Staging and the honest alarm

One pump with a deadband holds a normal day. But inflow isn't polite — a storm, a wash-down, a stuck valve upstream — and one pump can't always keep up. The professional answer isn't a bigger pump. It's a second pump that only shows up when things get serious — and an alarm for when even that isn't enough.

Staging: the same rung, higher up

The tank circuit runs P2 with the exact hysteresis rung you already know — just with higher numbers: in at 85, out at 50.

Read the two bands together:

• Level climbs past 60 → P1 starts and usually wins; the level turns around, drops to 30, done. P2 never moved.
• A heavy day pushes past 85 anyway → P2 joins, and now two pumps pull the level down.
• P2 lets go at 50 (its work is done), P1 carries on alone to 30.

The bands overlap on purpose: staggered starts mean the second motor only runs when the first is already losing, and staggered stops hand the tail of the job back to one pump. That's duty/assist staging, and it's two copies of one rung with different setpoints.

The alarm that refuses a seal

Rung 2 is different on purpose:

──────[ AI1 ≥ 95 ]──────( HN1 horn )

No seal. No deadband. The horn is loud at 96 and silent the moment the level drops below 95 — it always tells the CURRENT truth. For a high-high alarm that's exactly right: the sound means "the condition is happening now."

The other philosophy exists too — you built it in the alarm-acknowledge circuit of the simulator: a fault that latches and keeps a light on until a human acknowledges it, so nobody can miss that it happened. Which one a signal deserves is a design question you should now recognize on any print: does this alarm report a state, or record an event? A sealless compare reports; a latch records.

Try the whole choreography

In the tank circuit, slide AI1 slowly from 20 up to 96 and back down to 20. Watch the order: P1 in at 60, P2 in at 85, horn at 95 — then horn out at 94, P2 out at 50, P1 out at 30. Five thresholds, three outputs, one slider. Every sequenced start you'll ever wire is some arrangement of these bands.