Downtime – there’s nothing funny about that...

When systems fail, it is often a simple part that shuts an application down. And unlike the typical light-hearted approach of our blog, there’s nothing funny about that! In today’s post, David Almond, PVL’s head of sales and marketing explains why pressure switches are often overlooked.

Pressure switches are incredibly common in process plants, yet design engineers often, understandably, know little about them. As a process focussed design engineer, your specialisation might be robotics, heat transfer or any of a thousand other things.

A pressure switch detects a pressure change and at a predetermined level, at which it will activate an electrical contact. Traditionally electromechanical devices, pressure switches today may also be solid state devices, with one to four or more switch points, digital displays, analogue and digital outputs, and full programmability.

In many cases, they can act as open loop controllers in addition to simple switching mechanisms; providing analogue 4-20mA or digital output and interfacing with programmable logic controllers, distributed control systems and stand alone industrial computers.

Still nothing funny so far. But hold on, the last paragraph is a cracker.

Solid state pressure switches provide longer cycle life, improved accuracy to ±0.25%, resistance to shock and vibration, the ability to handle a wide range of system pressures, broad frequency response and long term stability.
However, electromagnetic interference (EMI) can corrupt signal data, something that does not affect electromechanical switches, which are either open or closed. Furthermore, solid state switches require a power source, but they are not subject to metal fatigue like electromechanical switches.

The frequency with which a switch is activated will affect its life, system downtime and maintenance schedule. A solid state switch should be selected whenever the cycle rate exceeds 50 cycles per minute. The switch point should be in the upper 25% of the operating range, whereas for an electromechanical switch, the switch point should be in the middle of the operating range.

Thus a system that requires a switch to activate at 140 psi should use a solid state switch with an operating range of 150 psi or an electromechanical switch with an operating range of 300 psi. If the switch is used to trigger an alarm, ±2% accuracy is sufficient, but where the error of various devices in a process is cumulative, then ±0.25% may be absolutely necessary.

Despite its design complexity, the pressure switch remains one of the simplest devices to integrate and maintain – providing you have the basics to hand.

As a result, we’ve recently authored this article in Industrial technology magazine, which tells you more. Read on at the magazine’s site, or call us on +44 (0) 1892 664499 to discuss the subject with one of PVL’s qualified and experienced engineers.

And, if you’ve been holding on this long for a joke about engineers, we don’t want to disappoint. There’s a great one here, even if we do say so ourselves.


For more information, contact...

PVL Limited
Unit 9, Lexden Lodge Industrial Estate, CROWBOROUGH TN6 2NQ. UK.
Tel : +44(0) 1892 664499
Fax : +44(0) 1892 663690
Email :
Website :

March 2012

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