Pressure switch

01

Pick the sensing element for cycle life and surges

Barksdale's pressure switch selection guide makes service life the first consideration: for one million cycles or fewer a bourdon tube or diaphragm switch fits, while above one million a piston switch should be used, except that where the pressure change is 20% or less of the adjustable range a bourdon tube or diaphragm switch can reach 2.5 million cycles before metal fatigue. A switch cycling more than once every three seconds should be a piston type, because a bourdon tube or diaphragm acts as a spring that heats and fatigues. [1]

Barksdale defines proof pressure as the highest surge the system will ever see and warns that a gauge's orifice can dampen surges that still reach a sensitive diaphragm or bourdon switch. Its diaphragm switches adjust from vacuum to 150 psi with proof pressure to 300 psi, bourdon tube switches to 18,000 psi with proof to 24,000 psi, and piston switches to 12,000 psi with proof to 20,000 psi. For greatest accuracy the set point should be in the upper 65% of the adjustable range and for best life in the lower 65%, so the best combination lies in the middle 30%; only diaphragm and bourdon tube switches suit steam service. [1]

Editorial calculation: the upper 65% and lower 65% of a range overlap between 35% and 65% of it, so on a 0–150 psi diaphragm switch the middle-30% zone runs from about 52.5 to 97.5 psi. [1]

Give the supplier the medium and its temperature, normal working pressure and the highest surge (proof) pressure, required set point and reset point or differential, rising or falling actuation, expected cycles and cycling rate, single, differential or dual-limit function, the electrical load (motor HP/FLA/LRA or pilot duty, voltage, AC or DC), contact form, enclosure or hazardous-area rating, process connection, and quantity.

02

Rate the contacts for the compressor motor

Condor USA's MDR11 air compressor switch is a 2-pole, normally closed single-phase switch rated for definite-purpose switching at 26 FLA/156 LRA up to 240 V, general-purpose 2 HP at 120 V and 3 HP at 240 V, NEMA A600 pilot duty, and 4.0 kW AC-3 at 240 V to EN 60947, with cut-out pressures up to 225 psi, adjustable range and differential, and unloader valves including ¼ in 90° compression, 90° push-connect and straight compression types. [2]

Its technical data list rated operational current of 20 A (EN 60947) or 26 A (UL/CSA) at 240 V AC, electrical life of at least 100,000 cycles (AC-3), mechanical life of at least 500,000 cycles, maximum 120 electrical and 600 mechanical cycles per hour, bursting strength at least 35 bar, medium temperature −5 to +80 °C for air, NEMA 1 enclosure and IP44 protection. Replacement settings are listed as on/off pairs such as 60/80 and 70/100 psi, with ¼ in NPT single or four-port connections. [2]

Editorial calculation: a 70/100 psi setting has a 30 psi (about 2.1 bar) differential, and 225 psi cut-out is about 15.5 bar; at the maximum of 120 electrical cycles an hour, 100,000 cycles would be reached in about 833 hours of operation. [2] Barksdale's housing options run from stripped switches for mounting inside an existing enclosure to housed, terminal-block and explosion-proof versions. [1]

An industrial bellows switch shows the settings a process buyer specifies. Danfoss's KP and KPI data sheet (2019.12) lists the KP 35 for gaseous media with a setting range of −0.2 to 7.5 bar, an adjustable differential of 0.7–4.0 bar, permissible operating pressure of 17 bar and maximum test pressure of 22 bar, on a G ¼ A connection. Its single-pole changeover (SPDT) contacts are rated AC-1 16 A, AC-3 16 A and AC-15 10 A at 400 V, and DC-13 12 W at 220 V (the KPI version for liquids is lower, AC-3 6 A at 440 V). It operates at −40 to 65 °C ambient (up to 80 °C for 2 hours) and −40 to 100 °C media, comes in IP30 or IP55 enclosures, and is vibration-proof to 4 g over 0–1,000 Hz when mounted on a back plate or wall bracket, but not recommended on an angle bracket where vibration occurs. [3]

Editorial calculation: on the KP 35 a cut-out at 6.0 bar with the minimum 0.7 bar differential cuts back in at 5.3 bar; with the maximum 4.0 bar differential it cuts in at 2.0 bar, so the differential setting sets how often a compressor or pump restarts. [3]

03

Compare what suppliers offer

Suppliers linked to this page list pressure switches; each card links to the page reviewed.

Listings include air pressure switches, electronic and digital pressure switches, vacuum switches for gripping systems, low-pressure switches for spray pumps and A/C refrigeration pressure switches; ask each for the sensing element, set-point range and repeatability, proof pressure and contact rating.

04

Match the switch element to the load and check setpoint repeatability

Ashcroft's B-Series switch data sheet (05/2018) lists adjustable setpoints of 15–100% of range and setpoint repeatability of ±1% of full range; it states that a setpoint shift of ±1% of range per 50 °F temperature change "is normal", with switches calibrated at 70 °F. The B4 watertight enclosure meets NEMA 3, 4, 4X, 13 and IP66, and the B7 explosion-proof enclosure meets Div. 1 & 2, NEMA 7, 9 and IP66, for ambient −20 °F to 150 °F (−28 °C to 65 °C). [4]

The contact rating comes from the switch element in the ordering code (table below). Process temperature limits depend on the diaphragm: 0 to 150 °F for Buna N or Teflon, 20 to 300 °F for Viton, and 0 to 300 °F for stainless steel or Monel. Deadbands are tabulated for Buna N; Ashcroft multiplies them by 1.4 for Viton, 1.2 for Teflon and 1.7 for stainless steel or Monel, and by 1.6 for dual switch elements. [4]

Editorial calculation: on the 3000 psi hydraulic range (adjustable setpoint 450–3000 psi, proof 12,000 psi), ±1% of full range is ±30 psi, and at 120 °F, 50 °F above calibration, the setpoint can shift another ±30 psi. [4]

05

Trace the pressure switch construction

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your switch

A pressure switch quotation needs values for your application. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the pressures, cycles and load together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Medium: [air, water, oil, steam; temperature]
Pressures: [working; proof/surge; set and reset]
Duty: [cycles; cycles per hour]
Function: [single, differential, dual]
Load: [motor HP/FLA/LRA or pilot; voltage]
Contacts and housing: [poles; N.O./N.C.; NEMA/IP/Ex]
Connection: [thread; ports; unloader]

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