Runs an output for a fixed share of every cycle — 25 % of every ten minutes, say. It reads no sensor, so it feeds at the same rate whatever the water is doing. Use it for a steady feed where there is no probe for the chemical and no water meter.
| Setting | Range on the controller | What it does |
|---|---|---|
| Cycle Length | 1 min to 2 h, in 1 min steps | How long one on-then-off cycle lasts. |
| Percent On | 1 % to 100 %, in 1 % steps | The share of each cycle the output is on. |
The on time is the cycle length × percent on; the rest of the cycle is off.
| Cycle Length | Percent On | On | Off |
|---|---|---|---|
| 10 min | 25 % | 2 min 30 s | 7 min 30 s |
| 1 min | 50 % | 30 s | 30 s |
| 2 h | 5 % | 6 min | 1 h 54 min |
| 5 min | 10 % | 30 s | 4 min 30 s |
100 % keeps the output on. To stop a Percent Timer feeding, disable the mode.
A cooling tower bleeds 5,000 gal/day. The inhibitor program calls for 100 ppm of product in the tower water, and the product’s specific gravity is 1.2. The metering pump delivers 1.0 gal/h at its current stroke setting.
The controller sets Percent On in whole percent, so choose 2 %. On a 60 min cycle that is 1 min 12 s on each hour, which feeds 0.48 gal/day — 115 ppm, not 100.
On a big pump, 1 % steps are coarse. Here 1 % gives 58 ppm and 2 % gives 115 ppm. Turn the pump’s own stroke or speed down so each step is smaller: at 0.3 gal/h, each 1 % is about 17 ppm, and 6 % gives 104 ppm. Better still, size the pump so the feed needs 10 % or more.
The same formula runs backwards to check a site: pump rate × 24 × Percent On ÷ 100 = product per day. Compare it with the drum’s actual draw-down over a week.
The 5,000 gal/day is the water the product leaves in — for a cooling tower, bleed plus drift. See Cycles of Concentration and Chemical Feed Methods.
Each cycle starts with the on period. The cycle restarts — beginning with a fresh on period — whenever the controller restarts or the mode is edited.
Pick a cycle length your equipment is happy with. The same percentage can be delivered as short frequent runs or longer infrequent ones. A shorter cycle means more starts per hour; a pump with a minimum run time or a relay rated for limited switching wants a longer cycle.
Because it does not measure anything, a Percent Timer keeps feeding when there is no flow, a tank is empty, or the system is shut down. Add an interlock for whatever should stop it:
While any interlock holds it off, the output goes to its safe state — normally off — and the cycle’s countdown is frozen. When the interlock clears, the cycle carries on with the time it had left. Feed time lost during the block is not made up. A during calendar event pause holds the output off.