Diaphragm-based systems offer an efficient alternative to valveless piston pumps in mid-range precision fluid dosing and metering applications in IVD devices.
|Diaphragm metering pump|
Largely by default, valveless piston pumps historically have reigned as the “pumps of choice” for midrange precision fluid dosing and metering in IVD systems. Other technologies-most notably peristaltic hose pumps and high-precision syringe pumps (the simplest form of a piston pump)-have been available as alternatives. But in comparison to valveless piston types, these alternatives have not made significant headway in IVD applications because of various issues that include the addition of unwanted complexity to systems, the need for more maintenance, and potential leakage.
Newly evolved diaphragm-based fluid pump technology now promises to give valveless piston pumps a run for their money. By combining innovative sealed diaphragm pump designs and advanced motor drives, diaphragm-based fluid pumps deliver enhanced opportunities for fluidics design engineers to optimize and more closely integrate the mid-range (5 µl to approximately 100 ml) precision fluid dosing and metering function into the overall IVD system control scheme.
In managing the need for midrange precision liquid dosing and metering in IVD systems, fluidics design engineers traditionally have narrowed the options to several potentially suitable pump types, each of which has its advantages and limitations.
Peristaltic hose pumps are one option. They use a set of rollers that squeeze a flexible tube, or hose, in a circular pump housing. Fluid is trapped in the section of the tube squeezed off by the rollers and forced through the tube as the rollers rotate. For the pump to provide accurate flow, the roller must squeeze the tube completely to prevent recirculation, placing high stresses on the tubing.
These pumps can accurately meter very low flows down to fractions of a milliliter and provide a seamless flow path in which the pump never contacts the fluid. These devices require no valves and can handle some particulates, and the tubes can be sterilized.
They are typically unsuitable, however, for high pressures; are prone to eventual leakage; and the tubing material limits the range of chemicals that can be handled. Inherently limited tubing life presents another disadvantage, and the need to replace hoses means more maintenance, increased costs, and lost productivity. In addition, more often than not, the accuracy of the pump in metering applications will decline as tubing wears and loses its flexibility. Ultimately, this device performs best if the flow rate allows the rollers to operate slowly and the pressure is low.
High-precision syringe pumps are another option. These systems basically operate with a piston that is drawn back in a closed chamber, creating a vacuum drawing in a fixed volume of fluid. The piston then moves forward and expels the fluid. In this way, either by controlling the stroke length of the piston or the piston stroking speed, accurate flow control can be achieved. Syringe pumps can meter up to the volume of one full stroke of the syringe and, by accurately stepping the piston, produce highly precise dispense volumes or flow rates.
|Peristaltic tubing pump|
A primary disadvantage of this type of pump, however, is that once the syringe is empty, the refill period allows no flow from the pump unless two pumps are in parallel using a complex control scheme to maintain continuous flow. As a result, a syringe pump will normally be unsuitable for continuous metering applications. These pumps additionally require sliding seals or close clearances around the piston to operate accurately, which leads to problems associated with seal and piston wear and contamination of the pumped fluid by wear particles. The wear is accelerated if the application requires frequent cycling, as in the case of continuous dosing or washing, and is particularly problematic when handling fluids with a tendency to crystallize. Appropriate materials for optimum chemical resistance will be limited with these types of pumps. Syringe pumps, therefore, are best suited to applications requiring very accurate dispensing of finite amounts of clean fluid.
For years, the valveless piston pump has served as the predominant technology for midrange precision fluid dosing and metering functions, offering relatively more advantages than disadvantages. Pumping is accomplished by the synchronous rotation and reciprocation of a ceramic piston in a precisely mated ceramic cylinder liner. One complete piston revolution is required for each suction and discharge cycle.
In a valveless pumping system, no valves are present to regulate the flow direction, and pumping efficiency remains uncompromised. The absence of valves reduces the potential for clogging, allows for reversible flow by rotation, promotes consistent flow without impact from valve deterioration, and prevents overshoot.
Other features and benefits of these types of pumps include:
• consistent performance regardless of viscosity changes, enabling the handling of a wide range of fluid types;
• high accuracy levels that ensure precision in delivery;
• drift-free operation, promoting long-term and consistent fluid delivery;
• a single moving part, contributing to high reliability;
• positive displacement, which presents limited change in delivery rates with increasing discharge pressure changes;
• dimensionally stable pump materials that underwrite consistent delivery rates immune to changes in fluids or temperatures.
Despite the advantages, drawbacks persist:
• the absence of valves tends to reduce reverse-flow leak tightness;
• complex stroke adjustment is required to achieve desired high accuracy levels and precise delivery;
• drift-free operation lasts only over a limited lifetime (84 million cycles for one manufacturer’s product);
• the positive-
displacement feature can develop dangerously high pressures;
• valveless piston pumps usually are orientation-sensitive, which limits design options for the device using the pump;
• the ability to adjust the stroke mechanically requires a complicated and costly configuration;
• a large size-to-delivery rate requires the pump to occupy an outsized portion of the device;
• the requirement for one complete revolution per dose volume limits the per-dose volume range;
• electronic controls generally remain relatively unsophisticated, bulky, and offer limited capabilities.
These pumps additionally require sliding seals that can wear out or degrade over time, potentially resulting in shorter life, long-term drift in delivery rate, leakage, and higher cost of ownership.
