Knitting needles and crochet hooks made according to the same gauging system and marked with the same gauge number — directly or on the packaging — can nonetheless differ to a perceptible degree in their actual diameters. This variation may be a simple result of careless sorting or otherwise insufficient quality control. However, it also has two significant nonrandom causes. One is that tools made in one country in compliance with its predominant standard, when intended for export, are marked with what is judged to be the nearest equivalent size in the standard of the destination country. Tools are also often labeled with the gauge designations of both countries.
The millimeter frequently appears either as the primary or alternate unit, rounded off to the nearest whole, half, or quarter. These increments are commonly used in countries where manufacturers work directly to a metric gauge (except for the finest-sized steel crochet hooks where the gradation is in tenths or five-hundredths of a millimeter). However, the sizes of hooks and needles produced elsewhere will not necessarily align with a scale divided exactly into quarter millimeters.
For example, I have two crochet hooks of the same highly regarded Japanese brand that are identical except for the size indications on their labels. When measured directly with slide calipers (explained below), the diameter of both hooks is 2.5 mm. One is intended for the domestic and European markets and labeled “4/0 — 2.50 mm.” The other is for export to the US, labeled “C-2 — 2.75 mm.”
Since these markings are on ergonomic handles I sacrificed the one on the latter hook, revealing the metal tool to be embossed “4/0 — 2.5.” The details of the Japanese gauge system are described in a post on the Japanese Knit and Crochet Pattern Help blog which says that a 4/0 hook can also be labeled as 2.25 mm. This means there is a ±10% tolerance in the indication of the actual size, gainsaying the widespread belief that millimeter markings are inherently more accurate than gauge numbers.
The second source of discrepancy between the nominal and actual diameters of hooks and needles is the precision with which the gauges used in their manufacture are calibrated against the underlying standard. (The term “gauge” designates both the measuring tool and the ordered system of numbers and dimensions that it incorporates.) This extends to the gauges commonly marketed to knitters and crocheters, which are typically accurate to about the same ±10% — even when marked in millimeters. (Anyone curious about slide calipers as an alternative, but less interested in background information about them, can skip directly to a how-to discussion below.)
This was a major industrial concern in mid-19th century England, when Imperial units of measurement were still in widespread international use but the push toward global metrication was gaining momentum. A leading participant in the debate, Joseph Whitworth, was among those who convincingly argued that the pivotal issue was the decimal representation of small linear measurements. In 1857, he proposed a standard wire gauge ranging from 0.001 to 0.500 inches, in increments gradually expanding from 0.001″ to 0.025″, with each represented size also serving as its gauge number. Continue reading “Calibrating tool gauges”