How to Choose a Microswitch for a Pressure Switch Application

How Microswitches Work Inside Pressure Switches

A pressure switch microswitch is the internal electrical switching element that changes state when the pressure switch mechanism reaches its operating point. The pressure-sensing portion of the instrument may use a diaphragm, piston, bellows, bourdon element, or another mechanical actuator. As pressure rises or falls, that sensing element moves. At the setpoint, the motion is transferred through a lever, spring, or linkage to the microswitch actuator. The microswitch then opens or closes the electrical contacts connected to the external circuit.

The microswitch is therefore not the pressure sensor itself. It is the electrical contact block inside the pressure switch assembly. Choosing it correctly means matching the contact arrangement, electrical rating, contact material, reset behavior, and environmental suitability to the circuit that the pressure switch will control.

A common contact arrangement is SPDT, or single-pole double-throw. An SPDT microswitch has one common terminal and two switched contacts: normally open and normally closed. In the unactuated state, the common terminal is connected to one contact. When the pressure switch mechanism actuates the microswitch, the common transfers to the other contact. This allows one pressure event to perform either of two functions: close a circuit that was open, or open a circuit that was closed.

For example, an SPDT pressure switch microswitch may be wired so that a pump stops when pressure reaches a high limit, or so that an alarm circuit energizes when pressure falls below a low limit. The same contact block can support either logic, depending on which terminals are used.

A DPDT arrangement, or double-pole double-throw, behaves like two SPDT switching elements actuated at the same time by the same mechanical event. Each pole has its own common, normally open, and normally closed contacts. The two circuits remain electrically separate, but they change state together when the pressure setpoint is reached.

DPDT behavior is useful when one pressure condition must control two independent circuits or loads. One pole might signal a control system input, while the other pole switches a relay, lamp, shutdown circuit, or redundant indication circuit. The key point is not the number of terminals alone, but the required circuit function: whether the pressure switch needs to switch one circuit, transfer one circuit between two states, or control two isolated circuits from the same pressure event.

When selecting the microswitch configuration, begin with the control logic:

In many industrial pressure switches, the microswitch cannot be selected in isolation from the full instrument. The available contact forms may depend on the pressure switch series, enclosure, hazardous-area rating, environmental sealing, temperature option, and reset style. Selection should therefore start with the electrical circuit requirement, then be checked against the pressure switch manufacturer’s available configurations.

Electrical Ratings for Pressure-Switch Microswitches

Electrical rating is one of the most important selection criteria for a pressure switch microswitch. A microswitch that has the correct contact form can still fail prematurely if it is asked to switch the wrong voltage, excessive current, or an unsuitable load type.

The rating printed in a catalog or data sheet is not a general permission to use the switch in any circuit below a single current number. Ratings are usually tied to specific conditions, such as AC or DC voltage, resistive or inductive load, and sometimes lamp, motor, pilot-duty, or control-circuit service. A microswitch suitable for one kind of circuit is not automatically suitable for another.

AC and DC switching are different because the arc formed when contacts open behaves differently. In AC circuits, the current crosses zero repeatedly, which helps extinguish the arc. In DC circuits, there is no natural current zero crossing, so the arc can persist longer. For this reason, a microswitch may have a lower DC rating than AC rating at comparable voltages, or it may require a different contact design for reliable DC interruption. The data sheet must be checked for the specific voltage and current in the actual circuit.

Load type also matters. A resistive load, such as a simple heater or resistor bank, is usually easier to switch than an inductive load. Motors, pumps, solenoid valves, relay coils, contactor coils, and transformers can create inrush current, stored magnetic energy, and voltage transients when switched. Lamps may also impose high inrush current when cold. Alarms, beacons, and control inputs vary widely depending on their internal electronics. A pressure switch used only to signal a PLC input may carry very little current, while one used to directly interrupt a pump motor circuit may face much more severe switching duty.

Overloading a microswitch can cause several failure modes. Contacts may burn, erode, or become contaminated by arcing products. A contact may burn open, leaving the circuit unable to complete when the pressure switch actuates. In other cases, contacts may weld closed, leaving the controlled circuit energized even after the pressure condition changes. Both conditions can create troubleshooting problems and may create safety issues if the pressure switch is part of an alarm or shutdown function.

A practical selection process should include at least these checks:

• Confirm whether the circuit is AC or DC.
• Identify the normal operating voltage and maximum possible voltage.
• Determine the steady-state current and any inrush or transient current.
• Classify the load as resistive, inductive, lamp, motor, control input, or another defined load type.
• Check whether the microswitch rating applies to the exact load type, not just to a similar voltage.
• Consider whether the pressure switch will switch the load directly or only drive an interposing relay or control input.
• Verify the final selection against the microswitch data sheet and the pressure switch manufacturer’s documentation.

