A digital pressure or vacuum switch can display a plausible value while its output logic, unit, setpoint or hysteresis is wrong for the machine sequence. Replacing or resetting the device without a parameter record can therefore create intermittent stops, delayed alarms or a false ready signal.
The correct values are configuration-dependent. This guide explains how to reconstruct and verify the switching function without inventing a universal pressure threshold. Work from the installed model manual, electrical and pneumatic drawings, PLC logic and measured cycle demand.
Decision matrix
| Parameter | Evidence | Why it matters |
|---|---|---|
| Measurement range | Complete model code and rated range | A similar display may use a different sensor range |
| Unit and display | Configured unit and decimal position | Prevents a numerically plausible but wrong interpretation |
| Output circuit | PNP or NPN, NO or NC, output 1 or 2 | Must match PLC input wiring and logic |
| Setpoint and hysteresis | P value, H value or window limits | Controls switching stability near the threshold |
| Response behavior | Filter, delay and peak/bottom hold settings | Can change when the PLC sees the event |
Define what the switch is proving
Identify whether the device confirms plant air availability, vacuum pickup, filter restriction or a station-specific process. Trace its port to the actual measurement point and its output wire to the PLC input. Record the input state during a known safe stopped condition.
Do not adjust the switch to conceal low supply, leakage or a blocked line. Compare the displayed value with a suitable independent instrument at a defined point. A pressure value upstream of a regulator or restriction does not prove what is available at the actuator or suction cup.
Capture the full configuration
Photograph every display page and record the complete part number before changing settings. SMC manuals show that supported output modes, units, response filters and set procedures vary across switch families. Use the manual for the exact suffix, not a similar front panel.
Record output type, normally open or normally closed behavior, setpoint, hysteresis or window limits, response time, display color rules, zero-clear state and any copy or security setting. Note whether the PLC expects a rising transition, falling transition or a maintained ready state.
Check wiring and pneumatic installation
Confirm supply voltage, common wiring, PNP or NPN compatibility and the output channel used. Inspect the connector pinout rather than relying on wire color. Electrical testing should follow safe isolation and qualified-person requirements.
Inspect tubing direction, port seal, contamination and mechanical stress on the sensor body. For vacuum service, confirm that the device range is appropriate for the negative-pressure measurement and that the port is not filled with dust or liquid. Zero adjustment must be made only under the conditions specified by the manufacturer.
Set thresholds from process evidence
Collect stable-machine data across startup, normal cycles and controlled stopping. Define a safe operating band with the machine designer or authorized technician, then choose setpoint and hysteresis that detect a real loss without chattering on normal ripple. Do not copy a threshold from an unrelated machine.
Where the switch supports window mode or separate outputs, document which function is active. Confirm alarm delay and PLC timer behavior together: two delays can hide a fault longer than intended, while no filtering can create nuisance trips. Safety-related pressure monitoring requires a validated safety design, not an ordinary process switch.
Validate transitions in the real sequence
With guards restored, observe the physical pressure or vacuum, switch indicator, PLC input and HMI message at the same time. Test the permitted transition into and out of the ready state and confirm that loss of the signal produces the intended controlled response.
Record the final configuration, independent gauge reference, test conditions and approver. Seal or password-protect settings only under the site control process. Use the linked demand test when the switch is correctly configured but available air still falls during production.
Technical references
- SMC digital pressure switch manuals — general component guidance; the installed machine documentation remains controlling.
Related machine and guide
Review the HN-180 high-speed paper cup machine for machine context, then use the compressed-air demand test guide for the adjacent check. Browse all Buyer Guides when building a wider project checklist.
Frequently asked questions
Is there one correct air-pressure setpoint for every paper cup machine?
No. It depends on machine design, measurement point, components and validated process demand.
Can the displayed value prove the output is correct?
No. Verify output type, channel, logic, threshold, hysteresis and the PLC input transition.
Why does a pressure input chatter?
Investigate real pressure ripple, leakage, tubing, response filtering and hysteresis before changing the threshold.
Should a replacement switch use the old numeric settings?
Only after confirming the replacement range, units, mode and scaling match the original approved function.
Review your machine configuration with HANNAI
Send the machine model, project reference, component labels, drawings, photos and a description of the required result. We will clarify which details must be confirmed before a quotation or technical recommendation.