What gauge minimises coring through a rubber stopper?

My setup is a refrigerator with a logger and a small work area I wipe down, nothing more.

These are treated as interchangeable and I do not think they are.

If both are acceptable I would like to know that, so I can stop thinking about it.

Is there a defensible reason to prefer one, or is this a coin flip?

The relevant detail is that for a 4 mm pen-style needle the gauge options are narrow and the choice is nearly made for you.

Typical outer diameters: 21G is about 0.82 mm, 23G about 0.64 mm, 25G about 0.51 mm, 29G about 0.34 mm and 31G about 0.26 mm. The gauge number and the diameter move in opposite directions.

Stopper coring — punching a disc of rubber into the solution — is a large-bore phenomenon. An 18G or 21G needle inserted straight and fast is the classic way to do it; inserting at a slight angle with the bevel up reduces the risk.

Higher gauge is not automatically better; it is thinner, which has costs as well as benefits.

Angle the bevel and insert gently to avoid coring the stopper.

Answer first: use the largest bore you tolerate for drawing and the smallest for injecting, because the two operations have opposite requirements.

A 30G or 31G needle through a butyl stopper leaves a track that reseals, which is why fine-gauge repeated entry is tolerable and coarse-gauge repeated entry is not.

The part that matters: very fine needles are more prone to bending and to blocking with any particulate, which is a practical argument for inspecting the solution before drawing.

If a solution will not draw through a 25G needle, the problem is the solution rather than the needle.

Big to draw, small to inject, never the same one twice.

Answering this needs to know the viscosity of what is being drawn, since a viscous solution through a fine needle is slow enough to encourage bad technique.

Drawing a viscous or foamy solution through a fine needle takes long enough that people rush the plunger, which causes more foaming. Using a wider drawing needle is the fix.

It helps to be literal here: fixed-needle insulin syringes are supplied in 29G to 31G and cannot be swapped for drawing, which is the trade-off against their much lower dead space.

Nothing here is medical advice.

Gauge numbers run backwards. Higher number, thinner needle.

Concretely, coring the stopper with a large-bore needle is the risk at the drawing end, and it is real.

For injecting, 29G to 31G is the usual range and the difference in perceived discomfort between them is small. Needle length matters more than gauge for comfort at these volumes.

Needle gauge to outer diameter correspondence is standardised and published; the inverse relationship between gauge number and diameter is the reason for the counter-intuitive labelling.

Length affects comfort more than gauge does at these volumes.

Start with the fact that gauge numbers run backwards — a higher number is a thinner needle — which is the source of half the confusion in this tag.

Flow through a needle scales with the fourth power of the internal radius under the Hagen–Poiseuille relation. Halving the radius reduces flow sixteen-fold at the same pressure, which is why a 31G needle draws so much more slowly than a 21G.

Butyl rubber closures are specified for resealing after piercing up to a stated gauge, which is the basis for the fine-gauge repeated-entry practice.

Flow goes as the fourth power of radius. That is why the difference feels so large.

edited 19 May 2024 by lyoph_cake — clarified the distinction between purity and content