Feeds in proportion to water use. Every time a water meter counts a set volume, the output runs for a set time — one dose per 100 gallons of make-up water, say. Chemical tracks load automatically: a busy system gets more, a quiet one less.
An input that accumulates volume:
A flow rate cannot be used — the mode counts throughput, so it needs a running total. Channels that do not accumulate are shown in the picker as Not a water meter.
| Setting | Choices on the controller | Starting value | What it does |
|---|---|---|---|
| Gallons/Dose | 1, 5, 10, 25, 50, 100, 150, 200, 250, 500, 750, 1000 | 100 | Volume that triggers one dose. |
| On Duration | 5 s to 10 min | 15 s | How long the output runs for each dose. |
| Max Pending | 1 to 10 | 3 | How many doses may queue when water arrives faster than they can be fed. |
The chemical delivered per dose is whatever the pump puts out over the On Duration. Size the two together: for a target feed, pick the gallons per dose, then set the On Duration to the pump run time that delivers the right amount for that volume.
A cooling tower’s inhibitor is fed on make-up water. The program calls for 50 ppm of product in the make-up (at 4 cycles the tower then holds about 200 ppm). The product’s specific gravity is 1.2. The pump delivers 1.0 gal/h — about 3.8 L/h, or 1.05 mL/s. The make-up meter gives 10 pulses per gallon.
So Gallons/Dose 100 and On Duration 15 s — which happen to be the starting values.
Check the queue. At the site’s peak make-up of 40 gpm, 100 gal passes every 2½ minutes and each dose takes 15 s, so the pump runs 10 % of the time and the queue never builds. The queue fills only when doses are owed faster than they can be fed — here, above 400 gpm.
Check the meter. With the K-factor set to 10 pulses/gal, the controller counts gallons correctly. Set it to 1 by mistake and every pulse counts as a gallon: the mode doses ten times too often. See Water meters.
The mode keeps a running count of the water that has passed since the last dose. Each time the count reaches Gallons/Dose, it starts a dose and takes that volume off the count.
Doses run back to back. If more doses are owed when one finishes, the output simply stays on — three owed doses at 15 s each are a single 45 s run, not three separate starts.
The queue is capped. At most Max Pending doses can be owed at once. Water beyond that is not fed for — it is written off, so that a surge, a backfill, or a stuck meter cannot trigger a long overfeed afterwards. When the queue is full the screen shows it and the controller raises Dose Backlog Full (232) — water is passing unfed.
A full queue means the pump cannot keep up. If the mode sits at its cap, the On Duration is too long for the flow, or the gallons per dose too small. Raising Max Pending only hides the problem.
Example. 100 gallons per dose, 15 s on, Max Pending 3. A slow trickle gives one 15 s dose every 100 gallons. A sudden 450 gallons queues three doses — one 45 s run — and the remaining 150 gallons are written off.
A Volume Dosing mode cannot drive an output whose safe state is on. The controller refuses the save with Relay safe state is ON - set it OFF, and refuses to set an output’s safe state to on while a dosing mode uses it. Set the output’s safe state to off, then save the mode.
A mode saved this way on earlier firmware still loads and runs, holding the output off when blocked. It can be disabled, renamed, or moved to a fail-off output, but not otherwise changed until its output fails off.
While an interlock holds the mode off, the output is off — always, whatever the output’s configured safe state, because a dosing pump is never safer running — and any dose in progress is frozen, resuming with the time it had left. The meter keeps counting, so doses owed during the block queue up — to Max Pending — and are fed once it clears.
If the meter stops responding, no new doses start. The mode then stops and its output goes to its safe state — see What every mode does.