What a pressure switch is
A pressure switch changes the state of a contact when the pressure reaches a figure that has been set. It does not report the pressure and it does not indicate it; it acts on it. The contact then drives a motor, a solenoid, an alarm or a lamp, and the fact that nothing else is required - no supply for the measurement, no controller, no loop - is the reason it remains in service on plant that has been updated around it.
It is worth separating the switch from what is near it, because the names are used loosely. A transmitter produces a continuous signal proportional to pressure and leaves the decision to the control system. A controller receives a signal, displays it and produces an output according to a set point held in its own electronics. A switch contains the decision itself, mechanically, and the set point is changed by turning a screw rather than by entering a number.
What decides a switch is a pair of figures rather than one. The actuation point is where the contact transfers; the differential is how far the pressure must return before it transfers back. Both are adjustable on most industrial switches, both have limits, and the pair together is what makes the device stable or useless on a given duty.
How a pressure switch measures
Pressure acts on a sensing element - a diaphragm, a bellows or a piston - and the element works against a spring. The spring force sets the pressure at which the element has travelled far enough to transfer the contacts, and the screw that adjusts it is the set point control. A second adjustment sets how far the element must travel back before the contacts return, and that is the differential, sometimes called the deadband or the reset point.
The differential is what makes a switch usable. Without a gap between the point where the contact makes and the point where it breaks, a pressure wandering either side of the set figure would cause the contact to chatter, which wears the contacts and produces nonsense at whatever it is switching. The differential is also a trade: a large one holds a state firmly but lets the pressure move further before anything happens, and a small one responds closely but is more exposed to the noise in the process.
Sensing elements differ in what they tolerate rather than in principle. A diaphragm suits low pressures and can be made from a material chosen for the medium; a piston tolerates higher pressures and is less affected by a dirty fluid; a bellows gives more travel and is common where a clear difference between the two set points is wanted. The element also decides the proof pressure the switch survives without being damaged, which is a different figure from the pressure it is set to act at.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Ranges, and the direction the switch can act in | A range that starts below atmospheric, or one that begins above it | Whether a vacuum duty is covered at all. A switch whose low end is above zero cannot be set to act on a vacuum however the screw is turned, so the direction of the process is settled before the maximum figure is discussed. |
| Adjustable differential | The band within which the differential can be set | Whether the process can run without the switch cycling. The band is normally a fraction of the range, so a range chosen far above the working pressure cannot be set finely, and one chosen too close leaves no room for the swing the process actually has. |
| Contact form | Single pole double throw, or a simpler make-or-break arrangement | How many circuits one switch can drive and in which direction. A changeover contact lets a single device stop one circuit and start another as it transfers, which is often what removes a relay from the panel. |
| Reset behaviour | Automatic, or manual after the contact has operated | How the plant recovers from the condition the switch has detected. Automatic reset restores the circuit as soon as the pressure returns; a manual reset holds the state until an operator has seen and acknowledged it. |
| Rating, enclosure and connection | Contact rating including inrush, ingress protection, and the process connection | Whether the switch survives the load and the location. A rating quoted for a resistive load does not cover a motor or a solenoid, and an enclosure rated for solid objects rather than water decides where the switch may be mounted. |
How to choose a pressure switch
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| The pressure in both directions | A switch that has to act on a vacuum is a different instrument from one that acts on positive pressure, and the two are often confused because the top of the range looks similar. | State the full cycle of the process, including start-up, shutdown and any evacuation step, not only the working pressure. |
| The figure at which the switch should act | It is the value the plant is protected by or controlled against, and it has to be settable within the band the instrument offers. | Give the actuation point and say whether it should be set before dispatch or adjusted on site. |
| How much the pressure is allowed to move after the switch acts | This is the differential, and it is what stops the switch cycling. Too small and the contact chatters; too large and the plant swings further than intended. | Give the acceptable swing, or say that the factory setting is acceptable and what the process does around it. |
| The load being switched | A contact rating is not a property of the pressure duty, and an inductive load draws far more at the moment of switching than it does once running. | Give the voltage, the current and whether the load is resistive or inductive, and say how often the switch operates. |
| Where the switch is mounted | The enclosure rating, the ambient temperature and the fluid temperature have separate limits, and the medium compatibility depends on the wetted parts. | State the location, whether it is exposed to water or weather, and what the medium is, rather than assuming a general-purpose enclosure covers it. |
The nearest alternatives
These are the types an enquiry for a pressure switch usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a pressure switch |
| Pressure transmitters | Reports the pressure rather than acting on it, leaving the decision to the control system, where the set point can be changed without touching the instrument. Chosen where the value itself is wanted, or where the switching logic is likely to change. |
| Pressure controllers | Reads a pressure and displays it, then produces an output against a set point held in its electronics. It can do more than one thing with the same reading, and it can be reconfigured; a switch does one thing mechanically and cannot be reprogrammed. |
| Vacuum pressure switches | Intended for duties entirely below atmospheric, where the whole scale is usable rather than half of it being a range the process never enters. |
| Differential pressure switches | Acts on the difference between two ports, for filter blockage and flow proving. A single-port switch cannot see a difference, so it cannot protect an element whose condition shows up as a rise in pressure drop. |
| Pressure gauges | Indicates the pressure and does nothing with it. Where a visible reading and a switching action are both needed, the two sit side by side rather than one standing in for the other. |
A switch set at a figure and a gauge at the same tapping point is a common pairing, because one acts on the pressure and the other lets an operator see why it acted.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- Medium and its temperature
- The full range the pressure passes through, in both directions
- The figure at which the switch should act
- The differential, or acceptable swing, around that figure
- Whether reset should be automatic or require an operator
- Contact function required, and the circuits it will drive
- Load voltage and current, and whether the load is inductive
- Process connection type and size, or tube size for a push-in fitting
- Enclosure requirement from the location
- Quantity, destination port and required delivery date
