Choose DPDT when the two circuits must be electrically isolated or carry different voltages; choose SPDT when a single alarm loop is all that is required and one common is acceptable.
Inside a DPDT pressure switch sit two independent SPDT contact sets, mechanically linked to a single sensing element, typically a diaphragm or piston driven by process pressure, so both throws change state at the same setpoint [S1][S2].
Contact architecture: one pole vs two poles under one housing
An SPDT pressure switch exposes three terminals: Common (C), Normally Open (NO), and Normally Closed (NC); the common terminal carries the only pole, and current is directed to either NO or NC at the trip point [S2]. A DPDT pressure switch doubles that to six terminals, 2C, 2NO, 2NC, and is, in effect, two SPDT switches operating in tandem from a common lever mechanism inside one housing [S1][S2]. The two SPDT circuits inside the DPDT are electrically isolated from each other, so each leg can carry its own voltage and current without cross-talk [S1].
This matters for pressure switch selection because the same setpoint can fire a 24 VDC PLC input on pole A and a 120 VAC relay coil on pole B without adding a second instrument or a separate interposing relay [S1][S3].
Decision matrix: SPDT vs DPDT on isolation, redundancy, wiring, cost
Four criteria tell you which one to specify:
1. Circuit isolation. DPDT keeps the two legs galvanically separate inside one body; SPDT shares a single common, so the two downstream devices must share a common reference, which rules out mixed-voltage or mixed-ground systems [S1][S6].
2. Failover / redundancy. DPDT supports a redundant signalling topology: if circuit A loses its signal path, circuit B is already wired to the standby channel, and the tandem lever guarantees both switch at the same pressure [S1]. SPDT cannot offer that without a second switch, and two separate switches will not share a setpoint as tightly as a single DPDT mechanism.
3. Wiring effort and footprint. DPDT replaces two SPDT switches plus a manifold bracket, dropping the panel cutout count from two to one and the wiring terminals from six to six but on a single device, which simplifies commissioning and documentation [S3][S4]. For a single alarm or single shut-down loop, SPDT is the lower-cost, lower-complexity answer.
4. Rating headroom. Both formats are routinely offered with contact blocks rated for high inrush currents to drive relays, motor contactors, and pump starters directly, so the electrical rating is not the deciding factor, the topology is [S1][S3].
Use cases that justify DPDT in process plants
Triple-redundant SIS and process-shutdown logic is the headline application: one DPDT pressure switch feeds a safety PLC on pole A and a separate annunciator or voter on pole B, both changing state at the same trip pressure because they share the same sensing element and lever [S1]. Pump control with permissive interlocks is another common case, where one pole starts a pump and the other pole proves the start command to a DCS, eliminating the need for a second pressure switch on the same line.
For more detailed transmitter-vs-switch trade-offs in the same loop, the piece on 0.04% vs 0.1% DP transmitter accuracy and price gap is a useful reference, since a DPDT switch is often paired with a lower-accuracy DP transmitter where the switch handles the trip and the transmitter handles trending.
When SPDT is the right (and only) answer
Single-loop alarm on a single voltage, single common, single PLC input: an SPDT pressure switch is the correct, lower-cost spec [S1][S2]. Wiring the NC contact for a normally-energised alarm and the NO contact for a normally-de-energised trip is a standard pattern that needs no second pole [S1]. Trying to use a DPDT here just adds four unused terminals and a larger housing for no engineering benefit.
For field verification of either topology, a hand pump paired with a pressure calibrator is the standard check; both SPDT and DPDT units show the same setpoint within the instrument's deadband, so the test procedure does not change with contact count.
Wiring pitfalls when a DPDT is misused as an SPDT
Treating the two commons as the same node is the most common wiring error: the two C terminals are independent poles, not a jumper pair, and tying them together defeats the isolation that justified the DPDT spec in the first place [S1][S6].
Second, the two internal SPDT sub-switches in a DPDT are not necessarily rated identically; if the data sheet lists different resistive and inductive ratings per pole, the lower-rated pole sets the system limit and must drive the heavier load [S1][S3].
Third, mechanical DPDT switches in outdoor or cold service benefit from stiffer contact springs and engineered contact materials, which is also the OEM guidance for SPDT service, since freeze-up of the mechanical internals is a known failure mode for both contact counts [S1].
What the data sheets and standards actually cover
Contact topology, SPDT, DPDT, SPST, DPST, is descriptive of the switch internals, not a standard number itself; the ratings that govern a real installation are the contact electrical rating (resistive and inductive amps at a stated voltage), the enclosure rating (NEMA 4/IP66 for washdown, NEMA 4X/IP66 for corrosive atmospheres), and the hazardous-area certification (Class I Div 1 / Div 2, ATEX 2014/34/EU, IECEx) when the switch sits in a classified area [S3]. Process-industry pressure switches with DPDT or SPDT contacts from major instrument vendors are commonly offered in Division 1, Division 2, and ATEX/IECEx variants, and the same contact topology is available across those certifications, so the explosion-protection method does not force a contact count [S3].
For background on how a pressure gauge or pressure transmitter pairs with a switch in a typical instrument loop, the encyclopedia entries cover the sensing and signal-chain side; the SPDT vs DPDT decision sits on the contact side of the same loop.
Field spec checklist before ordering
Confirm: required contact format (SPDT or DPDT), setpoint with deadband, proof pressure, wetted material compatible with the process, process connection size and type, electrical rating per pole (resistive and inductive), enclosure rating, hazardous-area certification, and ambient temperature range. A DPDT pressure switch that meets all of the above and uses two isolated SPDT sub-switches in one housing is the most flexible single-instrument answer for dual-circuit pressure control [S1][S3][S7].
Trackable next signals to watch: vendor datasheet revisions that publish per-pole electrical ratings separately for DPDT units, and the spread of IECEx-certified DPDT pressure switches for Zone 1 process service, both of which are decision-shaping when a dual-circuit safety function is being specced.