Sizing, rim form & glass choice
How to order tubes that fit the equipment already in the laboratory.
Test tubes are specified as outside diameter by length in millimeters, because that pair is what a rack, a centrifuge carrier, a heating block and a cap are built around. The other real choice is the rim. Everything else — glass type, wall thickness, whether it carries a mark — follows from what happens inside the tube.
Outside diameter is the dimension the equipment sees
A 16 mm tube goes into a 16 mm rack position, a 16 mm carrier and a 16 mm block whatever its wall thickness or capacity. That is why tubes are sold by OD and length rather than by volume. Ordering by nominal capacity is the fastest way to receive tubes that will not stand in the racks the laboratory already owns.
Rimmed tubes take caps; rimless tubes take plugs
A rolled or beaded lip strengthens the mouth and gives a snap cap or a screw closure something to grip. A plain cut rim suits cotton or foam plugs, fits closer in a rack and is easier to flame. Mixing the two in one stock leads to closures that fit some tubes and not others, which is a small annoyance repeated hundreds of times a week.
Glass choice is a heating question
Soda-lime tubes are cheap and adequate for room-temperature work and for teaching, and they crack when heated unevenly in a flame or a block. Borosilicate 3.3 takes direct heating and repeated autoclaving. A laboratory that heats tubes routinely spends less on borosilicate than on replacing soda-lime, and the two are indistinguishable once they are in a rack.
The round bottom is functional, not traditional
A hemispherical base has no corner for a pellet, a precipitate or the last of a sample to hide in, so contents drain and resuspend evenly. It also spreads thermal stress when a tube is heated from below. Flat-bottom tubes exist where a tube must stand unaided on a bench, and they trade both of those properties for it.
Wall thickness is a centrifuge decision
Standard-wall tubes are for handling and heating; heavy-wall tubes are for spinning. A tube in a centrifuge carrier takes a load it never sees on a bench, and a standard-wall tube used at speed fails inside the rotor where it makes a mess of everything else. Where tubes are spun, the wall grade belongs on the order rather than in a hope.
Marking spots and graduations are separate options
A fired enamel marking spot takes pencil and survives washing, which matters when a rack of forty tubes has to stay identifiable. Graduation marks are a different option and are approximate working scales, not calibrations. Ordering one when the laboratory wanted the other is common enough to be worth confirming explicitly on the first order.
