The Pressure Switch That Chattered
We used a pressure switch to detect when the FRL regulator output was at 6 bar. The switch turned on a green “air ready” light. On the bench, it worked. On the floor, the light chattered (flickered) — the switch toggled on and off rapidly. The problem: the switch’s setpoint was 6 bar, and the system pressure fluctuated (5.8 to 6.2 bar) as other machines cycled. The switch toggled at 6 bar every time the pressure crossed. The switch had no hysteresis (or it was set too tight). We adjusted the hysteresis (deadband) to 0.5 bar: the switch turns on at 6 bar and off at 5.5 bar. The chattering stopped. The mistake was setting the switch right at the fluctuating pressure, with no deadband.
Pressure switch selection and setup is about the setpoint and hysteresis. Set it wrong, and it chatters. This article covers the choices.
What a Pressure Switch Does
A pressure switch has a setpoint. When the pressure crosses it, the switch changes state (on/off). It’s used for:
- Air ready: Detect that the FRL pressure is up (before starting the machine).
- Vacuum OK: Detect that the vacuum gripper has vacuum (article 113).
- Overpressure alarm: Detect a blocked filter (pressure rises across it).
- Loss of pressure: Detect a leak (pressure drops).
Step 1: Setpoint and Range
The switch must be adjustable over the expected pressure range. For an air system at 6 bar, pick a switch with a range of 0–10 bar (so 6 bar is in the middle). Don’t pick a switch with a 0–2 bar range (it can’t reach 6 bar) or a 0–100 bar range (6 bar is at the bottom, poor resolution).
For a vacuum gripper (-0.8 bar), pick a vacuum switch (range -1 to 0 bar).
Step 2: Hysteresis (Deadband)
The hysteresis (deadband) is the difference between the turn-on and turn-off points. Without it, the switch chatters at the setpoint.
- Adjustable hysteresis: Set the deadband (e.g., 0.5 bar). The switch turns on at 6 bar, off at 5.5 bar. Pressure fluctuates between 5.8 and 6.2 — the switch stays on (no chattering).
- Fixed hysteresis: The switch has a built-in deadband (e.g., 3% of range). Simpler, but not adjustable.
Always set the hysteresis wider than the expected pressure fluctuation. If the pressure fluctuates ±0.2 bar, set the deadband to 0.5 bar.
The pressure switch rule: Set the setpoint in the middle of the switch’s range, and set the hysteresis (deadband) wider than the expected pressure fluctuation. The switch that chattered was at 6 bar with no deadband, and the pressure fluctuated ±0.2 bar. Set the deadband to 0.5 bar.
Step 3: Switch Type
- Mechanical pressure switch: A diaphragm or piston actuates a microswitch. Simple, cheap. But the contacts wear (mechanical life). Good for infrequent switching.
- Electronic (solid-state) pressure switch: A pressure transducer with a digital output (PNP/NPN). No moving parts. Adjustable setpoint and hysteresis via a keypad. Standard for modern machines.
- Pressure sensor (analog): Continuous output (4–20 mA or 0–10 V). Read by the PLC. The PLC sets the setpoint and hysteresis in software. Most flexible.
| Type | Adjustability | Best For |
|---|---|---|
| Mechanical switch | Fixed (or adjustable screw) | Simple, infrequent switching, low cost |
| Electronic switch (PNP/NPN) | Adjustable setpoint and hysteresis | Standard machine, repeatable |
| Pressure sensor (analog) | Setpoint in PLC software | Continuous monitoring, data logging |
Step 4: Wet vs Dry Contacts
A mechanical switch has wet (voltage-carrying) contacts. They switch the load directly (a light, a relay coil). But the contacts are rated for a certain current (e.g., 5 A). Don’t exceed it (the contacts weld).
An electronic switch has a solid-state output (PNP/NPN, 24 V, 100 mA). It goes to a PLC input. Don’t drive a heavy load directly (use an intermediate relay).
Step 5: Mounting and Protection
The pressure switch mounts on the air line (a tee). For pulsating pressure (near a pump or cylinder), add a damping restriction (a needle valve) or a small reservoir to smooth the pulsation. Without damping, the switch chatters from pressure spikes.
For a vacuum switch (article 113), mount it near the gripper (not at the generator) so it senses the actual gripper vacuum.
A Pressure Switch Checklist
- What pressure range? (Air, vacuum, overpressure?)
- Is the setpoint in the middle of the switch range?
- What is the expected pressure fluctuation? (±bar?)
- Is the hysteresis wider than the fluctuation?
- Mechanical or electronic switch?
- Is the output compatible? (PLC input or direct load?)
- Is there damping for pulsating pressure?
- Is the switch mounted at the right point? (Near the gripper?)
- Is the IP rating correct? (Dust, oil?)
- Is the setpoint labeled? (For maintenance?)
The Bottom Line
Pressure switch selection and setup is setpoint in range plus hysteresis. The switch that chattered was at 6 bar with no deadband, and the pressure fluctuated. Use an electronic switch (adjustable setpoint and hysteresis), set the deadband wider than the pressure fluctuation, and add damping for pulsation. The switch that stays put wasn’t the most sensitive one — it had the right deadband.
