The Pressure Switch That Chattered: Dead Band and Pulsation, Not the Switch
A pneumatic pressure switch detecting line pressure. It chattered (on-off-on-off) as the pressure fluctuated. The customer thought the switch was failing. The switch was fine. It was switching at the exact setpoint, and the pressure was fluctuating around that setpoint by 0.1 bar. This is about pressure switch dead band and system pulsation, not about the switch being defective.
Why pressure switches chatter
A pressure switch turns ON at the setpoint (e.g., 5 bar) and OFF at the same setpoint if there’s no dead band. If the pressure is hovering around 5 bar (fluctuating 4.9-5.1 bar), the switch toggles. It chatters. A dead band (or hysteresis) means the switch turns ON at 5 bar and OFF at 4.7 bar (a 0.3 bar gap). The pressure must drop below 4.7 bar before the switch turns OFF. Normal fluctuations (0.1 bar) don’t cause chatter.
The dead band settings
| Switch type | Dead band | Use for |
|---|---|---|
| Simple pressure switch (fixed) | 0.1-0.2 bar | Stable systems |
| Adjustable dead band switch | 0.2-1 bar (settable) | Fluctuating systems |
| Pressure transducer + PLC | Programmable (any value) | Precise control |
What we did
1. Increased the dead band. We set the switch’s dead band to 0.5 bar. It turns ON at 5.0 bar and OFF at 4.5 bar. The normal fluctuation (0.1 bar) doesn’t cause chatter. The switch stays ON as long as the pressure is above 4.5 bar.
2. Added a pressure gauge dampener. The system had pulsation from the compressor cycling. We installed a needle valve (restrictor) on the pressure switch line. The pulsation is dampened. The switch sees a steady pressure, not a bouncing one.
3. Considered a transducer. For precise pressure monitoring (not just on/off), a pressure transducer (4-20 mA) into the PLC is better. The PLC can apply its own filtering and setpoint logic. But for a simple “pressure OK” signal, the switch with adjusted dead band is fine and cheaper.
The pulsation source
Pressure fluctuation comes from: the compressor cycling (on/off), the downstream valves actuating (surges), and long lines with no accumulator. We added a small accumulator (1 L) near the switch. It smooths the pressure. The switch sees a stable reading.
The pressure switch rule: If a pressure switch chatters, increase the dead band (hysteresis). Add a dampener or accumulator. The switch isn’t failing — it’s switching at the exact setpoint while the pressure fluctuates around it. A dead band of 0.3-0.5 bar absorbs normal fluctuation. The switch that chattered wasn’t defective — it had no hysteresis.