With an all-new generation of diaphragm-based metering and dosing pumps, the universe of options has expanded. For the first time there is a viable, accurate, and practical liquid dosing and transfer alternative to the sliding-seal piston-based fluid-handling technologies.
• allows electronic adjustment of flows, eliminating any need for mechanical stroke length change;
• has no sliding or rotating seals to wear out or degrade over time;
• enables the use of a wide range of wetted, chemically inert materials, including PTFE;
• results in pumps that are extremely small in size relative to the flows delivered;
• delivers outstanding pressure
• provides low overall cost of ownership compared with conventional technologies.
Based on a simple diaphragm pump design combined with high-tech control elements, the dosing function can now be performed using a smaller package and comparably consuming less of the valuable space within an IVD system. In turn, system control schemes compatible with diaphragm-based pumps offer the opportunity to become more responsive to changes in dosing requirements, from batch-to-batch and down to dose-to-dose.
The evolution of diaphragm-based fluid pump technology as it relates to IVD dosing applications owes much to advances over the years in such areas as materials, motors, and electronic controls, among other factors. The story begins with the basics of diaphragm pump operation.
Diaphragm metering pumps effectively eliminate some of the disadvantages of piston-style pumps by replacing the piston with a flexible diaphragm. These pumps operate by means of an eccentric that moves the diaphragm up and down inside a chamber. On the down stroke, liquid is drawn into the chamber through a non-return valve. The valve closes as soon as the diaphragm starts to move upward, and this movement pressurizes the liquid and forces it out of the chamber through another non-return valve, producing flow. Electronically controlled inlet valve designs are available for increased precision.
Standard pump design provides self-priming, mixed media pumping capability, as well as the ability to operate dry (without liquid) indefinitely. Clamping is used to seal the diaphragm around the edge, eliminating potential leakage or contamination of the pumped fluid. Rotating or sliding seals of other pump types are replaced with mechanically superior static seals.
Several decades ago, high-tech engineered plastics, such as PVDF and PPS, and an increasing capability to design and fabricate complex molded parts at reasonable cost enabled repeatable precision in the production of key fluid-path elements. At the same time, the availability of new, chemically inert elastomers became more widely used within IVD systems, especially for elements in contact with fluids for processing and cleaning. These developments opened the door for application of elastomeric diaphragm technology for bulk fluid flow and aspiration functions in IVD systems.
The acceptance and utilization of diaphragm pump technology in production IVD systems over time led to continuous refinements, including use of a wider range of chemically inert materials, such as FFPM and FPM elastomers and PTFE and PEEK plastics; successive reductions in pump size; and experience with a wide array of motors and control schemes.
|Valveless piston pump|
Diaphragm and valve design elements similarly advanced over the years as the experience base increased, more mechanically versatile materials became available, and increasingly optimized flow paths and components were developed.
Innovations in motor technology and electronic controls also have played vital roles in contributing to advanced diaphragm pump technology.
Some 20 years ago, electronically commutated brushless dc (BLDC) motors became more commercially accessible for use as drives for diaphragm fluid pumps, and their adoption in IVD systems enabled an experience platform to refine and evolve the technology. In parallel, the electronic control industry continued to provide higher levels of data-processing power in smaller and less-expensive packages, both for BLDC and stepper motors.
Motor drives now can control more operating parameters, leading to higher-precision speed control in smaller and relatively less-expensive packages. In addition, sensors (such as the Hall-effect position sensor) can now be used to acquire motor speed information and diaphragm-positioning information to improve the control accuracy of the primary fluid-driver elements, effectively providing an interactive smart element governing the fluid-dosing function. This includes the ability to change the speed of suction and discharge stroke to avoid degassing, reduce pulsations, or improve delivery precision.
The result: Today’s new generation of diaphragm pumps combine the application-proven diaphragm fluid-pump concept with dynamically interactive controls to refine the delivery precision in fulfilling midrange fluid dosing functions in IVD systems.
The following are among the particular benefits that the new pump technology brings to fluidics design engineers specifying for IVD systems:
• leak-tight pump construction and statically sealed fluid path ensure containment of the pumped fluid without sliding seals to wear out or degrade;
• pumps can self-prime to exceptional levels (9 in. Hg);
• output pressure capability to 145 psig can be achieved;
• small pump size (compared with alternative pump designs) allows for miniaturization of the device using the pump;
• long-life pump drivers, such as BLDC or stepper motors and solenoids, promote extended service life and lower the cost of ownership (compared with motors using brushes, which are subject to wear and are shown to be the leading cause of brush-commutated dc motor failure);
• electronically controlled stroke volume enables real-time response to flow-change requirements and frequency of dosing;
• Hall-effect feedback (for smart pump operation) can be changed from simple to complex dosing applications using a basic electronic interface;
• chemical compatibility using readily available materials contributes to high value-to-cost ratio.
Diaphragm pumps offer benefits associated with relatively simple construction, oil-free operation without maintenance, a statically sealed fluid path, and uncontaminated delivery of the pumped medium. Acceptance and advances in pump design, motor technology, and electronic controls have extended their potential and versatility, helped contribute to the development of smaller and lighter pumps to accommodate ever-shrinking envelopes, and boosted overall capabilities.
Fluidics design engineers now have an alternative to consider when deciding how best to manage the need for midrange precision fluid dosing and metering in IVD systems.
David H. Vanderbeck is business development manager at KNF Neuberger Inc. (Trenton, NJ). He can be reached at firstname.lastname@example.org.