A pressure switch that chattered in a hydraulic clamping circuit. The switch was set at 100 bar to confirm clamp pressure. At 100 bar, the switch closed. The system started the next operation. But pressure fluctuated between 95 and 105 bar due to thermal expansion. The switch opened at 95 bar and closed again at 100 bar. The control circuit saw rapid on/off signals. The machine faulted on a “clamp pressure lost” alarm. The switch was not defective — it was too sensitive. This is about pressure switch dead band and control reliability.
The dead band
A pressure switch has two set points: the close point (where the switch activates as pressure rises) and the open point (where it deactivates as pressure falls). The difference between them is the dead band. For a standard mechanical switch, the dead band is about 5-10% of the range. For an electronic pressure switch, the dead band is adjustable down to 1%.
In the failed installation, the switch was set with a 2-bar dead band. It closed at 100 bar and opened at 98 bar. The system pressure cycled between 95 and 105 bar. The switch opened at 98 bar (within the fluctuation) and the alarm triggered. The dead band was too narrow for the pressure fluctuation.
Setting the dead band
The dead band should be wider than the normal pressure fluctuation. If the system fluctuates ±5 bar, the dead band must be at least 10 bar. The close point should be above the maximum fluctuation, and the open point below the minimum.
For the clamp circuit: set close at 105 bar (above the 105 bar peak) and open at 90 bar (below the 95 bar trough). The dead band is 15 bar. The switch stays closed through the fluctuation. It only opens if pressure genuinely drops below 90 bar. The chatter stops. The false alarm disappears.
The switch type comparison
| Type | Dead band | Repeatability | Use for |
|---|---|---|---|
| Mechanical piston | 5-10% of range | ±2% | General pressure confirmation |
| Diaphragm (adjustable) | 2-5% | ±1% | Clamp, safety circuits |
| Electronic (settable) | 0.5-2% | ±0.2% | Precision control, PID feedback |
| Pressure transducer (analog) | N/A (continuous) | ±0.1% | Closed-loop control, data logging |
The debounce delay
Even with a wide dead band, electrical chattering can occur at the set point. Most electronic switches have a built-in delay (100-500 ms). The output must remain in one state for the delay time before it changes. This filters out momentary pressure spikes. For mechanical switches, a relay with an RC snubber or a PLC timer (100 ms) debounces the signal. Without debounce, contact bounce creates multiple signals at the switch point.
The dead band rule: at least 2x the normal pressure fluctuation. The chattering switch wasn’t defective — it had a 2-bar dead band in a 10-bar fluctuating circuit. Set close above the peak, open below the trough. Add a 100 ms debounce. For critical circuits, use an electronic switch with adjustable dead band.
