Reading a Rothwell Absorption Figure (ISO 11948)

1. What does a Rothwell absorption figure actually measure?

ISO 11948-1, universally called the Rothwell method, measures the total absorption capacity of a urine-absorbing product: the pad is fully immersed in test fluid, allowed to reach saturation, drained under defined conditions, and weighed. The result is the mass of fluid the absorbent core can hold.

That is a theoretical capacity under optimal laboratory conditions, and the gap to what a wearer experiences is large and well documented. The working rule in the field is that practical absorption is around one third of the Rothwell figure. Core composition, product size, the type of incontinence, the pressure applied by the body and the wearer's posture all sit between the two numbers.

This matters commercially, not just scientifically. A Rothwell value is a legitimate specification and a legitimate basis for comparing your core against a competitor's. Presented to a purchaser as the volume the product will hold in use, it is a claim the method does not support and that the literature contradicts.

2. What is the difference between ISO 11948-1 and ISO 11948-2?

Neither part predicts leakage well on its own. Published work examining both concluded that they are limited predictors of leakage performance for small pads. If leakage is the property you are actually selling against, the laboratory figure is a screen, and in-use data is the evidence.

3. Why do two laboratories get different Rothwell values?

The method's repeatability and reproducibility were investigated across laboratories running the same apparatus, and the finding is worth carrying into any comparison you make. Reproducibility was good, but the differences between laboratories were systematic rather than random: the Swedish and Spanish laboratories typically exceeded the English one by around 13 % and 8 % respectively.

A 13 % offset is larger than most of the product-to-product differences anyone argues about. Two practical conclusions follow: a specification should name the laboratory or at least fix it, and a comparison between your product and a competitor's is only meaningful if both figures came from the same laboratory on the same apparatus. A difference of a few percent between two labs is not evidence of a better core.

4. Why is the ISO 11948 test fluid deliberately plain?

The synthetic urine used here has one job: to wet and swell the superabsorbent polymer reproducibly. The property being measured is the polymer's uptake, and polyacrylate SAP uptake is strongly sensitive to ionic strength and to divalent cations. Calcium and magnesium crosslink the gel and suppress swelling, which is exactly the confounder you do not want inside a capacity measurement.

So it is formulated as a defined, low-divalent, precipitate-free solution rather than as a realistic urine. A fluid that deposits struvite or calcium phosphate on standing would change ionic strength between the first pad of a run and the last, and the drift would look like product variability.

If your question is encrustation, blockage or deposit formation, this is the wrong fluid and ISO 11948 is the wrong method. No horizontal ISO or EN standard governs that work: it uses a urease-active urine, an AUM formulation of the kind used throughout the Stickler bladder-model literature, where precipitation is the phenomenon under study rather than an artefact to be eliminated. Our AUM fluid is built for that; the ISO 11948 fluid is not.

5. How much test fluid does an absorption campaign consume?

Total immersion consumes far more fluid than the pad retains, because the bath has to stay large enough that saturation is not limited by the volume available. Plan on the bath volume and the number of replicates, and remember that a high-capacity adult product can retain several hundred millilitres by itself.

This guide describes how the result should be read and specified. It does not reproduce the apparatus, drainage times or fluid composition of ISO 11948-1, which you need from ISO or your national body.