Pressure switches are one of the most commonly used fluid control components. We use them at home, in our refrigerators, dishwashers and washing machines. Whenever we handle a gas or liquid we almost always need to control the pressure.

Our home appliances do not demand high accuracy nor do they experience high cycle rates. By contrast, the pressure switches used in industrial machinery and systems must be rugged, dependable, accurate, and have a high lifecycle.

Most of the time we never think about pressure switches. They just show up with such machinery as paper machines, air compressors or pump sets. In this type of equipment, we depend upon pressure switches to act as safety devices, alarms, or as the control element within the system. But in most cases we give little consideration to this component when we make a purchase.

The basic function of a pressure switch is to detect a pressure change and convert it into an electrical signal function – typically on/off or off/on. Although there are many different types, pressure switches fall under two basic classifications:

-- Electromechanical

-- Electronic/solid state.

And while each may have its advantages, arriving at the correct pressure switch for your application is the same.

Electromechanical pressure switches have a sensing element which responds to changes in pressure and mechanically operates a snap-acting switch in response to the pressure changes. Different types of sensing technologies are used in the design of electromechanical pressure switches.

Diaphragm switches use an elastomeric or weld-sealed metal diaphragm which deflects with pressure changes; they act directly, or via a push-rod, on a snap-acting switch.

With a bellows or bourdon tube switch, the movement of the bellows, or sealed metal bourdon tube, is caused by pressure changes; this movement mechanically operates a snap-acting switch.

A piston switch design uses an O-ring sealed piston that moves in response to pressure changes, and directly or via a push-rod, actuates the electrical snap-acting switch.

Solid state pressure switches use the same technology found in analog pressure transmitters to sense changes in pressure. A weld-sealed metal diaphragm or O-ring sealed ceramic diaphragm with a piezoresistive strain gage-based sensing element is used to measure changes in pressure. Rather than harnessing the energy of the pressure changes to mechanically operate a switch (as with electromechanical pressure switches), solid state pressure switches electrically measure pressure changes and internal electronic circuitry is used to activate one or more solid state switched outputs.

Today, there are a wide variety of solid-state pressure switches having one to four or more switch points, digital displays, analog and digital outputs, and full programmability.

In addition to opening or closing the pressure switch circuit(s), they provide a proportional analog 4-20 mA signal or digital output. The analog signal can interface with PLCs (Programmable Logic Controls), DCSs (Distributed Control Systems) and computers.

Solid-state pressure switches provide a number of advantages over electromechanical switches, including:

-- Much longer cycle life

-- Improved accuracy to ±0.25%

-- High resistance to shock and vibration

-- The ability to handle a wide range of system pressures

-- Broad frequency response

-- Excellent long-term stability

ESS201 Series Intelligent Pressure Switch adopts the solid-state sensing technology and the major advantage lies in cycle life. Solid-state switches routinely have an operational life of 100 million cycles. One concern with solid-state switches used in industrial and process settings is electromagnetic interference which can corrupt signal data. One should select a solid-state pressure switch that is in compliance with approved electronic standards. Additionally, a solid state switch requires an input power source to function. EMI/RFI does not affect electromechanical switches because the circuit is a mechanical switch that is either open or closed.

Even with their list of attributes, however, the initial comparatively high price may probably prevent many users from upgrading to this technology. If one is designing a new system, the selection of a solid-state switch may, in fact, be the most cost-effective solution. When a system requires multiple switch points, a local gauge and transmitters using a solid-state pressure switch can reduce the installed cost by one-halfs