TDS is the early-warning system that says your refill is overdue. Here is how to test it cheaply and the threshold that genuinely matters.
Total Dissolved Solids (TDS) measures everything dissolved in your water: sanitisers, hardness, alkalinity, sweat residues, sun cream, the lot. It rises slowly and predictably. When it gets above a threshold, no amount of dosing or balancing will rescue the chemistry. The right answer is then a refill.
Every chemical you add. Every bather session. Every top-up of the original mains water with its inherent hardness. Even the slow leaching from rubber components.
Fresh mains water in a UK hot tub starts at around 200 to 400 ppm TDS depending on hardness. A six-week-old tub used regularly is typically at 800 to 1,200 ppm. By 12 weeks the same tub can be at 1,500 to 2,000 ppm.
Above 1,500 ppm in a hot tub, the water starts to feel slick or heavy. Above 2,000 ppm, the chemistry stops behaving predictably and chloramine production accelerates.
A TDS pen costs around 15 to 25 pounds and gives an instant reading. Calibrate it once with a sachet of standard solution and it stays accurate for months.
Test TDS at every refill, then again at six weeks. The trend tells you whether you are on track for a 12-week refill or whether your tub needs sooner attention.
TDS at 1,000 to 1,500 ppm: you have time. Plan a refill in the next two weeks but no rush.
TDS at 1,500 to 2,000 ppm: schedule the refill this weekend.
TDS above 2,000 ppm: refill now. No amount of shock or balancing will rescue water at this level. You are throwing chemicals into a tub that cannot hold them.
A partial drain (half) and refill drops TDS by around half. It is a useful interim measure if a full refill is inconvenient that week.
No. There is no consumer chemical that removes dissolved solids. Reverse osmosis can, but the equipment costs more than a year of refills.
Slightly. Soft water starts at lower TDS so the climb from 200 to 1,500 takes a little longer. The 12-week rule of thumb still holds for moderate use.