Measuring absorption: what the numbers really say

What is the Rothwell method (ISO 11948-1)?

ISO 11948-1 is the only international standard that tests a whole absorbent product. The protocol, known as Rothwell, is simple: the dry product is weighed, immersed in the test liquid for 30 minutes, drained for 5 minutes, then weighed again. The weight difference is the total absorption capacity.

It is a laboratory value obtained under ideal conditions: fully immersed, no pressure, no movement. It is the shared benchmark for comparing manufacturers and remains the most requested criterion in public and hospital tenders.

Water, saline, urine: three liquids, three results

The core of a product is made of cellulose and superabsorbent polymer (SAP). SAP swells by holding water through osmosis. Dissolved ions (sodium, chloride, urea, minerals) reduce that swelling: the more loaded the liquid, the less the polymer retains.

The values published on our product pages are total absorption capacities measured in 0.9 % NaCl saline solution, as ISO 11948-1 prescribes, because saline is closer to urine than water. A figure obtained in water is a different and much larger number: in our own data the saline value is about 22 % of the water value, so the two are never comparable. Real urine varies further from one person to another and hour to hour, with hydration, diet and medication.

In real use, an immersion test is not the whole story: body pressure, posture and the composition of urine all reduce what a product actually retains. That is not a product flaw, it is the difference between a laboratory immersion and a body that moves, sits and presses on the product. We publish no figure for in-use capacity, because we do not measure it.

Three measures that matter

Maximum absorbency (Rothwell) answers "how much". It is useful to rank products, provided the measurements were made with the same liquid.

Rate of absorbency, or ROA, answers "how fast". A void arrives in seconds: if the core does not absorb quickly enough, liquid escapes at the sides before it is held. It is the key measure against leaks, at night and for bedridden people.

Rewet answers "what the skin feels". Pressure is applied to an already loaded product and the moisture that returns to the surface is weighed. Low rewet means dry skin, hence fewer irritations and incontinence-associated dermatitis.

How Nateen calculates and displays absorption

To stop mixing water, saline and marketing figures, we display one number per product, calculated the same way for the whole range from the factory technical data sheet (TDS): theoretical maximum absorbency = (55 × SAP weight in g + 14 × cellulose weight in g) × adjustment coefficient, in millilitres, then converted to fluid ounces and cups. Both cores add up when the product has two.

The coefficient (1.05 today) is set in one place: changing it updates every value on the site at once. Laboratory measurements are generally higher than the calculation; we will publish the reports next to the calculated figure, with the test liquid used.

Why Nateen goes further

Common market practice is to display a single Rothwell value and keep the other results in-house. We believe a professional should see all three measures, with the test liquid used, before choosing.

Choosing well is more than one number

The right product depends on the person: mobility, body shape, type and timing of leaks, skin condition, how often a change is possible. A product too absorbent for the need is uncomfortable and costly; one too weak leaks.

Our rule: as discreet as possible, as absorbent as necessary. The Nateen assistant asks five questions and suggests a product and a size, never a diagnosis.