A pressure switch is a device that makes electrical contact (opening or closing) when a preset liquid or gas pressure is reached. The switch makes an electrical contact at the pressure rise or pressure drop from the preset pressure level. It is found in a wide variety of industrial and residential applications such as pressure switches, HVAC systems, well pumps, furnaces, etc.
PRESSURE SWITCH TYPES
Mechanic Pressure Switch
A mechanical pressure switch uses a spring and a diaphragm or piston to control the pressure at which the microswitch is triggered. The spring is the force that opposes the inlet pressure and the pretension of the spring is adjusted with a set screw or knob. Spring pre-tension is directly related to the pressure at which the switch makes an electrical contact. When the pressure drops, the switch is reset to its original state.
The difference between the switching point and the reset point is called hysteresis. Usually this is expressed as a percentage of the waypoint value, for example 20%. The manufacturer defines hysteresis and most mechanical switches are not user replaceable. These pressure switches usually have three different contacts: normally open (NO), normally closed (NC) and changeover (SPDT) contacts.
A mechanical pressure switch is more suitable for operating at higher voltages and amperes than an electronic pressure switch.
Electronic Pressure Switch
An electronic pressure switch uses an electrical pressure sensor to measure the change in inlet pressure. It has digital displays to set the switching function. The keypoint can be set by the manufacturer or programmed on site based on application requirements.
Switching point, output signals, hysteresis, delay time etc. can be adjusted by the user according to the requirements. Electronic pressure switches are suitable for automated and controlled equipment systems that require programmable function, digital display, flexibility, accuracy, ingress protection and stability.
The internal diagram of a pressure switch is shown below. The pressure switch shown is an example of a single-pole double-throw (SPDT) switch with a mechanical operating principle. All components are inside the switch case (F) and have an inlet pressure port (H). In short, the inlet pressure pushes a piston (D) against a spring (C) having a known resistive force. Next, the piston triggers the microswitch (A) and moves it between a normally closed (NC) and normally open (NO) position via an actuation pin (B) and an insulated release switch (E).
The trip adjusting nut (G) changes the spring pocket depth to adjust the pressure level at which the microswitch switches between NC and NO. This depth change allows the spring to increase or decrease the resistive force corresponding to a set pressure to trigger the microswitch. Inlet pressure (H) exerts pressure on the working piston (D), creating a force against the range spring (C). When the force of the inlet piston is greater than the counterspring force, it pushes the actuation pin (B) against the insulated trigger button (E). This button then moves the microswitch from the NC position to the NO position. If the pressure falls below the spring force, the button, pin, and plunger move away from the microswitch, breaking the connection. The link then switches from NO to NC.
Consider the following selection criteria;
- Fluid Type: The fluid type must be compatible with the housing and gasket material. It is suitable for use with nitrile butadiene rubber (NBR), air and hydraulic/machine oil. Ethylene propylene diene monomer rubber (EPDM) is suitable when the medium is water. Common media used with pressure switches include:
- Hydraulic oil
- Heating fuel
2. Pressure: It should be able to withstand the maximum working pressure. The diaphragm design works well for vacuum and low pressure applications. For high pressure applications a piston design would be a better choice.
3. Temperature: It should be able to work well in the maximum and minimum temperature range.
4.Repeatability: Repeatability or accuracy is the ability of the device to accurately return to the same setpoint for each repetition. The range of accuracy required will determine the pressure switch selection for your application. Diaphragm designs often provide greater accuracy than piston designs.
5.Hysteresis: Hysteresis is the difference between the switching point and the reset point. If the reset point is too large, the switch remains active for a long time. If the reset point is too short, the switch will frequently toggle between on/off. Hysteresis can be configured on an electric pressure switch, but is preset by the manufacturer on a mechanical pressure switch.
6.Pressure switch type: Use diaphragm design one for low pressure and vacuum applications. Use one with a piston design for high pressure applications.
7.Electric or mechanical pressure switch: An electric pressure switch is more expensive, but provides more control over settings such as pressure setpoint and hysteresis compared to a mechanical pressure switch.
A pressure switch can be used in a wide variety of domestic and commercial applications listed below:
- HVAC, gas cylinders, air pumps etc. It uses air compressor pressure switches to monitor and control the air pressure of the system.
- Oil pressure switches are used by engines as an actuator or sensor to determine if the engine oil pressure has dropped below the preset level.
- Furnace pressure switches function as industrial and residential safety devices. They detect negative pressure at the start of the furnace and turn off the furnace in case of low air pressure.
- Well pump pressure switches are used in residential and commercial buildings to bring water from the well and to ensure that there is sufficient water pressure without overpressure in the system.
- Water pump pressure switches in residential, commercial and agricultural applications automatically regulate the water flow.
- Vacuum pressure switches measure the vacuum or negative pressure in the system. They are found in residential boilers, electric heaters, air compressors and transmission systems.