For demanding loads, the safer design may be to let the pressure switch microswitch operate a relay, contactor, or control input rather than switching the high-energy load directly. This does not eliminate the need for rating checks, but it can reduce the electrical stress on the microswitch if the control circuit is properly designed.

Pressure Switch Deadband and Reset Behavior

Deadband is the pressure difference between the actuation setpoint and the reset or deactivation point. If a pressure switch actuates at one pressure and then resets only after pressure changes by a certain amount in the opposite direction, that difference is the deadband. It is commonly expressed as a pressure value, such as psi, bar, or kPa, depending on the instrument documentation.

Deadband prevents the switch from rapidly cycling when pressure is close to the setpoint. Without sufficient separation between actuation and reset, a small pressure fluctuation or vibration could make the microswitch chatter. Chatter can shorten contact life, create unstable control behavior, and cause nuisance alarms or repeated starting and stopping of equipment.

The pressure deadband is linked to mechanical movement inside the pressure switch. The pressure-sensing element must move far enough to operate the microswitch actuator. The microswitch plunger or lever also has its own pretravel, operating point, overtravel, and release point. Springs, linkages, and adjustment mechanisms in the pressure switch translate pressure change into that mechanical travel. As a result, the final deadband is a property of the complete pressure switch mechanism, not only of the microswitch contact block.

This distinction is important when choosing or servicing a pressure switch. Replacing a microswitch with a part that looks similar but has different actuator travel or operating force can affect switching behavior. Likewise, adjusting the pressure setpoint may influence the reset point depending on the design of the instrument.

Some pressure switches are factory set. In these designs, the manufacturer establishes the setpoint, deadband, or both during production or calibration. Other pressure switches allow end-user adjustment of the setpoint, and some models may allow deadband adjustment within specified limits. The available adjustment depends on the exact model and construction.

Before changing a setpoint or deadband, verify that field adjustment is permitted. The manufacturer’s instructions should state whether the adjustment is allowed, which mechanism is used, what tools or procedures are required, and whether recalibration or functional testing is needed afterward. In safety-related service, adjustment may also be governed by site procedures, regulatory requirements, or the design basis of the control system.

Reset behavior is another selection factor. Many pressure switches reset automatically when pressure returns past the reset point. Others use a manual-reset function, so the switch remains tripped until an operator physically resets it. Automatic reset is suitable for many control and indication applications. Manual reset is useful where equipment should remain shut down until the cause of the abnormal pressure condition has been investigated.

Additional Factors When Selecting a Microswitch

Electrical rating and contact configuration are essential, but they are not the only considerations. A pressure switch microswitch operates inside a mechanical assembly that may be exposed to vibration, temperature extremes, moisture, corrosive atmospheres, hazardous locations, or frequent cycling. The microswitch option should fit the whole operating environment.

Environmental exposure is a major factor. In clean indoor control panels, a standard microswitch may be adequate if the enclosure protects it from dust, moisture, and chemical vapors. In corrosive or challenging atmospheres, a hermetically sealed microswitch may be considered. Hermetic sealing can help isolate the contacts and internal mechanism from external gases or contaminants. This can be relevant in chemical processing, marine service, offshore equipment, or other locations where the surrounding atmosphere may attack exposed materials.

Hermetically sealed versions are not automatically the best choice for every application. Depending on the product line, a hermetically sealed microswitch may have different electrical ratings than a non-hermetic or heavy-duty alternative. In some cases, the sealed option may carry a lower current rating. This creates a trade-off: environmental protection may improve suitability for corrosive atmospheres, while electrical load requirements may point toward a heavier-duty contact block. The correct choice depends on the combined environmental and electrical requirements.

High-temperature service also affects selection. Elevated ambient temperature, process temperature conducted through the pressure connection, and heat from nearby equipment can all influence the switch assembly. Some pressure switch lines offer high-temperature microswitch options, special wiring, or separation features, but the allowable limits must be taken from the relevant manufacturer documentation. It is not enough to assume that a microswitch will survive because the pressure element is rated for the process. The electrical contacts, actuator materials, insulation, terminals, and enclosure components all have temperature limits.

For demanding electrical loads, heavy-duty AC/DC microswitch options may be required. These may be intended for higher current, tougher switching duty, or broader circuit compatibility, depending on the manufacturer’s offering. The selection should still distinguish between AC and DC service and between resistive and inductive loads. “Heavy-duty” is a useful description only when the published ratings match the actual circuit.

Manual-reset microswitches or manual-reset pressure switch mechanisms are appropriate where automatic restart would be undesirable. In a safety-shutdown application, for instance, the goal may be to keep a burner, compressor, pump, hydraulic system, or other equipment shut down until an operator verifies the condition and corrects the fault. Manual reset does not by itself make a system safe; it must be applied within a suitable control design. However, it can support operating procedures that require investigation before restart.

