When it comes to selecting a pump for use in a medical device, the requirements should be demanding. Any team working on a medical device will need to complete a thorough review of the selection criteria to ensure the best pump is chosen. Part of the decision-making process requires considering a variety of dispense specifications.
One dispense specification to consider is the pump’s dispense resolution, which is the smallest volume a stepper-motor driven pump can dispense. Most reciprocating pumps use a stepper motor to turn a leadscrew, which moves the piston back and forth. Several factors determine the resolution: the piston’s diameter, the lead screw’s pitch, and the stepper motor’s angle per step. The resolution can be further modified by microstepping the motor, which subdivides the motor’s step angle. Microstepping allows for finer resolution, enabling smaller possible dispenses and extremely steady flow throughout a given dispense.
Accuracy and precision requirements are an integral factor to a pump’s fluid circuit performance. Accuracy is usually expressed as a percentage of the target volume. Precision is expressed in a CV (coefficient of variation). For applications requiring high accuracy and small coefficients of variation, it is best to size the pump such that the dispensed volume is as close to the full stroke volume as possible.
The speed at which a pump dispenses can be a differentiator when considering multiple options. Generally, the smaller the dispense resolution, the slower the dispense speed. A fine-pitched lead screw and a small step angle can also lower the dispense rate. When applications require a higher dispense speed, a pump with a larger dispense volume may be necessary. However, using a pump with a faster dispense speed may have associated trade-offs, including lowering the accuracy and repeatability of each dispense.
The dispense speed is also affected by system parameters such as fluid viscosity and the tubing that connects the pump to the fluid reservoir and dispense point. Tubing with small inner diameters and long lengths can be very restrictive to flow. A pump quickly aspirating through such tubing can draw a vacuum great enough to pull entrained air out of the fluid. This issue is worsened when using fluids with high viscosities. These cases call for slower aspirations and larger tubing diameters to reduce the vacuum in the port head, preventing cavitation.
Improperly sized tubing can also cause problems. Using outlet tubing with too small a diameter may create a restriction, resulting in troublesome pressure spikes. This is because a positive displacement pump will increase system pressure to overcome downstream system resistance. If resistance is too high, it might be necessary to reduce the dispense speed or increase the tubing diameter. Otherwise, failures such as system leakage and motor stalls can happen.
Furthermore, an analysis of the pump’s function in an application – the dispense and aspiration volumes as well as operational sequences – should help to determine what specs are essential. For instance, some pumps intended for simple operations may only need to perform a single aspirate and dispense at a time. In cases like these, the balance between dispense speed and accuracy/repeatability should be the focus. Other pumps may be required to perform one large aspiration from a reservoir, followed by many small individual dispenses (or vice versa). As the divided dispenses get smaller, the importance of the pump’s resolution escalates. Further operations may have a pump perform many aspiration and dispense cycles to meet a large volume target. For these high-speed bulk transfers, the pump’s volume relative to its allowable size must be considered, and the accompanying tubing must be scrutinized.
Selecting the right pump for your application is a multifaceted task that demands careful consideration of various dispense specifications. The choice of dispense resolution, accuracy, precision, dispense speed, and tubing all directly impact the performance and efficiency of your fluid circuit. Each factor has its trade-offs and must be weighed against the specific requirements of your application. At The Lee Company, we understand the intricate dynamics involved in choosing the ideal pump to achieve your desired results. Our global sales force can discuss your application and discover the optimal pump solution for your project. We are committed to providing the highest quality and most precise fluid control components to ensure your success.