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Limit Switches vs. Proximity Switches- What’s the difference?

April 1, 2014
Switch Tops for Waukesha W60 Valves

Switch Tops for Waukesha W60 Valves

Control of today’s process systems range from complete computer systems  to operators manually controlling valves. One area of process control is valve automation. While valve automation can seem like a very daunting task, especially when compared to simple pull handles, there are a few common tools whose understanding can go a long way to helping users feel comfortable automating their processes. This post fill focus on limit and proximity switches, what they are, and how they are different.

Let’s start by clearing something up- the term “limit switch” is a misnomer. Limit switches do not limit the travel of a pneumatic actuator, instead they indicate when an actuator has reached, or has not reached, a specified point of actuation. “Position indicating switch” is probably a more accurate term. A limit switch, also known as a mechanical or micro switch, is a mechanical switching device that is made up of an input shaft coupled to the valve actuator. As the actuator cycles, the limit switch input shaft strokes and actuates the switches. Position feedback switches are used in a variety of applications such as light indication, valve sequencing, alarms, and interlocks.

Electric limit switches are usually characterized by the number of poles and throws they contain. A pole is the component of the switch that is moved by the switch to make or break an electrical connection. The total number of possible electric connections that can be made by a given pole are called throws. A limit switch can be either a single pull, single throw (on/off), single pole, double throw (either normally open or normally closed), or double pole, double throw (two moving components, both either normally open or normally closed).  These switches are often coupled to a visual indicator that will locally indicate valve position to operators while also providing electrical feedback to a DCS. Limit switches are economical and available in 120 VAC as well as 12-48VDC and are available in all of our WCB, ITT, ball, and butterfly valve offerings.

Another way of communicating a valve position is with a proximity switch. A prox switch serves the same function as a limit switch, the means by which a proximity switch communicates position, however, is quite different. A proximity switch operates when a metallic or magnetic object is brought into close proximity to the switch sensing area. A proximity sensor uses a coil to generate an electromagnetic field which is propagated by an oscillator. A detection circuit detects a change in the electromagnetic field when an object enters it and relays it to the output circuit, which produces the output signal. In a pneumatically actuated valve, the moving object is generally the actuator stem.

Automated ITT Sanitary Diaphragm Valve with Switch Housing

Automated ITT Sanitary Diaphragm Valve with Switch Housing

Proximity sensors operate without any physical contact from the object itself. This makes them ideal for use in dirty or wet environments. Prox switches are less sensitive to dust, moisture, moisture and oil than limit switches and their internal solid state circuitry makes them better suited for applications with high levels of vibration than the mechanical limit switches. Like limit switches, prox switches are available in 120 VAC as well as 12-48VDC and are available in all of our sanitary valve offerings.

In sum, both limit and proximity switches serve the same purpose- to indicate valve position. The means by which they accomplish this, however, are quite different. While limit switches are simple and economical, proximity switches can better suited for harsh environments. Future posts will focus on other valve automation elements, including solenoids and positioners.

Most automated sanitary valves today, be it stem seat, ball, butterfly are diaphragm types are available with wither limit or proximity switches. If you have any questions about how to figure out what position your valve is in, contact a Holland Sales Engineer today.

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