Other selection details may include terminal type, wiring space, enclosure rating, mechanical life, expected switching frequency, approvals, hazardous-area classification, and compatibility with the pressure switch model. These factors should be reviewed as part of the complete pressure switch assembly rather than treated as afterthoughts.

Gold Contacts Versus Silver Contacts in Microswitches

Contact material should be selected according to circuit energy, voltage, current, and signal reliability requirements. In pressure switch microswitches, silver and gold contacts are common options, but they serve different needs.

Silver contacts are widely used in standard microswitch configurations for many general switching duties. Silver and silver-alloy contacts are typically suited to circuits with enough voltage and current to maintain a reliable contact interface during switching. They are often used where the pressure switch controls relays, lamps, alarms, solenoids, or other conventional industrial loads, provided the published rating matches the circuit.

Gold contacts are commonly preferred for low-voltage, low-current, or low-energy signal circuits. Gold resists oxidation and can provide more reliable signal continuity where the circuit energy is too low to break through films or contamination that may form on other contact materials. This makes gold contacts important in instrumentation and control applications where the pressure switch is not switching a power load but instead providing a status signal.

PLC input circuits are a common example. Many PLC digital inputs draw very small current compared with traditional relay coils or pilot lights. If the input signal level is too low, silver contacts may not always provide the most reliable long-term signal behavior. In such cases, a gold-contact microswitch may be recommended by the pressure switch or microswitch manufacturer.

The selection is not simply “gold is better” or “silver is stronger.” Gold contacts used for low-energy signals may not be intended for higher-power switching. If a gold-contact microswitch is used above its intended rating, the gold layer may be damaged, and the switch may no longer provide the low-level reliability for which it was selected. Silver contacts may be more appropriate for higher-energy circuits, but they may be less suitable for dry-circuit or very low-energy signals.

A useful way to think about the choice is:

Avoid applying universal voltage or current thresholds unless they come from the relevant manufacturer’s chart or data sheet. Different microswitch families use different contact materials, plating thicknesses, spring forces, and ratings. The correct contact material should be chosen from the published guidance for the specific pressure switch microswitch option.

Microswitch Options for Industrial Pressure Switches

Industrial pressure switch product lines often offer multiple microswitch options because one contact block cannot serve every application equally well. A pressure switch used for a clean control-room signal has different requirements from one installed on offshore equipment, in a chemical plant, or near high-temperature process piping.

Available options may vary by pressure switch series. One series may offer SPDT and DPDT contact forms, gold-contact versions, hermetically sealed microswitches, high-temperature options, manual-reset mechanisms, or heavy-duty AC/DC switches. Another series may offer only a smaller set of choices. This is why selection should begin with the complete instrument specification, not just the desired microswitch.

Typical industrial environments where pressure switches are used include offshore platforms, chemical processing plants, pulp and paper facilities, steel production, power generation, and water or wastewater treatment systems. These settings can involve vibration, moisture, corrosive vapors, washdown, outdoor exposure, temperature variation, and electrically noisy loads. In some locations, hazardous-area requirements may also apply. The microswitch, enclosure, wiring method, process connection, and approvals must be suitable for the installation.

Corrosive or hazardous service conditions may require special switch, enclosure, or sealing choices. A hermetically sealed microswitch may help protect the internal contact mechanism from certain atmospheres, while a suitable enclosure may protect against external ingress. Hazardous locations may require certified enclosures, sealing fittings, intrinsically safe circuits, explosionproof construction, or other protection methods depending on the classification and local code requirements. Specific suitability must always be verified in the manufacturer documentation and the applicable installation standards.

For technical buyers, the best approach is to define the application in terms of operating requirements:

• Pressure range, setpoint, and reset behavior.
• Whether automatic or manual reset is required.
• SPDT, DPDT, or other contact logic.
• AC or DC circuit type.
• Voltage, current, inrush, and load category.
• Low-energy signal needs, including PLC or DCS inputs.
• Contact material requirement, such as gold or silver.
• Environmental exposure, including moisture, corrosion, and dust.
• Temperature conditions around the switch body and wiring.
• Enclosure and approval requirements.
• Expected switching frequency and service criticality.

The pressure switch microswitch should be selected as part of the full pressure switch assembly. The sensing element determines when the pressure event occurs. The adjustment mechanism determines the setpoint and deadband behavior. The enclosure and sealing determine environmental suitability. The microswitch determines how the pressure event is translated into an electrical signal or control action. Reliable selection requires all of these parts to be compatible with the process, the electrical circuit, and the operating environment.