Pressure Switch Perfected: Your Ultimate Guide to Top 20 Interview Questions & Answers
Navigating the intricacies of pressure switch functionality and settings is a critical skill for any technician or engineer. Whether you are a seasoned professional or a fresh candidate, a strong understanding of these components is often a key focus in technical interviews. This comprehensive guide delves into the top 20 most frequently asked interview questions about pressure switches, providing you with clear, concise answers to help you confidently demonstrate your expertise.
1. What is a pressure switch and what is its basic working principle?
A pressure switch is an electromechanical or electronic device that operates an electrical switch when a predetermined fluid (liquid or gas) pressure is reached.
Working Principle: At its core, a pressure switch operates by converting pressure into mechanical motion. This is typically achieved through a sensing element like a diaphragm, bellows, or piston. When the process pressure changes, it causes the sensing element to move. This movement, upon reaching a specific setpoint, actuates a set of electrical contacts, either opening or closing an electrical circuit.
2. What are the main components of a mechanical pressure switch?
A typical mechanical pressure switch consists of:
- Sensing Element: A flexible component (diaphragm, bellows, or piston) that directly interfaces with the process fluid and moves in response to pressure changes.
- Spring: A calibrated spring that opposes the force of the sensing element. The pre-set tension of this spring determines the switch’s setpoint.
- Operating Mechanism: A lever or linkage system that transfers the movement of the sensing element to the electrical contacts.
- Electrical Contacts: These are the switch’s output, which can be Normally Open (NO), Normally Closed (NC), or a combination of both (SPDT – Single Pole Double Throw).
- Adjustment Screws: Used to set the cut-in and cut-out pressures.
3. Can you explain the terms “cut-in,” “cut-out,” and “differential” pressure?
These are fundamental settings for a pressure switch:
- Cut-in Pressure: This is the low-pressure setpoint at which the electrical contacts of the switch close, completing a circuit and typically starting a pump or compressor.
- Cut-out Pressure: This is the high-pressure setpoint at which the electrical contacts open, breaking the circuit and stopping the pump or compressor.
- Differential (or Hysteresis or Deadband): This is the pressure difference between the cut-out and cut-in points. It prevents the switch from cycling too frequently (chattering) in response to minor pressure fluctuations. For example, if the cut-in is 30 PSI and the cut-out is 50 PSI, the differential is 20 PSI.
4. How do you adjust the settings on a pressure switch?
Adjusting a pressure switch typically involves two adjustment screws:
- Range Screw: This larger screw adjusts both the cut-in and cut-out pressures simultaneously, effectively shifting the entire operating range up or down. Turning it clockwise generally increases the pressure settings.
- Differential Screw: This smaller screw adjusts the cut-out pressure only, thereby changing the differential. Turning this screw clockwise typically increases the differential, meaning the cut-out pressure will be higher relative to the cut-in pressure.
Crucially, always disconnect power before making any adjustments.
5. What are the different types of sensing elements in a pressure switch, and where are they used?
The choice of sensing element depends on the application’s pressure range and media:
- Diaphragm: A flexible, circular membrane. Ideal for low-pressure applications and vacuum sensing. Commonly found in HVAC systems and for sensing air pressure.
- Bellows: A one-piece, collapsible and expandable metallic unit with deep folds. They offer a longer stroke and are suitable for low to moderate pressure ranges.
- Piston: A solid cylinder that moves within a housing. Best suited for high-pressure applications, such as in hydraulic systems, due to their robust construction.
6. What is the difference between a pressure switch, a pressure transmitter, and a pressure transducer?
While related, these devices have distinct functions:
7. What are “Normally Open” (NO) and “Normally Closed” (NC) contacts in a pressure switch?
This refers to the state of the electrical contacts when the switch is in its “normal” or unactuated state (i.e., when the pressure is below the setpoint):
- Normally Open (NO): The electrical circuit is open, and no current flows. When the setpoint pressure is reached, the contacts close, completing the circuit.
- Normally Closed (NC): The electrical circuit is closed, and current flows. When the setpoint pressure is reached, the contacts open, breaking the circuit.
The choice between NO and NC depends on the desired control action (e.g., to energize or de-energize a device at the setpoint).
8. What is “deadband” in the context of a pressure switch?
“Deadband” is another term for the differential or hysteresis. It is the range of pressure between the actuation point (setpoint) and the de-actuation point (reset point). A deadband is essential to prevent rapid cycling or “chattering” of the switch if the pressure is hovering around the setpoint. This improves the stability of the system and prolongs the life of the switch and the controlled equipment.
9. What are some common applications of pressure switches?
Pressure switches are ubiquitous in various industries:
- HVAC Systems: To monitor and control refrigerant pressure, air pressure in ductwork, and water pressure in boilers.
