When systems fail, engineers tend to focus on the big, the
expensive or the exotic. However, it is often a problem with
a simple, inexpensive part that shuts an application down
or affects performance. Here, David Almond, head of sales
and marketing at switch distributor PVL, explains why he
believes pressure switches are one of the most overlooked
technologies of all

Pressure switches are one of the most
common types of component in
process plants. Yet they are also one
that many engineers know little about.

Despite this, a clear understanding of the
basics and a reasoned, methodical
approach will make the selection process
much easier.

In its simplest form, a pressure switch is a
device capable of detecting a pressure
change, and, at a predetermined level,
opening or closing an electrical contact.

The most common electromechanical
pressure switches consist of a sensing
element and an electrical snap-action
switch. A number of different types of
sensing elements can be used but they have
one thing in common: they move in
response to changes in the system pressure.

Through their movement they directly act
on the opening and closing of the snapaction
switch’s contacts.

The current market provides a wide
variety of solid-state pressure switches, with
one to four or more switch points, digital
displays, analogue and digital outputs, and
full programmability.

In many cases they cross the line from
simply being a switch to becoming an openloop
controller. In addition to opening or
closing the pressure switch circuit or
circuits, they provide a proportional
analogue 4-20 mA signal or digital output.

The analogue signal can interface with
PLCs (Programmable Logic Controllers),
DCSs (Distributed Control Systems) or
stand alone industrial computers.

Solid-state pressure switches provide a
number of advantages over
electromechanical switches, including a
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 and excellent long-term stability.

However, the biggest advantage lies in cycle
life. Solid-state switches routinely have an
operational span of 100 million cycles.

Speed, range and switch point The frequency with which the switch is
activated will have direct impact on switch
life, system downtime and the maintenance
schedule. Due to their design,
electromechanical switches are subject to
metal fatigue although solid state switches
aren’t.

Cycle speed will also affect switch life and
preventative maintenance programmes once
the design is used in anger. A solid-state
switch should be selected whenever the
cycle rate exceeds 50 cycles per minute so
that metal fatigue is not a problem.

Establishing the right relationship
between the switch point and the operating
pressure range of a switch is also important.

When a solid-state pressure switch is
selected, the switch point should normally
be in the upper 25% of the operating range.

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 pressure switch with
an operating range of 150 psi, or an
electromechanical switch with an operating
range of 300 psi.

Accuracy, pressure points and
housing Pressure switch accuracy is defined as the
ability of the switch to operate repetitively
at its set-point. If the switch is used to
trigger an alarm, ±2% accuracy is sufficient.

If one is controlling a process where the
error of various devices is cumulative, then
±0.25% accuracy may be absolutely
necessary. Accuracy is referenced at the
high end of the operating pressure range
and decreases at lower pressure.

Once the required accuracy is
established, we should decide on the
number of switch points need. When
sensing pressure at one point, it is normal
that only one switch point is required.

Nevertheless, it’s not unusual for a system to
require two or even four switch points to be
monitored, controlled or alarmed. In
designing a system, one could select a single
switch for each switch point, or a single
pressure switch capable of handling as many
as three separate switch points
If one considers all of the factors that
have to be taken into account when
designing a pressure switch into an
application, the hidden complexity in one of
the manufacturing and process industry’s
simplest components is revealed. However,
despite this hidden complexity, this kind of
switch remains one of the simplest to
integrate and maintain. The job of the
design engineer is to make that process
even easier.