Pressure Switch: Important Facts To Know

Pressure switchespressureswitch can be used in very many industrial and domestic applications.

For instance, the pneumatic switches can be used to switch on/off the following: the light alarms if the aircraft’s cabin pressure goes below the desired level, gas compressor and water pumps amongst others.

On the other hand, the hydraulic types can be used to switch on vehicles’ alarm light if the engines oil pressure goes below the safe level, to control automatic lock up and transmission systems among other uses.

How do they operate?

These switches exist as either normally closed or normally open. Their principle of operation is almost the same.

For a normally open pressure switch, assuming that it’s a piston type; the pressure entering through the switch’s input acts on the diaphragm or the piston (depending on the switch).

The pressure exerted results in some force which make the plunger to move alongside with the contact disks/plates which closes the switch between the contacts (open switch).

The force exerted by the pressure must be greater than the pre-loaded force of the springs. When this pressure goes down, the force reduces and as a result, the springs pull back the disks leaving the circuit open.

The reverse takes place in the normally closed switches. In some cases such as that of the micro-switch, this change over process of the normally closed and the normally open can be integrated together in one switch. This makes the micro-switch a perfect choice for very many application.

The operation principle may vary from one switch to the other. However, the bottom line is that when a given amount of pressure which is more than the preset pressure enters the switch’s input, the switch either opens or closes depending on its initial state.

Important factors to consider while dealing with pressure switches

  1. pressureswitchesHysteresis; it’s used to describe the difference between various switching points. This is mostly when the pressure is rising and wh falling. It depends majorly on the switching point.
  2. Switching frequency; this gives the information about the number of switching cycles/minute. It can be given as 100/minute. This will also depend on both the operating conditions and the type of switch.
  3. The set point tolerances; the operating temperature and pressure also determines the accuracy and lifespan of the switch. Remember, the switching points can change with time due to either the aging effects or temperature.
  4. Membrane; the choice of material will depend on its main application. For instance, EPDM is ideal for brake fluid,Viton® is a perfect choice for both high pressure and temperature, silicone is suitable for extensive temperature range and buna-N is the most common since it’s considered as the standard material.
  5. The switch’s life; diaphragm switches have a longer lifespan. The piston types are also durable. The lifespan depends on the cycle speed (the higher the speed, the shorter the lifespan), pressure levels, rate of change in pressure, the electrical load on the switch, hydraulic shock and deadband setting. A deadband refers to difference in the re-actuation and actuation of the switch.
  6. Switch points of a pressure switch; a switch can have more than one switching point. These may include low, high, low-low and high-high. The operator must be able to handle all these switch points. The set points must also be adjustable to suit an application.

The switch’s housing is also important since it protects it from possible hazards.

Moreover, calibrating the switch is also necessary to ensure that it operates optimally and accurately. With all these in mind, it’s possible to get a switch which suitable for a specific application.

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Pressure Switch

What exactly is a pressure switch?

This is a device with the ability to detect any changes in pressure and uses the change to either open or close an electrical contact.

It can either be when pressure falls or rises. It will only act when a given magnitude of pressure has been attained at its input.

It’s configured to respond in a certain manner depending on the pressure level. It’s mostly used to monitor various industrial processes. These switches are represented in a circuit diagram as shown below:

pressure switch 1

Broadly, these types of switches can be classified as either solid state or electromechanical switches.

pressure switches 2

The diagram shows the internal structure of a pressure switch.

The electromechanical types

These types of switches have two major components; the sensing element and the electrical snap-action switch.

They’re very many types of electromechanical pressure switches which can be classified depending on the type of sensing technology used. Some of the most common types include:

The diaphragm switches; the weld-sealed diaphragm acts as the switch and they can withstand a pressure of up to 150 psi. They have an accuracy of ±0.5% and are also characterized with low cycle rates.

The bourdon tube switches; they can operate in pressures between 50 psi to 18,000 psi. Like the diaphragm types, they have an accuracy of ±0.5% and are also characterized with low cycle rates.

The Dia-seal piston switches; they can operate in pressures which range from vacuum to about 1,600 psi. Their accuracy is ±2% and can go up to 2.6 million cycles.

Piston switches; they have an accuracy of ±2% and they can also operate at pressures of about 12,000 psi.

The solid-state switches

Most electromechanical switches had a lot of setbacks and hence, there was need to get more efficacious pressure switches. The first solid-state switch was introduced by 1980.

They have better accuracy (±0.25%); resistant to vibrations and shock; can handle very many pressure systems; durable; stable and have a broad frequency response.

Despite all these advantages, in an electromagnetic setup, most signals transmitted by these sensors can be corrupted. Furthermore, their initial cost is also higher.

Characteristics of pressure switches

While choosing switches, there are a number of factors one must have in mind. These include ease to use, reliability, accuracy and ability to fulfill the required objectives. Some of the most basic characteristics of these switches include:

The switching frequency; this directly affects the lifespan of the switch. For cases where the switch is turned ON and OFF more frequently, going for solid-state switches with about 100 million cycle life is desirable than the electromechanical switches such as bourdon tube which have about 1 million cycle life.

Cycle speed; go for a solid-state switch if the cycle rate is above 50 cycles/minute and a bourdon tube if the cycle rate is less than 25 cycles/minute. It will also determine the life span of the switch.

Operating pressure range; it will determine the accuracy of the switch. For instance, in a system where the switch should be active at 140 psi, it will require a switch whose operating range is 150 psi.

Proof pressure; while determining this pressure, all surges and spikes must be included. This is the maximum pressure the switch can be exposed to. They must not exceed the normal operational pressure. Other characteristics include the accuracy and the switching points.

These switches are very vital in an industrial setup since they can be used to stop various operations which tend to exceed the recommended limits. They make the automation process easier.