Temperature of the liquid changes the volume

Temperature of the liquid changes the volume an air-displacement pipette delivers, even when the thumb repeats a perfect forward stroke. The decision is when to equilibrate, when to accept that a reagent must stay cold and change the way you interpret the dial, and when a warm weighing room has fooled the balance rather than the piston. The stroke itself is taught in accurate micropipetting technique.

The cushion expands and contracts

The piston sets a geometric air volume. The liquid moves because that air moved. Air's volume depends on temperature and pressure. A warm barrel around a cold plug of liquid, or a cold pipette dipped into a warm buffer, puts a temperature gradient inside the cushion during the stroke. The amount of liquid that ends in the tip is then not the amount a calibration at one temperature predicted. Hand heat on a small pipette during a long series is the same mechanism in miniature.

The liquid's own density moves too. That shows up twice: once in the volume that left the bottle, and again when you weigh the delivery and convert mass to volume. Near room temperature, water is about 0.998 grams per millilitre. Use a table for the temperature you measured. A Z-factor copied from a cooler calibration day will make a warm series look biased when the water was simply less dense.

ISO 8655-1:2022 includes user recommendations for these influences. It is a terminology and recommendation standard, not a certificate that your pipette is accurate on a hot afternoon. Numerical tolerances remain in ISO 8655-2:2022 and in the manufacturer specification. Compare a check done at a stated temperature with the specification you claim.

What to equilibrate, and what must stay cold

Equilibrate the pipette, the tip box, and any liquid whose volume is a result and whose chemistry allows it to sit at the bench temperature. Water for a gravimetric check belongs in this group. Buffers for a standard curve usually do too. Tips stored in a sunny window or against a cold wall should come to the same place as the pipette before a quantitative plate.

Some liquids must stay cold: enzyme stocks, ligations, certain competent-cell steps. Taking them from ice and pipetting them slowly with a warm tip lets the cushion change while you work through the rack. The practical controls are a pre-wet at the start of that cold series, a short contact time, a tip that is not huge relative to the aliquot, and a refusal to mix those tubes into a calibration story that was collected at room temperature. If the assay can use a master mix that has been brought to a stated temperature, do that, and keep only the enzyme cold. Follow the manufacturer's protocol for the enzyme. This page does not replace it.

Humidity is the decoy. It changes evaporation from open plates and from weigh boats, and it changes static on plastic. It is a weaker direct driver of the air cushion than temperature. If replicates scatter only on the balance, suspect the weighing. If they scatter in the assay while the balance is calm, suspect the liquid temperature and the stroke. The interim check pattern in checking a pipette between calibrations is how you separate those.

A decision path when morning and afternoon disagree

Yesterday's check passed in an air-conditioned room. This afternoon the room is warmer and the standard curve has shifted. Do not turn the pipette's adjustment screw to chase the weather. First, repeat a short water series with equilibrated water and record its temperature. Convert with the matching density. If that series meets the specification, the pipette is behaving and the assay liquids need the same patience. If it fails, the instrument or the tip is out of quantitative service until a fuller check says otherwise.

Branch for cold reagents separately. A water series can pass at the bench while an ice-bucket enzyme is still being delivered by a changing cushion. Watch the first and the last tube of a practice row into a tared cold-compatible vessel if the volume justifies it, using that liquid's density. If they diverge, standardise a faster rhythm or a smaller number of tubes per tip, and write it down. Do not publish the dial and the ice as if they were the equilibrated calibration.

Hand warmth and sunlight on a barrel are the same branch. Move the instrument, let it sit, then measure.

Failure modes

A standard curve prepared in a cool morning and samples pipetted in a hot afternoon are not the same volume scale. A certificate from a controlled room remains a true report of that day. Check again on the warm bench. Stopping short of the first stop to chase the difference hides the thermal error inside a new systematic error.

Safety

Cold racks sweat. Condensation on a lid is water you did not pipette. Wipe without contaminating a sterile tube mouth. Follow the institutional assessment. This guide is research technique.

In a warm laboratory the working room may sit well above the temperature printed on a temperate calibration. Record that condition. Keep the balance out of direct sun, let it warm up after cooling returns, and repeat a water check when the room you care about is back. A hill station adds altitude to heat. Check the pipette where it works.

What an enquiry should include

State the volume range, the liquid temperature you must work at, and whether the calibration document should say the temperature of the test. Name air displacement or positive displacement, the tip family, and any solvent exposure. Use the scientific instruments catalogue and the laboratory consumables catalogue, and send the thermal condition with the quotation request.