Key Takeaway
Reading a pressure switch means understanding when and how it activates. Most switches have labels that tell you the cut-in and cut-out pressure settings. For example, a “30/50 psi” switch turns ON at 30 psi and OFF at 50 psi. Some switches have an adjustment screw for tuning. The switch itself doesn’t show live pressure — you need a separate pressure gauge nearby for that. To test how it works, watch the pressure gauge while running the system. If the switch turns the pump or compressor ON and OFF at the expected values, it’s working correctly. Also, listen for a clicking sound — that means the switch is triggering. If pressure passes the set limit and the switch does nothing, it may be faulty. So, you don’t read the pressure switch directly — you read its action in response to pressure. Always refer to the label and test against a gauge.
Setpoint Labeling and Markings
Every pressure switch includes labels that indicate its operating range and factory-set pressure setpoints. These are typically displayed in PSI, bar, or MPa. You may also see symbols for NO (Normally Open), NC (Normally Closed), and terminal connections. Understanding these markings is vital for proper setup and safety. For adjustable switches, you’ll find knobs or screws for fine-tuning the cut-in and cut-out values. Misreading a label can lead to incorrect installation, which may cause short cycling or unsafe pressure levels. Always refer to the datasheet or user manual when configuring or replacing a switch. Clear interpretation ensures proper performance, accurate switching, and long-term reliability.
Range Indicators: PSI/Bar/MPa
“Units matter. Confuse them, and you’re asking for trouble.”
Pressure switches often use three main units: PSI (pounds per square inch), Bar, and MPa (megapascal).
You’ll often see something like: “Range: 0.2 – 8.0 bar”. This is the full operating range of the switch. Never try to set a switch beyond this range, or you’ll either damage it or it won’t function correctly.
Some switches show dual units. Like: “0.5 – 6.0 bar / 7.2 – 87 psi”. That’s helpful when working in multinational environments or using imported components.
If you’re setting or adjusting a switch, make sure your tool (gauge, meter, or software) matches the unit shown on the pressure switch. Converting wrongly between bar and psi can cause delays, safety risks, or equipment damage.
Quick Conversion:
1 bar = 14.5 psi
1 MPa = 10 bar = 145 psi
MPa is mostly used in hydraulic systems where pressures go really high. For water pumps, compressors, or HVAC systems, you’ll mostly deal with Bar or PSI.
Also, be aware of tolerance – if the label says “±0.1 bar”, then it’s not going to trigger exactly at 5.0 bar. It might do so at 4.9 or 5.1 depending on conditions.
Reading the range isn’t just about limits. It tells you where the switch is most accurate and what application it was designed for.
Understanding NO and NC Contacts
“NO or NC? Know this before wiring anything.”
Every pressure switch has contacts that either open or close depending on pressure. And these are marked as:
NO (Normally Open)
NC (Normally Closed)
Let me simplify this:
Normally Open (NO): The circuit is open when the switch is at rest (no pressure or below setpoint). It closes (conducts) when pressure goes above the setpoint.
Normally Closed (NC): The circuit is closed at rest. It opens when the pressure goes above the setpoint.
You can think of NO as a trigger and NC as a stop signal.
This is critical when connecting it to alarms, contactors, or PLCs. Get it wrong, and your motor might start when it should stop!
You’ll find these labeled near terminals or in the wiring diagram. Sometimes you’ll see both NO and NC in one switch, especially in SPDT (Single Pole Double Throw) configurations. That gives you more flexibility.
Golden Rule:
Always test the contact behavior with no pressure applied. A multimeter in continuity mode will help (more on that next).
And if it’s a mechanical switch, a little pressure from your finger might be enough to feel the click or change in contact.
Never assume – always verify. Because miswiring one relay could stop an entire machine line.
Using a Multimeter to Confirm Function
“Your best friend? A basic multimeter.”
Even if the label tells you NO or NC, don’t trust it blindly. Use a multimeter to double-check contact functionality.
Set your multimeter to continuity mode (the one that beeps when there’s a closed path). Connect the two leads to the terminals marked NO and COM, or NC and COM.
Here’s how to read it:
If you’re testing NO + COM: You should hear a beep (continuity) only when pressure is applied above the setpoint.
For NC + COM: You should hear a beep when no pressure is applied.
If it beeps under the wrong condition, the switch might be wired differently or may be faulty.
Also, for adjustable switches, try gently increasing pressure manually (like via a hand pump or by activating the line) and listen for the click.
A multimeter helps you detect:
Wrong terminal wiring
Contact failure
Stuck actuator
Identifying Faults via Label Readings
“The switch won’t work? The label knows why.”
You can often spot faults by going back to the basics – reading the label properly.
First, check the voltage and current ratings. A pressure switch designed for 24VDC control cannot handle 230VAC motor loads directly. It will either burn out or behave unpredictably.
Next, check the media compatibility. Some switches are labeled for air, others for liquid, steam, or refrigerant. If used with the wrong medium, the diaphragm or sensing element can corrode or rupture.
Many times, engineers misread the differential range. Let’s say the switch is designed for a setpoint of 6 bar and a differential of 2 bar. That means it resets only at 4 bar. If your system doesn’t drop that low, the switch never resets. And then you’ll think it’s faulty, when it’s actually working fine.
Look for temperature ratings too. A switch labeled “-10 to 60°C” will fail or drift outside that window. High temp steam applications require specialized switches with extended ranges.
Lastly, don’t ignore manufacturing date codes. A switch sitting unused for 10 years may have hardened diaphragms or oxidized contacts.
Reading faults isn’t always about testing. Sometimes, the label itself explains the problem.
Conclusion
To use a pressure switch confidently, you must know how to read its labeling and settings. Start by locating the setpoint range, often written in PSI, bar, or MPa. This tells you what pressure levels the switch is designed to operate within. Next, identify the NO and NC terminals—these show how the switch will behave under pressure. Some switches include adjustable dials or screws with markers. Others may use charts to explain how turns translate to pressure changes. If you see unclear markings or suspect calibration drift, use a pressure gauge and multimeter to verify actual switching points. Accurate reading is especially critical when replacing a switch or integrating it into a new system. Misreading a switch can lead to short cycling, underperformance, or complete system failure. Taking time to understand your switch’s specs ensures consistent operation, better safety, and fewer surprises down the line.