Right Sizing a Syringe Pump

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Right Sizing a Syringe Pump Syringe pump technology has been widely used in fluidic system designs for several decades and they are a popular choice because they offer a high level of flexibility and versatility within a single device. This means that the device can be optimized to perform at a wide range of fluid delivery functions on command.

Instrument designers are challenged to select a fluid delivery device, such as a syringe pump, for their application and must take into consideration the full scope of the instrument’s requirements, including the physical, environmental and chemical.

This paper will focus on understanding key terminology, such as precision, accuracy, resolution and speed, that is necessary in order to right size a syringe pump for a particular application.

Precision Refers to the repeatability of the results when tested under the same conditions. This is measured as the standard deviation over the mean (CV - Coefficient of Variation).

Accuracy Refers to how close the actual result is to the desired result. This is measured as Âą % error from the desired result.

Resolution Refers to the number of single incremental steps available within a full stroke. These are the mechanical points to which movements can be made.

Speed Refers to the number of steps per time period possible and determines the dispense time and flow rate. Typically measured in half-steps per second.

The diagrams on the right illustrate varying combinations of precision and accuracy performance in the context of a bull’s eye.

It is clear to see from this illustration that low precision is a larger problem as it produces unpredictable results. Low accuracy however, is only a risk when also combined with low precision. Low accuracy, when combined with high precision, can be confidently corrected in practice by a system calibration since there is a high confidence in the resulting performance, although it is off-target.

High accuracy and high precision is of course the most desired in a syringe pump but low accuracy and high precision is a close second.

High Mean value

Precision

These performance characteristics become critically important in applications where the instrument output is highly dependent on having received an exact amount of a fluid. Failing to achieve this could have several negative effects, including wastage of expensive reagent, reduced instrument throughput and inaccurate assay results.

It is possible to have a syringe pump with high accuracy but low precision, and vice versa. High accuracy on this illustration is represented by close proximity to the bull’s eye and high precision is demonstrated by a cluster of results.

Mean value

Target value

Target value

Low

Target value

Mean value

Mean value

Low

Precision & Accuracy Syringe pumps are selected for their unmatched ability to deliver precise amounts of fluids. Precision and accuracy are two different but basic performance charactieristics of a syringe pump that must be well understood in order to guarantee confidence in fluid delivery within a more complex system such as an instrument.

Accuracy

Target value

High

Figure 1 - Illustration of Precision and Accuracy

Resolution & Speed Resolution indicates the number of single incremental steps available within a full stroke of a syringe. These are the mechanical points to which complete stop movements can be made.

1/8th mL

1/8th mL

Resolution: 4 0.25mL/step

Resolution: 16 0.0624mL/step

Figure 2 - Impact of resolution choice The pumps in Figure 2 are assumed to have resolutions of 4 and 16 (left and right) for illustration purposes only. The resolution number denotes the possible mechanical stopping points along the syringe stroke length. When commanded to move to a position in between these designated stopping points, the pump will actually stop at whichever resolution point is closest. In this example, where a user is attempting to dispense 1/8mL, it is easy to see that the pump with resolution of 16 is a better option as it as there is a stopping point much closer to the desired dispense volume.

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The resolution of a syringe pump is relevant but independent to its accuracy and precision performance; depending on the output expected from the syringe pump, the resolution can have an influence on the accuracy of the output. While the resolution defines the step increments that can be taken, the accuracy measures how close to the desired step movement the pump actually moved.

The higher the resolution on a syringe pump, the smaller the possible movements available; which directly translates to a smaller volume of liquid being dispensed with increased accuracy (% of error). When selecting a syringe pump for automated use within an instrument, it is important to carefully consider what dispense volumes are required and to select the pump resolution and syringe size that most closely aligns with this desired output.

The IMI Norgren Cadent 3 pump is available in 6,000, 12,000 and 24,000 half-step resolution options.

Microstepping, the ability to electronically move through micro increments between the mechanical stop points, is also influenced by the choice of resolution, resulting in 96,000, 192,000 or 348,000 step increments respectively, by microstepping at 1/16th halfsteps.

The speed setting on the syringe pump, along with the resolution, becomes an important parameter particularly when operating the pump in a continuous flow mode. Adjusting this will help to fine tune the hardware towards achieving the desired flow rate.

Syringe pumps are versatile fluidic devices that allow for a broad range of performance outcomes. System designers need to become familiar with the various programmable parameters in order to select the optimal configuration for each specific application.