Hermetically Sealed Switch A switch element whose con-tacts are completely sealed from the environment to provideadditional safety and reliability. Contact arc cannot cause anexplosion, and atmospheric corrosive elements cannot affectthe contacts. Manual Reset Switch Pressure or Temp-erature switch inwhich contacts remain actuated even after the actuating vari-able returns to normal. On Ashcroft manual reset switches, abutton must be pushed to reset the contacts.
Temperature Switches - Type 400, B-Series - 7297 PRODUCT SELECTION INFORMATION
National Electrical Manufacturers Association (NEMA) Thisgroup has defined several categories of enclosures, usuallyreferred to as types. Further, they designate certain featuresand capabilities each type must include. For example, among
other features, a NEMA 4 enclosure must include a threadedconduit connector, external mounting provision and cover gas-kets. When selecting a NEMA 4 enclosure from any manufac-turer, a buyer is assured of receiving these features. NEMA 4 Watertight and dusttight enclosures intended for useindoors or outdoors to protect the equipment against splash-
ing, falling or hose-directed water, external condensation andwater seepage. They are also sleet-resistant. NEMA 4X Watertight, dusttight and corrosion-resistant enclo-sures with same qualifications as NEMA 4, but with added cor-rosion resistance. NEMA 7 Enclosures for indoor Class I, Division 1 Hazardouslocations with gas or vapor atmospheres. NEMA 9 Enclosures for indoor Class II, Division 1 Hazardouslocations with combustible dust atmospheres. Normal Switch Position Contact position before actuatingpressure (or variable) is applied. Normally closed contactsopen when the switch is actuated. Normally open contactsclose when the switch is actuated. Normally Closed Refers to switch contacts that are closed inthe normal switch state or position (unactuated). A pressurechange opens the contacts. Normally Open Switch Refers to the contacts that are open inthe normal switch state or position (unactuated). A pressurechange closes the contacts. Overpressure Rating(s) A nonspecific term that could referto either burst or proof pressure, or both. SELECTION Before making your selection, consider the following: Proof Pressure The maximum pressure which may beapplied without causing damage. This is determined understrict laboratory conditions including controlled rate of changeand temperature: This value is for reference only. Consult facto-ry for applications where switch must operate at pressuresabove nominal range, or reference calibration temperature (70F). 1. Actuator 4. The Switch Element The actuator responds to changes in pressure, temperature ordifferential pressure and operates the switch element in
response to these changes.The actuator is normally exposed to process fluid and musttherefore be chemically compatible with it. The following may
be used to help select actuator type:For nominal pressure ranges 0-15 psi through 0-3000 psi,Dressers standard actuator is a diaphragm-sealed piston. Inthis actuator, process pressure acting on the piston area causesit to overcome the adjustment spring force and actuate a snap-
action switch. A diaphragm and O-ring seal the process mediafrom this mechanism. These are available in various materials,i.e.: Buna N, Teflon and Viton. The standard process connection
is stainless steel. Optional Monel pressure connection is available.For H Finally, the electrical switching element must be compatible withthe electrical load being switched. For ease of selection, all elec-
trical switching elements are snap acting, SPDT (single pole-double throw), or 2 (SPDT). Refer to catalog pages for switchelement choices. Select a switch element with electrical rating
that exceeds the electrical rating of the device being controlled
by the switch. For better reliability and safety, optionalHermetically Sealed switching elements may be specified. ADDITIONAL SWITCH TERMINOLOGYAccuracy (see repeatability) Accuracy normally refers to con-formity of an indicated value to an accepted standard value.
There is no indication in switch products; thus, instead, the
term repeatability is used as the key performance measure.Ashcroft Repeatability (Accuracy) The closeness of agreement among
2 O Pressure and Differential Pressure Ranges, adiaphragm actuator is used. In this design, the standard pres-sure connections are carbon steel. Diaphragms are available inViton, Buna N and Teflon. Always review process temperature
limits before making seal selections. Optional stainless steel
pressure connections are available (option XTA).For High Differential Pressure Actuator Ranges, 3-15 to 60-600psid, a Dual Diaphragm-Sealed Piston Actuator is used. Thisactuator is designed to for high static-pressure applications.The standard pressure connections are nickel-plated brass.
