Piston Pump Accuracy, Repeatability and Resolution Explained
Piston-pump specifications often list capacity, full-stroke steps, accuracy and repeatability together. These terms all relate to dispensing performance, but they describe different questions.
A common mistake is to treat more motor steps as automatically higher liquid accuracy, or to call theoretical volume per step the minimum reliable dose. Real delivery also depends on mechanics, valves, tubing, bubbles, fluid properties, motion control and the measurement method.
1. What is piston-pump resolution?
Resolution is the smallest theoretical displacement increment commanded by the drive. For a stepper-driven pump, theoretical volume per step can be estimated from nominal capacity and full-stroke steps.
Theoretical volume per step = nominal capacity ÷ full-stroke stepsFor the displayed EA-500-PMMA configuration: 500 μL ÷ 2,000 steps ≈ 0.25 μL/step.The 0.25 μL value is a geometric command increment, not proof that 0.25 μL can be dispensed reliably in every liquid and fluid path. Resolution does not state actual error or minimum validated dose.
2. What is piston-pump accuracy?
Accuracy expresses how close the delivered result is to the target volume. It must be interpreted with the tested stroke, target volume, fluid and temperature, complete fluid path, measurement method and calculation definition.
- Full stroke or partial stroke
- Target dispensing volume
- Fluid and temperature
- Valves, tubing, fittings and outlet included in the test
- Gravimetric, photometric or other method
- Mean bias, maximum single error or another definition
3. What is piston-pump repeatability?
Repeatability describes the spread among repeated results under the same conditions. It may be reported as range, standard deviation, relative standard deviation or coefficient of variation.
Good repeatability does not guarantee accuracy. Repeated results near 96 μL for a 100 μL target can be tightly grouped but biased. Conversely, a mean close to 100 μL can hide excessive scatter.
| Observed result | Accuracy | Repeatability | Engineering meaning |
|---|---|---|---|
| Mean near target; results concentrated | Good | Good | Stable quantitative performance |
| Mean off target; results concentrated | Poor | Good | Calibration or systematic-bias correction may be needed |
| Mean near target; results scattered | Apparently good | Poor | Random variation is too large |
| Mean off target; results scattered | Poor | Poor | Inspect pump, valves, path, fluid and control |
4. Why more steps do not automatically improve liquid accuracy
Step count sets command granularity, while fluid travels through a complete system. Fixed and transient effects can separate theoretical piston displacement from delivered volume.
1. Mechanical backlash and transmission error
Lead screws, nuts, couplings and moving parts can have clearance. After direction reversal, some commanded motion may take up clearance before effective liquid displacement begins.
2. Valve response and timing
If piston motion and valve switching are not coordinated, backflow, air aspiration, unsettled pressure or retained liquid can occur.
3. Bubbles and tubing compliance
Compressible gas and expanding tubing absorb displacement and release it later, producing under-delivery, delay and interaction between cycles.
4. Fluid and environment
Viscosity, surface tension, volatility, dissolved gas, particles, crystallization and temperature affect aspiration, discharge and droplet detachment.
5. Motion and dispensing method
Aspiration/dispense speed, acceleration, settling time, priming, probe position and contact or non-contact delivery can matter more than added microsteps.
5. Why full-stroke performance cannot be extrapolated to small doses
Backlash, valve timing and bubble compression may be small relative to a full stroke but large relative to a short stroke. A pump can therefore show different accuracy and repeatability at different stroke ratios.
| Test point | Volume | Accuracy | Repeatability |
|---|---|---|---|
| 100% stroke | 500 μL | ≤0.5% | ≤0.5% |
| 2% stroke | 10 μL | ≤2.0% | ≤1.5% |
6. How to validate accuracy and repeatability
- Define critical working volumes: List minimum, normal and maximum doses and identify which one controls assay, ratio or yield.
- Define acceptance metrics: Separate mean bias, maximum error, standard deviation, CV and any industry-specific requirement.
- Build the actual fluid path: Include production valves, tubing, fittings, filters, probes or nozzles wherever possible.
- Use the real or a representative fluid: Match viscosity, surface tension, volatility and particles, and record temperature, backpressure, speed and priming.
- Choose a suitable method: Control balance resolution, density, evaporation and environmental effects for gravimetry; use appropriate photometric methods for very small volumes.
- Report each working point: Publish mean, bias and dispersion separately at minimum, normal and maximum volumes.
7. How to read FOREACH EA, SM and TM specifications
| Series | Current information | Selection focus |
|---|---|---|
| EA precision piston pump | 2,000 full-stroke steps in base configurations; full-stroke and 2% stroke data under specified conditions | Choose capacity around the critical working volumes |
| SM miniature piston pump | 2,000 full-stroke steps; full-stroke repeatability ≤0.5% under specified conditions; confirm accuracy by configuration | Balance space, volume, fluidic components and system validation |
| TM ultra-compact piston pump | Displayed configurations use 2,540 full-stroke steps; confirm accuracy and repeatability by configuration | Do not infer dispensing performance from step count alone |
Step counts cannot be compared without capacity, piston diameter, transmission and test conditions. The largest pump that covers the maximum dose is not automatically the best choice for the smallest critical dose.
Review the fundamentals in What Is a Precision Piston Pump?.
Frequently asked questions
Is theoretical volume per step the minimum dispensing volume?
No. It is a theoretical displacement increment. Minimum reliable dose must be tested with the actual fluid, valves, tubing, backpressure and controls.
Are more motor steps always better?
They improve command granularity but do not independently guarantee accuracy or repeatability.
Why can a repeatable result still be inaccurate?
A fixed systematic bias can produce tightly grouped results that are all offset from the target.
Can full-stroke accuracy predict performance at 10% or 2% stroke?
Not directly. Fixed errors and transients can occupy a larger proportion of a short stroke.
How can small-volume performance be improved?
Select an appropriate capacity, prime and degas, optimize tubing and speeds, coordinate valves, stabilize temperature and calibrate with the target fluid.
Conclusion: define the working volume before comparing pumps
Resolution, accuracy and repeatability answer different questions. Resolution is a theoretical command increment; accuracy is closeness to target; repeatability is consistency among repeated results.
Define minimum, normal and maximum working volumes first, then verify accuracy and repeatability at those strokes. Motor step count supports motion design but cannot establish minimum reliable dose or complete-system performance.