The Fundamentals Of Pressure Controls

Pressure controls serve various functions, which may be divided into control and protection functions. Examples of control functions are compressor cycling, pumpdown, or defrost control. Protection functions included pressure limiting and cutout against excessive pressures, against loss of charge or for freeze protection.

Pressure controls (such as those manufactured by Alco, a division of Emerson Climate Technologies) are equipped with display scale and pointers to indicate the approximate settings. The display scale is printed in relative pressure units “bar” and “psi.” For precise setting of the controls, external gauges must be used.


All pressures are understood as gauge pressures. The Alco PS1/PS2 and TS1 controls, for example, sense pressure by means of bellows that expand or contract when exposed to medium pressure. PS3 controls use a diaphragm rather than a bellows.

High-pressure limiters and pressure cutouts approved according to DIN 32733 feature a double bellows design. The inner bellows serves as the operating bellows and is enclosed by the outer bellows, which features larger surface area.

Should the inner bellows leak, the larger surface area of the outer bellows creates a larger force and causes the pressure control to a pre-empted cutout. This represents a fail-safe function.

Standard controls for refrigeration applications are equipped with bronze bellows and can be used with all common HFC, HCFC, and CFC refrigerants. For ammonia applications, controls with stainless steel bellows are available.


A variety of pressure connectors — including male and female flare-type connectors, capillary, and solder connectors — are available. The standard connector is a 7/-16 inch -20 UNF male flare connector, which, in its high-pressure versions, is equipped with a snubber to protect against pressure pulsations.


PS1/PS2/PS3 and TS1 controls are equipped with high-rated double snap action contacts designed for chatter-free and reliable operation. All contacts throughout this range of controls are designed as single pole double throw (SPDT) contacts. One contact may be used for control and the other contact for alarm/status indication or auxiliary control. In addition, dual pressure control PS2 comes with two independently actuated SPDT contacts, providing for even further application flexibility by allowing for a variety of wiring options.

Gold-plated contacts are available on request for low electrical loads (for example, in electronic signaling DC applications).


PS1/PS2 and TS1 are adjustable controls with either internal or external adjustment spindles for range and differential. By turning the range spindle, the upper setpoint is defined and by adjusting the differential spindle, the differential, and hence the lower setpoint, is changed. (The upper setpoint does not change.)

The dependency between upper and lower setpoint is always as follows:

Lower setpoint = upper setpoint differential.

The following two rules should always be kept in mind:

  • An adjustment of the range spindle always affects both upper and lower setpoint.

  • An adjustment of the differential spindle affects the lower setpoint only.


    All PS1/PS2 and TS1 controls have contacts labeled 1, 2, and 4 where 1 refers to the common pole, 2 refers to the lower setpoint, and 4 refers to the upper setpoint.


    On temperature/pressure rise above the upper setpoint, contacts 1-2 open and contacts 1-4 close. On decreasing temperature/pressure, lower setpoint contacts 1-4 open and contacts 1-2 close.


    On decreasing temperature or pressure below the lower setpoint, contacts 1-4 open, contacts 1-2 close and latch. Only on pressure rise above upper setpoint and after pressing the manual reset button contacts 1-2 will open and contacts 1-4 will close again.


    On increasing temperature or pressure above the upper setpoint, contacts 1-2 open, contacts 1-4 close and latch. Only on falling temperature or pressure below lower setpoint and after pressing the manual reset button, contacts 1-4 will open and contacts 1-2 will close again.

    For operational safety, all PS1/PS2 with manual reset are designed as trip-free controls. Pressing the manual reset button while the pressure has not reached its reset threshold will not operate the electrical contacts.

    The contact function for controls with internal and external manual reset is alike. The only difference between the two is that for internal manual reset the cover has to be undone, whereas the external reset controls can be reset without removing the cover.

    As a dual pressure control, PS2 has two complete sets of contacts. Their function is the same as on single pressure controls PS1, with the only difference that the contact labels are preceded by an additional index. One side of the control is labeled 11, 12, and 14 whereas the second side is 21, 22, and 24.

    The contact function of controls with convertible reset is as described above, but depends on the actual position of the convertible reset toggle (i.e., automatic or manual reset position).

    Maier is vice president of Engineering for Alco Controls. For more information, contact Alco at

    Publication date: 11/04/2002

  • Did you enjoy this article? Click here to subscribe to The NEWS Magazine

    You must login or register in order to post a comment.



    Image Galleries

    2015 AHR Expo

    Photos from the 2015 AHR Expo at McCormick Place in Chicago.

    Posted: February 19, 2015


    NEWSMakers: Andy Patron
    Andy Patron, partner with FMI Corp., discusses the impact a national labor shortage may have on the mechanical contracting industry. Posted on March 27.
    More Podcasts


    NEWS 03-23-15 cover

    2015 March 23

    Check out the weekly edition of The NEWS today!

    Table Of Contents Subscribe

    HVAC Trends

    The NEWS regularly reports on HVAC industry trends in our weekly issues. What trends do you see?
    View Results Poll Archive


    2015 National Plumbing & HVAC Estimator

    Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

    More Products

    Clear Seas Research


    Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.


    Magazine image
    Register today for complete access to Get full access to the latest features, Extra Edition, and more.


    facebook icontwitter iconyoutube iconLinkedIn i con