Technical Foundation
What Is a Pressure and Vacuum Switche?
A pressure or vacuum switch converts a mechanical pressure signal — sensed through a diaphragm, piston, or bourdon tube — into an electrical contact state change at a defined set point, used to protect equipment, sequence control logic, or signal an alarm condition. Unlike a pressure transducer, which outputs a continuous analog signal, a switch is a binary go/no-go device: it trips a set of contacts (typically SPDT) when pressure crosses the set point, with deadband (differential) separating the trip point from the reset point to prevent contact chatter near the threshold.
Adjustable switches allow field-setting the trip point within a rated range via a calibration screw, while fixed switches are factory-set for a single application like a specific compressor cut-out pressure. Differential pressure switches sense the pressure drop across two points — commonly used for filter clog monitoring — rather than a single absolute pressure, and wetted-part material must be compatible with the process media, since a switch rated for air or water isn't automatically suitable for hydraulic fluid, fuel, or corrosive process chemicals.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Set Point Range | e.g. 20-200 PSI adjustable, or fixed 145 PSI | Switch must be selected so the target trip point falls within its rated adjustment span |
| Deadband/Differential | Gap between trip and reset pressure | Too narrow a deadband causes contact chatter near the set point; too wide delays needed reset response |
| Switch Action | SPDT, DPDT; increasing or decreasing set point action | Determines whether the switch trips rising or falling pressure, and how many circuits it controls |
| Wetted Material | Brass, stainless steel, Buna-N diaphragm | Must be chemically compatible with the process media — mismatch causes corrosion or seal failure |
| Process Connection | 1/4 NPT, 1/8 NPT common sizes | Must match the port thread on the equipment being monitored |
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Applications
Where Pressure and Vacuum Switches Get Used
Pressure and vacuum switches are specified wherever equipment needs to respond automatically to a pressure threshold being crossed.
Air Compressor Control
Pressure switches cycling compressor motors on and off between a cut-in and cut-out pressure to maintain tank pressure.
Hydraulic Systems
High-pressure switches protecting pumps and cylinders by signaling overpressure or confirming system pressure before a sequence proceeds.
HVAC & Air Handling
Differential pressure switches monitoring filter loading and proving airflow across fans and dampers before burner ignition.
Water & Wastewater Systems
Pressure switches controlling well pump cut-in/cut-out cycling and low-pressure alarm signaling.
Vacuum Process Equipment
Vacuum switches confirming chuck or fixture hold-down pressure before allowing a machine cycle to start.
Refrigeration Systems
High and low pressure switches protecting compressors from operating outside safe refrigerant pressure limits.
FAQ
Pressure and Vacuum Switches Questions, Answered
What's the difference between a pressure switch and a pressure transducer?
A pressure switch is a binary device — it trips a mechanical contact open or closed when pressure crosses a set threshold, giving a simple on/off control signal. A pressure transducer outputs a continuous analog signal (typically 4-20mA or 0-5V) proportional to pressure across its full range, allowing a controller to read the actual pressure value rather than just a threshold crossing. Use a switch for simple go/no-go control or protection; use a transducer where the actual pressure value needs to be monitored or logged.
What does deadband or differential mean on a pressure switch?
Deadband is the gap between the pressure at which the switch trips and the pressure at which it resets — for example, a compressor switch might cut out at 150 PSI and not cut back in until pressure drops to 125 PSI. This prevents rapid on/off cycling (chatter) right at the set point, which would otherwise cause excessive wear on the switch contacts and the equipment it controls.
Can I use a standard air pressure switch on a hydraulic system?
Not generally — hydraulic systems run at much higher pressures than compressed air (often 1,500-5,000+ PSI vs. 100-200 PSI) and use fluid that may not be compatible with a switch's diaphragm and seal materials. Always select a switch explicitly rated for the pressure range and wetted-material compatibility of the specific hydraulic fluid in use.
Why would I need a differential pressure switch instead of a single-point switch?
A differential switch senses the pressure difference between two points rather than an absolute value, which is the correct approach for monitoring conditions like filter loading — where the absolute pressure varies with system demand, but the pressure drop across the filter is what actually indicates it needs replacement. A single-point switch can't distinguish a dirty filter from normal system pressure variation.
How do I know if a pressure switch is rated for increasing or decreasing pressure action?
Switches are typically specified as either normally closed opening on pressure rise (used for high-pressure cutoff/protection) or normally open closing on pressure rise (used to signal 'pressure achieved,' like a compressor pressure-proving switch). Check the manufacturer's datasheet for the specific action — installing a switch with the wrong action wired into a safety circuit can leave the protected condition unmonitored.
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