Reading the Pressure Switch Spec Sheet: Beyond the Setpoint
A pressure switch on an FRL regulator. Setpoint: 6 bar. The switch was supposed to signal the PLC when the system reached operating pressure. Instead, the output was chattering on and off — the PLC saw a rapid pulse train, not a steady signal. The customer’s machine wouldn’t start. The setpoint was right. The problem was in the spec sheet, not the switch.
The two numbers nobody reads
Every pressure switch has two critical specs beyond the setpoint:
Hysteresis (dead band). The difference between the actuation pressure and the release pressure. A switch set at 6 bar with a 0.5 bar hysteresis turns on at 6.0 bar and off at 5.5 bar. That’s intentional — it prevents chattering. But if the system pressure fluctuates between 5.6 and 6.1 bar (which it does when the compressor cycles), the switch bounces on and off through the hysteresis band.
Repeat accuracy. The switch’s setpoint drifts with temperature and cycle count. A ±0.1 bar accuracy means the switch might actuate at 5.9 or 6.1 bar depending on the day. Combined with hysteresis, the actual actuation band can be 5.5–6.2 bar.
What the chatter actually was
The system pressure cycled: the compressor filled to 7 bar, the regulator held 6 bar, then a cylinder actuated and the pressure dropped to 5.7 bar, then the compressor kicked back in. The pressure switch was right in the middle of this fluctuation. It turned on at 6.0, off at 5.5 (hysteresis), on again at 6.0, off at 5.5. The PLC counted 12 pulses in 10 seconds. It interpreted this as a fault and wouldn’t start the machine.
The fix: two options
Option A: Widen the hysteresis. If the switch has an adjustable hysteresis (some electronic switches do), set it to 1.0 bar. The switch turns on at 6.0 bar and off at 5.0 bar. The system pressure never drops below 5.5 bar, so the switch stays on. No chatter. This is the right fix for a “system ready” signal — you don’t need precision, you need stability.
Option B: Move the setpoint. If the switch has fixed hysteresis (mechanical diaphragm type), set the setpoint below the operating pressure. For example, set it at 4.0 bar. It turns on when the system reaches 4.0 bar and off at 3.5 bar. The system always runs above 5.5 bar, so the switch stays on. The setpoint isn’t measuring “6 bar” anymore — it’s measuring “pressure is above 4 bar, which is enough to run.”
The spec sheet table
| Spec | What it means | Why it matters |
|---|---|---|
| Setpoint range | The pressure at which it switches | Pick within range, not at the edge |
| Hysteresis | On/off pressure difference | Too narrow = chatter; too wide = late signal |
| Repeat accuracy | Day-to-day variation | Budget this around the setpoint |
| Max pressure | Overpressure rating | Don’t exceed (water hammer kills diaphragms) |
| Switching frequency | Max cycles/sec | Mechanical switches wear out at high frequency |
| Electrical life | Cycles before contact wear | Pick electronic for long life |
Mechanical vs electronic
Mechanical pressure switch (diaphragm or piston). Cheap, simple, but hysteresis is fixed (typically 10–20% of setpoint), and the contacts wear after 10⁵–10⁶ cycles. For a signal that switches a few times per shift, it’s fine. For a system that cycles 10 times per minute, the contacts will weld or burn out in months.
Electronic pressure switch (piezoresistive sensor + solid-state output). Adjustable hysteresis, 10⁸+ cycle life, and often has a display for reading the actual pressure. More expensive (3–5× the mechanical), but it’s the right choice for a “system ready” signal that needs to be stable.
We replaced the mechanical switch with an electronic one, set the hysteresis to 1.0 bar, and the chatter stopped. The PLC saw a steady “pressure ready” signal. The machine started on the first press.
The selection rule: If the switch signal needs to be stable (machine interlock, system ready), use an electronic switch with adjustable hysteresis. Set the hysteresis wider than the normal pressure fluctuation. If the signal is a simple high/low alarm (not an interlock), a mechanical switch is fine.
