Flow, level & control


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.


Fig 6. Main calculations for 0.0054 to 10.00ml/min low flow calibration rig


piston connection to be accurate and consistent, becomes crucial; as does fine accurate control and measurement of the motor rotation.


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


20


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 per cent 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.


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 per cent 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 per cent. 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.


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.


January 2026 Instrumentation Monthly


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