Absorbency Testing
Absorbency is the number buyers lean on hardest and understand least. Two products quoted at the same capacity can behave completely differently in use, because capacity is measured in several incompatible ways and the method is rarely stated.
Why one number means nothing
"Absorbency: 200 ml" is not a specification. It is a result, and a result only means something alongside the method that produced it. The same product can be reported at very different figures depending on four choices:
- What is measured. How much liquid the core takes up, or how much it keeps after being drained.
- What liquid is used. Saline, distilled water or a synthetic urine give different results on the same sample.
- How the sample is prepared. Whole product or core only, flat or folded, one layer or the full construction.
- How long it is given. A capacity reached in three minutes and one reached in thirty are not the same claim.
A supplier who answers "200 ml" without being asked which method is either quoting a marketing figure or quoting a test they did not run. Either way you cannot compare it with a second supplier's number.
The methods behind the number
These are the tests you will encounter. Ask for the result by name, not just the figure.
| Test | What it measures | Why it matters when worn |
|---|---|---|
| Free swell capacity | Liquid taken up by a freely swelling sample, usually after draining | The headline number, and the least predictive of leaks |
| Centrifuge retention capacity | Liquid retained after the sample is spun under a defined force | Closer to what stays in the core once the wearer sits or moves |
| Demand absorbency / acquisition rate | How quickly liquid is taken in under a set pressure | Predicts surface wetness on a sudden flow rather than total capacity |
| Rewet under pressure | Liquid that comes back to the surface under a defined load | The best single predictor of the leak and dampness a wearer notices |
| Strike-through time | Time for a set volume to pass the topsheet | Explains the "wet for a moment" feeling at the start of a flow |
| Wicking distance | How far liquid travels along the core in a set time | Decides whether the front of the product saturates while the back stays dry |
Free swell is the number most often quoted and the one least connected to performance. Rewet and acquisition rate are what a buyer should be asking for.
What liquid is used
Absorbency depends on the liquid. Distilled water is absorbed faster than saline, and saline is absorbed faster than a synthetic urine with a realistic surface tension. Results from different liquids cannot be compared, and a supplier switching liquids between quotations can make a product look better without changing anything about it.
So state the liquid in the request, and check that the report states it too. If two suppliers report wildly different figures, the liquid is the first thing to rule out — before you conclude that one core is genuinely better.
Rewet under pressure
Most complaints about this product are not "it could not hold enough". They are "it felt wet" or "it leaked when I sat down". Both are rewet and compression behaviour, and neither is captured by a capacity figure.
The test is simple in principle: load the product, apply a defined weight for a defined time, then measure how much liquid transfers back to a dry layer on the surface. What matters when comparing suppliers is that the load, the time and the absorbing layer are all stated — otherwise the figures are not comparable.
When a supplier claims a product is suitable for overnight or heavy flow, this is the test to ask for. Capacity alone does not support that claim.
A test you can run yourself
You do not need a laboratory to rank samples. A comparative test run identically on each sample is enough to tell you which one behaves better, and it costs almost nothing:
- Use the same liquid for every sample, and record what it was.
- Use the same volume for every sample. Pour it in one go, or in timed doses if you want to imitate a flow rather than a spill.
- Put the same weight on every sample for the same time. A stack of identical books is a perfectly good load, as long as it does not change between samples.
- Press the same dry layer on top, then weigh it. The increase is the rewet figure.
- Record everything: sample, liquid, volume, load, time, result. Two samples measured on different days with different volumes tell you nothing.
This will not produce a certified number. It will reliably tell you which of five samples you would rather wear, which is the decision you are actually making before you place an order.
What to put in the test request
- The name of the test, not just the figure you expect.
- The liquid and its concentration.
- The sample form: whole product, or core only.
- The load applied and the time held, for rewet and demand tests.
- The standard followed, if any, with its edition.
- Whether the report covers the exact SKU you are ordering or a similar one.
Ask for the report for the SKU, not for the product family. A report attached to a different core weight is a different product's evidence.
Frequently asked questions
- What absorbency should I specify?
Specify the test and the minimum result you need, not a bare number. For a worn product, a rewet figure under a stated load is more useful than a capacity figure, because it tracks the leaks buyers actually complain about.
- Why do two suppliers report very different absorbency for similar products?
Usually the method or the liquid differs, sometimes the sample form. Rule those out before assuming one core is better. Ask each supplier which test, which liquid and whether the sample was the whole product or the core alone.
- Is a higher absorbency figure always better?
No. More capacity usually means more superabsorbent and a thicker core, which changes fit, comfort and cost. A product can be over-specified for daytime use and still leak overnight if rewet and wicking are poor.
- Do I need a laboratory to compare samples?
No. A consistent comparative test on identical volumes and loads will rank samples usefully. Use a laboratory when you need a certified figure for a tender, a claim or a regulatory submission.
Specify absorbency properly
Post the test method and target you need, so quotes come back with comparable evidence instead of a single marketing figure.