Choosing the right fluid dispensing pump for a given application is critical. Whether it’s accuracy and precision or the need to perform for millions of cycles, understanding the most suitable types of pumps available is the first step. This article describes several pumps commonly used for medical manufacturing applications. It examines their advantages and disadvantages. In addition, it discusses how to maximize accuracy by minimizing fluid slip, an important factor in the design of any positive displacement pump. It also looks at the importance of testing the fluid prior to determining what pump is best for the application.
Linear Pumps vs. Rotary Displacement Pumps
There are essentially two types of drive systems that control fluid delivery using positive displacement. Linear drive systems and rotary drive systems. Positive displacement pumping refers to a pump that retracts in a cavity to generate volume on the suction side and extends into the cavity displacing the fluid on the discharge. This is a constant for each cycle. Both linear and rotary positive displacement pumps provide exceptional accuracy and precision. However, each method has its’ advantages and disadvantages.
Calibration. In a rotary application, the volume of displacement is a factor of the pump angle relative to the motor axis. Once the volume is calibrated, the pump module can be locked into position. The displacement is adjustable so it should be checked to confirm that calibration is still within specification. In a linear application, mechanical calibration is not required. The pump module is set in a static location, and volume is determined by how far the piston retracts in the cavity. There is no mechanical set point to change volume; only software is used to adjust this parameter.
Cycle Time. Rotary pumps can produce faster dispenses because the rotary valving motion and linear displacement motion are performed simultaneously. A single cycle is controlled by one revolution of the motor. Multiple revolutions will produce larger volumes based on the pumps fixed volume. Cycle time and the ability to produce a more constant flow rate (based on pump revolutions) are the rotary pumps’ strengths. The dispense profile of a rotary pump includes pulsations which are due to the sinusoidal waveform. This offers the unique advantage of firing off small volumes of fluid very quickly. An example of a rotary application would be dispensing microliter range dots of fluid onto a substrate passing through a high-speed automation system.
By contrast, a linear pump must retract from the suction port to draw fluid into the chamber valve to the discharge port, and then eject the fluid. In general, larger volumes require more time, depending on pump size. The linear dispense creates a flat dispense profile with no pulsation throughout the entire chamber capacity of the pump module. If more volume or a longer dispense is required, two pumps may be run out of phase so that one pump is discharging while the other is on the intake stroke. An example of a linear application would be dispensing a constant line over a distance, such as a diagnostic reagent, or precisely filling a substrate with slow absorption rates. Continuous web applications are ideal for this tandem approach.
Syringe Pumps vs. Ceramic Displacement Pumps
Syringe and ceramic displacement pumps are ideal for precise aspirating and dispensing of samples and reagents. They are often used for both analytical and IVD instruments. There are some differences between traditional syringe pumps and ceramic displacement pumps that are important to mention.
A traditional syringe pump is a device that uses a syringe (usually a glass barrel with a Teflon plunger) to move the fluid. The syringe pump is easily replaceable when it fails (which can be often). It also enables the user to change syringe sizes to accommodate different volumes. The majority of syringe pumps work in conjunction with a motorized valve to which the syringe attaches. The timing of the syringe movement and the valve activation (two motors required) are controlled by built-in electronics. The glass barrel and Teflon plunger of the syringe are wear components that must be replaced regularly.
In contrast, a ceramic displacement pump is a single motor mechanism that utilizes an internal ceramic piston to precisely aspirate and dispense fluids. Unlike the syringe pump, there are no serviceable parts to a ceramic displacement pump. The internal sealing mechanism combined with the polished ceramic piston generally last the entire life of the instrument. These pumps often exceed 10 million cycles. Since internal valving is an option (usually two-way solenoid valves are utilized), ceramic displacement pumps are specified to do exactly what is needed for each individual pipetting application. The style depends on the application, but here are some general guidelines.
Consider a syringe pump when:
The instrument can benefit from being able to change syringes easily.
The instrument uses relatively low cycles during the life of the instrument.
Consider a ceramic displacement pump when:
The instrument will perform high cycles (many millions) over its lifetime.
Multiple pumps with specific functions are required within the instrument.
The instrument uses a pump where the mechanism is not easily serviced or replaced.
The instrument requires high precision without periodic service calls.
Minimizing Fluid Slip
Fluid slip is a term commonly used to describe the migration of liquid around the internal moving parts of gear, lobe, and vane pumps. It is the volumetric difference between physical component displacement and liquid throughput. Slip loss refers to the liquid that passes through the clearance space (approximately 0.00005 in.) between the piston and the cylinder wall. Since this clearance represents a restrictive passage of essentially constant dimension, the slip rate is determined by viscosity, pressure, and time.
Assuming constant liquid viscosity and pressure, slip will be a smaller factor in a high-repetition rate pump (short time between strokes) than in a low-repetition rate pump. As viscosity increases and pressure decreases, time becomes a less significant contributor to slip loss. The clearance can be modified to compensate for viscosity. The clearance between the piston and cylinder wall can be optimized for any liquid in order to minimize fluid slip.
To minimize fluid slip, the reservoir height and tip height need to be considered. As a general rule, the reservoir height and tip height should be equal. A fluid’s viscosity plays a part in this determination. One way to determine the optimal position of the reservoir and tip is to prime the fluid lines and observe for any fluid movement during idle time. If the fluid moves back in the line, raise the reservoir. If the fluid drips from the tubing, lower the reservoir. The goal is to achieve zero pressure differential over the pump.
Fluid slip is an important factor to consider through the design and build stages of any pump or pump component. It can be critical to the performance of an application. Testing is highly recommended so that necessary parameters are met for each application.