25kHz were effectively controlled to 0.04% of the set point over the entire 1000:1 range of operation. This ensured accurate flow and volume control as illustrated in the example mass calibration of one of the 45ml/min calibrator (shown in Fig 4.) giving an effective calibration variation of only -0.01%.


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