Weighing moisture loss: a cheap daily number for each tub

The hygrometer said 84 percent. The scale said the tub had lost 62 grams since Tuesday. Both were telling the truth, and only one of them was useful. The hygrometer was reading the air 8 cm above a floor that had gone dry, and the scale was reading the whole tub, including the floor.

Relative humidity is a local, fast-moving number. It swings with the room, the time of day and where the sensor hangs. Mass is slow, cumulative and impossible to fake. If you want to know whether a substrate is actually losing water, weigh the tub. It costs one kitchen scale and about fifteen seconds a day.

The number that matters

Every tub has a daily water loss in grams, and that number is a property of the tub, not the weather. It is driven by exposed surface area, ventilation, temperature and the water content of the substrate. Change the lid and the number changes. Change the substrate depth and it changes. Change nothing and it stays remarkably constant, which is what makes it a baseline.

Once you know the number you can predict. A 30 by 20 cm tub losing 11 g a day will be 77 g lighter in a week, and if your floor holds about 300 g of usable water, that is a quarter of the buffer gone. You can plan a rehydration instead of reacting to a dry floor.

Setting a baseline in three days

Weigh the tub with the animal in it, lid on, at the same time each day, ideally before the morning mist. Write the number on tape on the side. Do it three days running and take the average daily difference. Do not weigh after feeding, and do not weigh right after misting, or you are measuring your own routine rather than the tub.

For small tubs the loss is a few grams a day and a 1 g scale is worth having. For large vivariums the scale needs to go to 20 kg and the loss will be in tens of grams. Either way, the same three-day routine applies.

What the trend is telling you

A stable daily loss is a healthy tub. A loss that doubles overnight means a lid has been left ajar, a vent has opened, or the room heating has come on. A loss that halves without any change on your part usually means the substrate has stopped releasing water freely, which happens when the gel has given up most of its reservoir and the remaining water is bound tightly in the fibre.

That second case is the one people miss. The floor still feels damp, the hygrometer still reads 75, and the tub has quietly stopped buffering. Then a hot weekend arrives and the humidity falls off a cliff in six hours. The scale showed it coming for a week.

• Weigh at the same time, before misting
• Record in grams, not percent
• Watch the seven-day total, not the daily wobble
• Treat a 40 percent drop in daily loss as a signal to check the floor

When to act and when to leave it alone

Rehydrate when the cumulative loss reaches about a third of the estimated usable water in the floor, not when the surface looks dry. For a 30 by 20 cm tub at 5 cm depth, that is somewhere around 200 to 250 g of cumulative loss. Pour water at the edge in two goes, an hour apart, and reweigh the next morning. The difference tells you how much the floor actually took up, which is usually less than you poured.

Leave it alone when the daily loss is stable and the cumulative number is under that threshold, even if the surface looks dry to you. Surface appearance is the least reliable indicator in the whole system, and reacting to it is how keepers end up with permanently wet floors and fungal problems. If you want the underlying mechanics, the SAP product data gives release behaviour by grade, which explains why some floors hold their daily loss flat for months and others taper off.

Put a scale under your most valuable tub tonight and write the number down. In a week you will have a baseline, and in a month you will wonder how you ever managed without it.