ATO reservoir safety calculator
An auto top-off (ATO) pump that sticks on can empty its whole reservoir into the tank. Check how far salinity would fall, and the biggest reservoir your tank can take.
Formulas and sources last checked . How we check
Calculator
Result
If the whole reservoir goes in32.3 ppt
- Drop
- 2.7 ppt
- Largest safe reservoir
- 28.1 litres
- One fill lasts
- about 6.3 days
OK: A full dump would keep salinity at or above 32.0 ppt.
How big should an ATO reservoir be?
The worst case is the whole reservoir of fresh water going in at once. Salinity then falls in proportion to the extra water. To stay above a floor, the reservoir must be no bigger than the system volume times the fraction by which salinity can fall. From 35 down to 32 ppt that is about 9 per cent of net water volume.
after a dump = salinity x volume / (volume + reservoir)largest safe reservoir = volume x (salinity now / lowest safe - 1)
Worked example: 300 litres and a 25 litre reservoir
- 300 litres at 35 ppt, 25 litres of RO/DI in the reservoir: 35 x 300 / 325 = 32.3 ppt if it all goes in.
- Largest reservoir that keeps it at 32 ppt or above: 300 x (35 / 32 - 1) = 28.1 litres.
- At 4 litres of evaporation a day, one fill lasts about 6 days.
How do I make an ATO fail safe?
Bulk Reef Supply (2024) keeps most reservoirs at or under 10 US gallons (about 38 litres) whatever the tank size, and says top-off water must be RO/DI at 0 TDS. It also warns against plumbing an RO/DI unit straight into the reservoir without a float valve and shut-off. A typical 50 to 100 US gallon tank (about 190 to 380 litres) uses roughly one 5 US gallon jug a week, it says.
What are the limits?
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