Serological pipettes are a different tool

The symptom is a millilitre problem treated with microlitre habits, or the reverse. Someone blows out a pipette that was meant to drain, or they try to set 250 microlitres on a serological pipette by squinting at a graduation, or they keep using a tube with a crack because the morning is busy. Serological pipettes are a different tool from a micropipette. The next check is the marking, the controller, and the plastic itself. Piston technique for true microlitre work stays in accurate micropipetting technique.

Two accuracy classes

A micropipette selects a volume with a piston and a dial. Its errors are discussed in the ISO 8655 pipette framework, and they are small relative to a microlitre decision when the instrument is fit and the tip seals. A serological pipette is a graduated tube, typically in a millilitre range, driven by a rubber bulb or an electronic controller. You read a meniscus against a printed line. The accuracy class is coarser. That is acceptable for feeding a culture flask or aliquoting a wash. It is a poor way to build a standard curve that will be interpreted to a few percent.

ISO 8655-1:2022 names piston-operated apparatus and gives user recommendations for that family. A serological pipette is not that family, even though both move liquid. Do not file a serological lot number under a piston-pipette calibration and feel that the framework has been satisfied. NIST's Office of Weights and Measures is a reminder that the measurement system should match the claim. A meniscus read at eye level is the claim a serological pipette can support. A dial is the claim a micropipette can support.

Blow-out, drain-out, and the controller

Look at the upper end of the pipette before you fill it. Blow-out serological pipettes are meant to deliver the liquid below the mark and then the last drop, using the controller to expel. The graduations were printed for that finish. If you stop when the liquid reaches the tip and leave the drop, you have shorted every aliquot in the same direction.

Other volumetric pipettes are drain-out. You touch the tip to the wall, let them drain, respect any waiting time the marking calls for, and you do not blow the residual film into the vessel. The film is part of the calibration. Blowing it out adds volume the mark did not include. The troubleshooting step is to read what this object says, not what the last pipette you held said.

The controller is the safety tool and the pace tool. It should seal on the mouthpiece without a chewed filter or a cracked collar. Set a speed that does not foam serum-containing media into bubbles. A bubble under the meniscus makes the graduation optimistic. Viscous additives need a slower aspirate and a pause before you trust the line. Tip fit here means the controller's holder, not a micropipette cone. A holder that leaks makes you draw harder and overshoot the mark.

Never mouth-pipette. The convenience is not a method. Institutional biosafety expectations, including the advisory material in the WHO laboratory biosafety manual and CDC BMBL, treat mouth pipetting as an exposure route to close. Your own assessment decides containment. This article does not.

Symptom, likely cause, next check

If the flask is short of medium, check three causes in order. First, the reading: eye level, no bubble, the mark you think you used. Second, the delivery contract: blow-out performed when the pipette requires it, and not performed when it does not. Third, the object: cracks, chips, faded graduations, a cotton plug that has sucked up liquid. Any of those retires the pipette. Do not rinse a cracked one and return it to the can.

If the problem is a microlitre assay that someone tried to run with a serological pipette, stop and move the work to a piston pipette in the right range. Do not average ten squints at the bottom of a 5 millilitre graduation and call it 100 microlitres. If the problem is a piston pipette being used to dispense 10 millilitres of wash in a hurry, stop and use a serological pipette or a dispenser class meant for that volume. Heroic repetition is where miscounts live.

If the liquid is viscous, watch the film. A fast controller leaves a thick coat and the mark under-delivers. Slow down. If the liquid is volatile, work in the ventilation you already assessed, cap vessels, and do not treat a serological pipette full of solvent as a pen you can gesture with. A drip from the tip while you talk is lost volume and an exposure.

Failure modes

Using a serological pipette as a stirrer bends and cracks it. Laying it down full runs liquid into the cotton plug and then into the controller. That plug is then a contamination path and a volume sponge. Eject the pipette into the waste the assessment requires. Do not rest a used one back in a clean pouch.

A controller with a tired battery that surges will overshoot marks late in the day. Charge it, or finish critical marks with a paced hand on a bulb you can control. Do not "correct" by eye after a surge and continue a dilution series.

Safety

The liquid is the hazard. Cultures belong in the containment you assessed. Cracked plastic fails without warning. Do not cut a serological pipette to make it fit a tube. This page is research troubleshooting. It is not a clinical filling instruction and not a diagnostic method.

What to specify

State the volume range in millilitres, individually wrapped sterile or bulk, blow-out style if that is what your method reads, and the controller type the mouthpiece must match. Micropipettes, if you need them for the microlitre step beside this work, belong in the same enquiry as a separate line so the classes do not get merged. Consumables are in the laboratory consumables catalogue. Controllers and micropipettes are in the scientific instruments catalogue. Send the marking style you rely on with the quotation request.