Reservoir top-up calculator
Plants drink water faster than they take up nutrients, so a reservoir's EC drifts as it empties. This works out what to top up with so the tank lands back on the EC you want.
How it is calculated
At the strengths used for feeding, EC mixes very nearly in proportion to volume: two equal volumes at EC 1.0 and 2.0 make EC 1.5. So the EC the top-up needs, to fill a tank of capacity C holding V at ECnow, is:
EC top-up = (C × EC target − V × EC now) ÷ (C − V)
Bringing a strong tank down with water whose own EC is ECwater:
water to add = V × (EC now − EC target) ÷ (EC target − EC water)
When that will not fit, drain first. Keep this much, then fill to the top with water:
keep = C × (EC target − EC water) ÷ (EC now − EC water)
The share of target strength subtracts your water's own EC, because that part is not nutrient.
Reading the drift
Which way EC moves tells you how the plants are feeding:
| As the tank empties | It usually means |
|---|---|
| EC rises | Plants are taking more water than nutrients: top up weaker |
| EC holds steady | Uptake matches the feed: top up at target |
| EC falls | Plants are hungry: top up stronger |
These are rules of thumb. Some nutrients are taken up faster than others, so a tank topped up for weeks drifts in balance even when its EC looks right; a full change every one to two weeks resets it.
Cultidock's water pack is designed to watch EC and tank level for you. Status: sensors chosen, the water-sensor bus is not yet tested with probes. The water sensor pack