Flow Control: Metering Fluids in Medical Devices

Medical Device & Diagnostic Industry Magazine
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An MD&DI May 1999 Column

Proper pump selection is key to achieving efficient flow control in medical devices that handle fluids.

The positive displacement metering pump is generally the first choice for providing precise and repeatable flow in many medical device fluid-dispensing applications. Liquids ranging from test samples to reagents and wash fluids may be transferred, dispensed, or metered, depending on the application. As devices are designed to process smaller and smaller volumes of fluids, the requirements for precise metering pumps become greater. This article describes established and new metering pump technologies to enable manufacturers to select a suitable pump to meet the fluid-handling needs of a given application.

METERING PUMP DESIGNS

Many medical devices pump fluids. Fluid transfer is sometimes accomplished using either centrifugal or positive displacement pumps. Centrifugal pumps transfer energy to a fluid through a spinning impeller, converting the impeller energy to fluid pressure, which moves the fluid. Because these types of pumps are pressure and fluid dependent, they are not typically used for metering because they cannot maintain precise flows under changing inlet and discharge conditions. They do, however, provide high flow rates at low pressures.

A liquid-metering pump shown with control board (KNF Neuberger Inc.).

Positive displacement pumps trap a fixed volume of fluid and move this fluid by means of gears, pistons, diaphragms, vanes, or other devices. These pumps, which typically operate at lower speeds, are less sensitive than centrifugal pumps to changes in discharge and suction conditions. Flow can be regulated by adjusting speed and displacement. Such features have made positive displacement pumps an obvious choice for metering fluids.

A metering pump can therefore be defined as a positive displacement device designed to provide a precise and repeatable flow within a specified capacity range. Capacity typically can be adjusted within the turndown range of the pump, usually from 10:1 to as high as 1000:1 on some pump models. Accuracy, repeatability, and linearity across the turndown range are the functions that differentiate a metering pump from other positive displacement pumps.

Positive displacement metering pumps are normally classified as rotary or reciprocating. Rotary pumps include gear, lobe, vane, and roller (peristaltic) pumps. Reciprocating pumps include diaphragm, piston, and bellows pumps.

Rotary Metering Pumps. Gear-type rotary pumps use gear teeth, lobes, or vanes to trap a fixed volume of fluid. A rotary motion carries the fluid from the inlet to the outlet of the pump. In a gear pump, for example, two meshing gears rotate in a closed cavity with minimal clearance between the gear teeth and the pump casing. Fluid captured between each tooth and the casing at the inlet port is carried to the outlet port. By maintaining precise volumes between the teeth and low leakage rates between each tooth and the casing, a gear pump can provide accurate metering. It is easy to achieve flow control by controlling the rotational speed of the gears. The accuracy of the control speed of the motor often determines the pump's flow accuracy. Lobe and vane pumps operate similarly, but substitute smooth lobes or vanes for the gear teeth.

Rotary pumps that use gears or lobes provide low pulsation and continuous flow. They can produce high pressures and handle high viscosities. These pumps require no valves and can handle shear-sensitive fluids. However, rotary pumps have surfaces that can rub and wear, and they require dynamic seals, which can also wear out and leak. They are also not self-priming and cannot operate dry. Rotary pumps have limited chemical resistance so that they can accommodate the requirements of the gear materials and casing. Highly accurate rotary metering pumps are therefore expensive because of their tight pump tolerances and motor requirements.

Roller or peristaltic-type pumps eliminate some of the problems presented by gear pumps by using a set of rollers that squeeze a flexible tube in a circular pump housing. The fluid trapped in the section of the tube squeezed off by the rollers is forced through the tube as the rollers rotate. For the pump to provide accurate flow, the rollers must squeeze the tube down completely to prevent recirculation, placing significant stress on the tubing.

Peristaltic pumps can accurately meter very low flows down to fractions of a milliliter. These pumps require no valves and have a seamless, sterilizable flow path in which the fluid never contacts the pump. They can handle some particulates and are easy to maintain. They are not suitable for high pressures, however, and tubing life presents some concerns. Tubing selection requires a balance between flexible materials with long life and materials that provide adequate chemical resistance. As the tubing wears and loses its flexibility, the accuracy of the pump in metering applications also degrades.

Reciprocating Metering Pumps. Reciprocating pumps displace a fixed volume of fluid through the reciprocating motion of a piston, a diaphragm, or a bellows. In the simplest example, a piston is drawn back in a closed chamber, creating a vacuum that draws in a fixed volume of fluid. The piston then moves forward and expels the fluid. In this way, accurate flow control can be achieved by controlling either the stroke length of the piston or the piston's stroking speed. A motor-driven eccentric or a linear magnetic drive (solenoid) can supply the reciprocating motion.

Piston pumps often require seals or tight clearances around the piston to operate efficiently. This introduces seal and piston wear. Wear particles can contaminate the pumped fluid. These problems also limit the material selections necessary for optimum chemical resistance.

The simplest form of the piston pump is the syringe pump, which is designed to meter up to the volume of one full stroke of the syringe. By accurately stepping the piston on a syringe pump, precise flow rates can be obtained in microliters. Once the syringe is empty, however, the pump provides no flow during the refill period. A syringe pump, therefore, is not suitable for continuous metering applications.

Diaphragm metering pumps compensate for some of the disadvantages of piston-style pumps by replacing the piston with a flexible diaphragm. Because clamping around the edge seals the diaphragm, the pump uses no dynamic seals, which can wear. This eliminates leakage and contamination of the pumped fluid.

