Our tube-ladder piece covered how the spacing between ratings in a graduated set gets engineered — the deliberate steps in wall thickness and diameter that separate a 10 LBS tube from a 20 LBS tube. This piece looks at a related but distinct question: the variance within a single rating, and why manufacturing tolerance means no two tubes are ever perfectly, atom-for-atom identical.
What "tolerance" means in manufacturing generally. No manufacturing process produces parts to one mathematically exact dimension, every single time. Instead, engineers specify an acceptable range — a tolerance — within which a measured dimension is considered fully compliant with spec. This is standard across manufacturing broadly, from machined metal parts to molded plastics to, in this case, extruded elastomer tubing.
How this applies to extruded tubing specifically. As covered in our tube-ladder piece, resistance tubes are formed by pushing elastomer material through a die to produce a consistent cylindrical shape. "Consistent" here means consistent within a tolerance band for wall thickness and outer diameter — not identical to the micron across an entire production run. A tube measuring at one edge of its tolerance range and another tube measuring at the opposite edge can both legitimately carry the same printed rating, provided both fall within the range the rating was tested and specified against.
Why tighter tolerance costs more. Holding a narrower tolerance range consistently across a high-volume production run generally requires more precise equipment, more frequent calibration, and more rigorous in-line quality checks — all of which add cost. This is part of why tolerance specifications aren't uniform across the market: a manufacturer targeting a tighter tolerance is making a deliberate cost/precision trade-off, not simply doing the same job better for free.
What this means practically. Two tubes carrying an identical printed rating, potentially even from the same production batch, can sit at different points within their shared tolerance range — which can translate into a subtle difference in stretch feel between them, even though both are fully within spec and both accurately represent their labeled rating. This is the expected, normal behavior of a manufactured product working within a tolerance system, not an indication that one unit is defective and the other isn't.
The takeaway. Manufacturing tolerance is a designed-in range, not a flaw in the process — and understanding that a rating represents compliance within a range, rather than one exact identical value, explains why slight unit-to-unit variation is normal in any manufactured resistance product, this brand's or otherwise.