Eighty-Four Percent Humidity: Nine Weeks Before the Powder Cakes

There is a hygrometer on the wall of the sluice room in a care home in Kent, and on the morning it was read it said 84 percent. The 25 kg sack of powder in the corner had been opened in June and was in use until September. By August, staff were breaking it up with the handle of a mop, and by September they had thrown away the last third of it because the lumps would not break down in the pan.

Nobody had done anything wrong, in the sense that the bag was in a cupboard and the cupboard had a door. But a sluice room is a wet room with a door, and superabsorbent powder left open in a wet room will absorb water from the air. The only question is how long it takes.

What moisture actually does

A superabsorbent particle in humid air takes up water at its surface first. The surface becomes tacky, neighbouring particles stick, and the mass sets into a crust. That is why a caked bag looks normal from the outside and behaves like gravel from the inside.

The damage is twofold. First, the caked material does not disperse in urine. A 20 g lump dropped into a pan will hydrate on its outside, form a barrier layer, and leave a dry core that never contributes to the set. Second, the water the powder has already absorbed is water the site paid for. A bag that has taken up 10 percent moisture weighs 10 percent more and absorbs 10 percent less.

How fast it happens

Rough numbers for an opened 25 kg sack in a room at 20 °C:

Sixty-five percent is the practical threshold. Below it, an opened bag in a sealed tub is stable for months. Above it, nothing you do to the bag will keep the surface free-flowing for long, because the air in the headspace is already loaded.

The barrier question

Packaging claims are worth reading carefully here. A three-layer foil laminate with a metallised layer has a moisture vapour transmission rate low enough that a sealed sachet is effectively stable for two years or more. A single-layer polyethylene bag, even a thick one, is roughly two orders of magnitude more permeable. The difference is not cosmetic: it is the difference between a two-year shelf life and a nine-month one.

For an opened bulk bag, the closure matters more than the bag. A folded-over top with a clip is better than nothing and much worse than a rigid, gasketed lid. The best arrangement seen in a care home was a 25 kg sack decanted into four 5 kg screw-top buckets, each labelled with the date of decanting, kept in a dry store and carried to the sluice in 1 kg lots. The sack never entered the wet room at all.

The counter-intuitive purchase decision

Sites that buy on price per kilo sometimes choose a supplier whose powder arrives at 8 to 12 percent moisture, which is within most specifications and completely normal for the grade. That moisture is free water in the bag. On a 25 kg order it can be 2 to 3 kg of water, and the site is paying powder prices for it. Over a year of 300 kg, that is 30 kg of water at £8 a kilo.

The check is simple: weigh a fresh, unopened bag from a new delivery and compare it with the declared net weight on the label. A bag that is consistently 4 percent over its declared weight is either generously filled or holding moisture. A reputable supplier will have it.

What to do with powder that has already caked

Light surface caking can be broken up and used, provided the lumps are crushed to below about 3 mm and the dose is increased by 10 percent to compensate for the water already absorbed. Heavy caking — a bag that has to be cut open and levered apart — should go back to the supplier or be written off.

Storage rules that survive a busy shift

• Keep bulk in a dry store, off the floor, away from external walls and hot water cylinders.
• Decant weekly into a lidded container small enough to be used up in that week.
• Never store the scoop inside the powder. Moisture travels down the handle.
• Label every container with the decant date and use oldest first.
• Put a cheap hygrometer in the store and in the sluice. If the sluice reads above 70 percent, the powder should not live there.

The Kent home moved its stock to a dry cupboard in the laundry, installed a £6 hygrometer, and stopped throwing away a third of every bag. The saving was roughly £180 a year on a 42-bed site — modest, but it came with a second benefit that mattered more: the gel started setting in the time the care plans said it would. Barrier and shelf-life figures sit with the packaging and shelf-life specifications, which is what to send a store manager who wants the sack kept in the sluice.