The Pressure Switch That Chattered (No Dead Band)
We used a pressure switch (article 122) to control a pump (the pump turns on when the pressure drops, off when it’s high). On the floor, the pump chattered (it turned on/off rapidly). The problem: the pressure switch had no dead band (the on/off points were the same). The pressure hovered around the setpoint (the switch toggled). We either adjusted the dead band (the switch’s differential) or set the on/off points apart. We set the dead band (the switch turns on at 4 bar, off at 6 bar — 2 bar apart). The pump cycled (not chattered). The mistake was setting the on/off points the same (no dead band).
Pressure switch dead band — the gap between on/off. This article covers the setting.
The Dead Band (Differential)
A pressure switch turns on (at a low pressure) and off (at a high pressure). The gap (between on and off) is the dead band (differential). If the dead band is zero (on = off), the switch chatters (the pressure hovers, the switch toggles). A dead band (e.g., on at 4 bar, off at 6 bar) lets the pressure vary (the switch doesn’t chatter).
Step 1: The Setpoints (On and Off)
Set two points: the low (on) and the high (off). For a pump (filling a tank): on at 4 bar (low, the tank is empty), off at 6 bar (high, the tank is full). The pressure varies (4–6 bar). The pump cycles (not chatters).
The dead band rule: Set the on/off points apart (a dead band). The switch that chattered had on = off (same point). Set on at 4 bar, off at 6 bar (2 bar apart). The pressure varies (the switch doesn’t chatter).
Step 2: The Adjustable Dead Band
Some pressure switches have an adjustable dead band (a screw). Set the dead band (e.g., 1 bar, or 2 bar). A small dead band (0.5 bar) chatters (for a fluctuating pressure). A large dead band (2 bar) is stable (but the pressure varies more). Pick the dead band (for the application).
Step 3: The Hysteresis (Built-In)
A pressure transducer (article 122, analog) has a hysteresis (in the PLC, article 181). The PLC reads the pressure. It turns on (at 4 bar) and off (at 6 bar). The hysteresis is in the program (the on/off are different). This is flexible (set in software). A mechanical switch (fixed) has a built-in dead band.
Step 4: The Pulsation (Stabilize)
The pressure (in a pump line) pulsates (the pump cycles). Add a damper (a pulse dampener, a small tank) to stabilize. A fluctuating pressure (no damper) makes the switch chatter (even with a dead band). Add a damper (stabilize the pressure).
| Setting | Behavior |
|---|---|
| On = off (no dead band) | Chatter (rapid toggle) |
| On 4 bar, off 6 bar (2 bar) | Stable (cycle) |
| Small dead band (0.5 bar) | Chatter (fluctuating) |
| Large dead band (2 bar) | Stable (pressure varies) |
A Pressure Switch Checklist
- Does the switch chatter? (Toggle?)
- Is the dead band set? (On/off apart?)
- Is the on point low? (4 bar?)
- Is the off point high? (6 bar?)
- Is the dead band adjustable? (Screw?)
- Is the pressure fluctuating? (Damper?)
- Is the hysteresis in the PLC? (Analog?)
- Does the pump cycle? (Not chatter?)
- Is the pressure stable? (Gauge?)
- Is the switch rated? (Cycles?)
The Bottom Line
Pressure switch dead band sets the on/off apart. The switch that chattered had on = off. Set on at 4 bar, off at 6 bar (dead band). Add a damper (stabilize). The switch that cycled (not chattered) wasn’t the most sensitive — it had a dead band.