LIQUID DIAPHRAGM PUMPS

Liquid diaphragm pumps use an eccentric to move a diaphragm up and down inside a chamber. On the down stroke, liquid is drawn into the chamber through a nonreturn valve. The valve closes as soon as the diaphragm starts to move upward. This movement compresses the liquid and forces it out of the chamber through another nonreturn valve, thus producing flow (Figure 1). This pumping concept is effective for handling either liquid or gases. A typical pump design provides self-priming, mixed-media pumping capability. It can also operate dry (without liquid) indefinitely. These pumps are mechanically simple and enable pump designers to select wetted parts made from chemically inert materials.

Figure 1. Operation of a diaphragm liquid-metering pump.

Traditional Diaphragm Liquid-Metering Pumps. Flow control is normally exercised by changing the pump stroke or, alternatively, by changing the rotation speed of the motor. In the case of ac motors, rotation is normally from 2800 to 3200 rpm, but with dc motors, rotation speed can range from 1200 rpm to 4000 rpm. For relatively low flow rates, stepper motors have been used, which have speeds up to approximately 300 rpm. Conventional diaphragm metering pumps are reliable, self-priming, and chemically resistant.

Diaphragm metering pumps dispense accurate volumes per stroke. Flow is discontinuous (there is no flow when the diaphragm is on the down stroke), however, and if the stepper motor is run at very low speeds to attain small volumes, there are long periods when no liquid is dispensed. In fact, during half of each shaft revolution, the pump does not deliver fluid. The result is pump output that produces flow delivery approximating a square-wave form for solenoid-powered pumps or linear-magnetic-drive­powered pumps (Figure 2a), and approximating the classical sine-wave form for motor-driven reciprocating pumps that use an eccentric (Figure 2b).

Figure 2. Flow characteristics of various diaphragm liquid-metering pump designs: (a) pump with a linear magnetic drive; (b) pump with an eccentric drive and ac motor; and (c) pump with an eccentric drive and stepper motor.

Until recently, diaphragm metering pumps have produced inconsistent flow at very low flow rates. New pumps now combine diaphragm pump technology with advanced stepper-motor drive technology to produce near pulseless flow.

Electronically Controlled Diaphragm Metering Pumps. The principle behind electronically controlled diaphragm metering pumps involves the use of electronics to precisely control motor speed throughout a single stroke cycle. This allows the well-established and robust design of diaphragm pumps to integrate microprocessor-controlled stepper-motor technology to compensate for the inherent oscillation in the pumped output.

In this concept, the electronically controlled pump, not unlike traditional diaphragm liquid-metering pumps, is an oscillating, positive displacement pump. An eccentric converts the rotary motion of the drive shaft into an oscillating movement of a connecting rod, which in turn transmits its motion to the diaphragm. Combined with inlet and exhaust valves, the diaphragm motion produces the pumping or metering action.

The diaphragm metering pump is driven by a stepper motor drive, which turns at maximum speed for the suction (down) stroke, and then controls the speed of the diaphragm during the delivery (up) stroke. The motor speed is preset in the motor-controller processor based on the relationship of the rotary motion of the pump eccentric to the linear speed of the diaphragm surface. When the rotary motion of the eccentric translates to a small change in the diaphragm speed, the motor runs faster. As the rotary movement of the eccentric produces a higher diaphragm speed, the motor slows. This sine-wave compensation produces a smooth and quasicontinuous pump output over each motor revolution (Figure 2c).

For example, when a pump with sine-wave compensation is operated at low to moderate flow rates, the pumping time lost to the suction stroke is less than 1% of the total pump delivery time. Unlike traditional diaphragm metering pumps with linear magnetic drives or eccentric-driven diaphragm metering pumps with conventional motors, this compensation produces a smooth, quasicontinuous delivery. Compensation is accomplished with a motor-position feedback that monitors the rotary position of the stepper motor. Position sensing, along with the inherent ability of the stepper motor design to resolve each revolution of the motor into many small steps, makes it possible to control the angular speed within a single revolution (sine-wave compensation). With this method, the slightest change in rotation speed is recognized. The microprocessor controller also enables the pump to be connected to and controlled by external digital and analog control signals. The microprocessor controller also provides the pump's operational data outputs. Flow rates between and 80 ml/min ensure accurate dosing.

MATERIALS AND APPLICATIONS

Because liquid-metering pumps are sometimes used with aggressive media, and because chemical inertness is sometimes a concern, all pump parts that come into contact with the liquid (e.g., valves, gaskets, diaphragms, and head material) should be constructed of corrosion-resistant material. The use of polytetrafluorethylene (PTFE), polyvinylidene (PVDF), perfluorinated elastomer or other combinations normally used in diaphragm pumps are generally acceptable for most applications.

The ability of pumps to provide chemical inertness, durability, and mixed-media pumping in a nearly continuous, precise flow is ideal for many medical applications. For in vitro diagnostic devices, the delivery of reagents (especially newer, more chemically active materials) can be accomplished reliably with feedback control from the main system control. Electronically controlled pumps are also suitable for clinical analyzers and flow cytometry applications that need precise, nearly continuous and controlled delivery of sample transport media such as sheath fluids. Histology and other microscopy preparation applications can use the high-precision delivery of staining materials and other sample-preparation materials. In high-throughput diagnostic screening systems, the pumps can provide reliable delivery of washing fluids for assay microplates. Reliable delivery of fluid is also particularly valuable in molecular biology, where multiple fluid dosing streams are necessary for sequential or parallel synthesis.

CONCLUSION

A variety of methods are available to accurately meter fluids in medical device applications. Selecting the appropriate pump depends on the control and accuracy required for the device. Recent pump designs have combined the advantages of traditional diaphragm liquid-metering pumps with electronically controlled stepper-motor drive technology, broadening the fluid-metering options available to medical device designers.

Eric Pepe is general sales manager and George Halfinger is regional manager/liquid pump specialist with KNF Neuberger Inc. (Trenton, NJ).

Opening photo by Roni Ramos

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