Diaphragms are available in Viton, Buna N and Teflon. Alwaysreview process temperature limits before making seal selec-tions. Optional stainless steel pressure connections are avail-
able (option XUD).For all temperature ranges the standard Ashcroft
pressure and temperature switch accuracy is 1% of nominal range. 2. Enclosure Automatic Reset Switch Switch which returns to normal statewhen actuating variable (pressure or temperature) is reduced. The enclosure protects the switch element and mechanism fromthe environment and has provisions for mounting and wiring. AllAshcroft switch enclosures are epoxy-coated aluminum or stain-
less steel for maximum corrosion resistance. Choose betweenwatertight NEMA 4, 4X for most industrial applications and explo-sion-proof NEMA 7/9 for most process applications.Ashcroft enclosures include watertight cover gaskets, externalmounting holes and one or two Adjustable or Operating Range That part of the nominal rangeover which the switch setpoint may be adjusted. Normally about
15% to 100% of the nominal range for pressure and differentialpressure switches and the full span for temperature switches. Burst Pressure The maximum pressure that may be appliedto a pressure switch without causing leakage or rupture. This is
normally at least 400% of nominal range for Ashcroft switches.Switches subjected to pressures above the nominal range canbe permanently damaged. Consult factory for switches that
must operate at pressure above nominal range or reference cali-bration temperature (70F). Deadband The difference between the setpoint and the resetpoint, normally expressed in units of the actuating variable.Sometimes referred to as differential. a number of consecutive measurements of the output setpointfor the same value of the input under the same operating condi-tions, approaching from the same direction, for full-range traverses.Ashcroft
temperatureactuator operates on the vapor pressure principle: the vapor
pressure in a sealed thermal system is applied to a sensing ele-
ment, which in turn actuates a switch. This is known as a SAMA
Class II system. Various filling materials are used, including
Propane, Butane, Methyl Alcohol, N Propyl Alcohol and Xylene.
High overtemperature capability is possible with this type of
system. The interface between liquid and vapor is the point at
which sensing occurs. This is the sensitive portion of the
bulb. Bulb extensions and capillary are normally filled with
vapor, and have little effect on the setpoint, regardless of ambi-
ent temperature variations; therefore, no ambient compensation
is required. For best results, the bulb should be mounted within
60 degrees of vertical to assure the liquid remains in the bulb. Division 1 A National Electrical Code Classification of haz-ardous locations. In Division 1 locations, hazardous concentra-tions of flammable gases or vapors exist continuously, intermit-tently or periodically under normal conditions; frequently because
of repair or maintenance operation/leakage or due to breakdownor faulty operation of equipment or processes which might alsocause simultaneous failure of electrical equipment. Explosion-
proof NEMA 7/9 enclosures are required in Division 1 locations.
pressure and temperature switch repeatability is 1% of nominal range. Note: It is usually measured as nonrepeatability and expressedas repeatability in percent of span or nominal range. It does notinclude hysteresis or deadband. Division 2 A National Electrical Code Classification of Haz-ardous locations. In Division 2 hazardous locations, flammableor volatile liquid or flammable gases are handled, processed or
used, but will normally be confined within closed containers orclosed systems from which they can escape only in case ofaccidental rupture or breakdown or in case of abnormal opera-
tion of equipment. Either Nema 7/9 explosion-proof enclosures
or any enclosure with hermetically sealed switch contacts maybe used in Division 2 locations. Reset Point The reset point is the Pressure, Temperature orDifferential Pressure Value where the electrical switch contactswill return to their original or normal position after the switchhas activated. Explosion Proof A term commonly used in industry referringto enclosures capable of withstanding an internal explosion of aspecified gas without igniting surrounding gases. Strict installa-tion practices in accordance with the national electrical code are
also required for safety. Fixed Deadband The difference between the setpoint and thereset point of a pressure or temperature switch. It further signi-
fies that this deadband is a fixed function of the pressure switchand not adjustable. Setpoint The setpoint is the Pressure, Temperature or Differ-ential Pressure value at which the electrical circuit of a switchwill change state or actuate. It should be specified either onincrease or decrease of that variable. (See also reset point.) Single-Pole Double Throw (SPDT) Switching Element ASPDT switching element has one normally open, one normallyclosed, and one common terminal. The switch can be wiredwith the circuit either normally open (N/O) or normally closed(N/C). SPDT is standard with most Ashcroft pressure and tem-perature switches.
3 ⁄
4 NPT electrical conduit holesfor ease of installation. Pressure switches may also be mounteddirectly to the process by means of the standard Snap Action In switch terminology, snap action generallyrefers to the action of contacts in the switch element. Thesecontacts open and close quickly and snap closed with sufficientpressure to firmly establish an electrical circuit. The term distin-guishes products from mercury bottle types that were subjectto vibration problems.
1 ⁄
4 NPTF oroptional
1 ⁄
2 NPT pressure connection. Note: When installing Ashcroft switches, refer to instructionsheets included with each switch, the National Electrical Code,and any other local codes or requirements to assure safety. 3. The Switching Function Static Pressure For
differential pressure switches, static pressurerefers to the lower of the two pressures applied to the actuator. Next, consider the switching function. Most applications for alarm
and shutdown are satisfied by single setpoint, fixed deadbandmodels. For high/low or alarm and shutdown, the dual setpoint
models may be selected. For pump, compressor, level and othercontrol applications, an adjustable deadband model is often thebest choice. Consult your Ashcroft representative for dual setpoint
or adjustable-deadband pressure and temperature switches.
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