Pipettes: choosing one, specifying multichannel pipettes, electronic pipettes including an electronic multichannel pipette or multichannel pipette electronic model, positive displacement pipettes, when a micro pipette is the right pipette volume class, and what a pipette service and calibration contract has to cover under ISO 8655

The purchase, then the contract

1. Air displacement or positive displacement. Air displacement handles aqueous liquids and is what most labs mean by a pipette. Positive displacement uses a disposable piston in contact with the sample and is what viscous, volatile, dense or contaminating liquids require. Using the wrong one is a systematic volume error, not a preference.
2. Channel count and spacing. Single channels suit method development and awkward vessels; eight and twelve channel heads suit plates and are where most of the throughput gain sits. Adjustable spacing heads bridge tubes and plates and cost considerably more, so buy them for a workflow that genuinely moves between formats.
3. Ergonomics are a real specification. Plunger force, tip ejection force and weight decide whether a technician can work a full day without strain, and repetitive strain in a pipetting role is a genuine occupational risk rather than a marketing angle. Have the people who will use the instrument hold it before the order goes in.
4. Tip fit is a lock-in decision. Every manufacturer will tell you generic tips fit; the seal at the tip cone is what sets the delivered volume, and a poor fit shows up as imprecision the calibration certificate did not predict. Decide deliberately whether you are buying into that manufacturer's tips for the life of the instrument.
5. The service contract. A pipette service should state the calibration standard applied, the number of volumes tested, the number of replicates at each, the acceptance criteria, and whether adjustment is included or charged when a unit fails. A certificate that reports a single volume tells you very little about the instrument.

What a calibration certificate should actually show

Look for the measured mean and the measured variability at each volume tested, against stated acceptance limits, along with the ambient conditions and the balance used. A certificate that reports only pass or fail has withheld the numbers that let you see an instrument drifting before it fails.

Ask whether the service is performed at your bench or at the vendor's site, and what the turnaround is. A fleet away for weeks is a hidden cost that rarely appears in the quoted price per instrument.

How often, and who decides

Interval is set by use and by consequence, not by the calendar alone: an instrument used constantly on a regulated assay is checked far more often than one used occasionally for buffer transfers. Most labs run a formal external service on a fixed cycle and a quick internal gravimetric check between them.

The internal check is the part that catches problems early and costs almost nothing beyond an analytical balance and ten minutes. Labs that do it find worn seals and damaged tip cones long before the annual service would.

Common questions

How often should pipettes be calibrated?

Set the interval from how heavily the instrument is used and what depends on the result. A regulated or high-throughput instrument is typically serviced far more frequently than an occasionally used one, with a quick internal gravimetric check between external services.

What does a pipette service include?

At minimum a clean, a seal and tip cone inspection, a multi-volume multi-replicate gravimetric test against stated limits, adjustment where needed and a certificate reporting the measured values. Confirm whether adjustment and parts are included or billed separately.

Can I use generic tips?

Often, but the tip cone seal sets the delivered volume, so any change of tip should be re-verified gravimetrically on the instruments concerned. Labs under an accredited scheme generally validate the specific tip and instrument pairing.

When do I need a positive displacement pipette?

For viscous, volatile, dense, foaming or highly contaminating liquids, where the air cushion in a conventional pipette introduces a systematic error or lets vapour reach the instrument.

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