- Well Pump Systems: To automatically turn the pump on and off to maintain water pressure in a storage tank.
- Air Compressors: To control the motor and maintain the desired air pressure in the receiver tank.
- Hydraulic Systems: To monitor and control hydraulic pressure for safety and operational purposes.
- Pneumatic Systems: To control the flow of compressed air.
- Industrial Processes: For alarm and shutdown functions to protect equipment from over-or under-pressure conditions.
10. How would you troubleshoot a well pump pressure switch that is not working correctly?
Troubleshooting steps would include:
- Safety First: Disconnect power to the pump.
- Check for Power: Verify that the switch is receiving power.
- Inspect Contacts: Visually inspect the electrical contacts for signs of burning, pitting, or welding. Clean or replace if necessary.
- Check the Sensing Tube: Ensure the small tube leading to the pressure switch is not clogged with sediment or debris.
- Verify Pressure Tank Pre-charge: The air pressure in the bladder tank should be 2 PSI below the cut-in pressure of the switch.
- Test the Switch Operation: Carefully, with power restored, monitor the pressure gauge as the system operates to see if the switch cuts in and cuts out at the correct pressures.
11. What are the key safety precautions to take when working with pressure switches?
- Disconnect Power: Always de-energize the circuit before opening the cover of a pressure switch. Use a multimeter to verify there is no voltage.
- Depressurize the System: Release the pressure from the system before removing or installing a pressure switch.
- Wear Personal Protective Equipment (PPE): This may include safety glasses, gloves, and other appropriate gear.
- Follow Lockout/Tagout (LOTO) Procedures: Ensure that the equipment cannot be accidentally re-energized while you are working on it.
- Use the Right Tools: Use insulated tools when working with electrical components.
12. What is the difference between a fixed and an adjustable differential pressure switch?
- Fixed Differential: The pressure difference between the cut-out and cut-in points is pre-set by the manufacturer and cannot be changed. These are simpler and often used in applications where a specific, non-critical differential is acceptable.
- Adjustable Differential: These switches have a separate adjustment screw that allows the user to change the differential. This provides more flexibility to fine-tune the system’s performance and prevent issues like short cycling.
13. Can a pressure switch be used for safety applications?
Yes, pressure switches are frequently used in safety-critical applications. For example:
- High-Pressure Cutout: To shut down a system if the pressure exceeds a safe operating limit, preventing equipment damage or a hazardous situation.
- Low-Pressure Cutout: To shut down a system if the pressure drops below a safe level, which could indicate a leak or a loss of lubrication.
In these cases, the switch is often part of a safety interlock system.
14. What factors should you consider when selecting a pressure switch for a specific application?
Key selection criteria include:
- Pressure Range: The switch’s operating range must be suitable for the application’s normal and maximum pressures.
- Fluid Compatibility: The materials of the wetted parts (sensing element, port) must be resistant to corrosion or degradation from the process fluid.
- Temperature Range: The switch must be able to operate reliably within the process and ambient temperature ranges.
- Electrical Rating: The switch’s contacts must be able to handle the voltage and current of the controlled circuit.
- Enclosure Type: The housing should be appropriate for the environment (e.g., waterproof, explosion-proof).
- Accuracy and Repeatability: The required precision of the switching point.
15. What is an SPDT pressure switch and what is its advantage?
An SPDT (Single Pole Double Throw) pressure switch has three electrical terminals: a common (C), a normally open (NO), and a normally closed (NC).
Advantage: This configuration provides greater flexibility. It allows you to control two different circuits with a single switch. For example, you could use the NO contact to turn on an alarm and the NC contact to turn off a motor simultaneously when the pressure reaches the setpoint.
16. What can cause a pressure switch to “chatter” or cycle rapidly?
Chattering is usually caused by:
- A very small differential (deadband): The switch is too sensitive to minor pressure fluctuations.
- Vibrations: Mechanical vibrations can cause the switch contacts to bounce.
- Pressure Pulsations: Rapid fluctuations in the process pressure.
- Improper Sizing: A switch that is not correctly matched to the system’s characteristics.
17. How does an electronic pressure switch differ from a mechanical one?
18. What is the purpose of a snubber in a pressure switch installation?
A pressure snubber is a device installed at the inlet of a pressure switch to dampen the effects of pressure spikes, surges, and pulsations. It helps to protect the delicate sensing element from damage and ensures a more stable and accurate pressure reading, thereby improving the reliability and lifespan of the switch.
19. Can you use a pressure switch to measure vacuum?
Yes, specific types of pressure switches, often called vacuum switches or compound pressure switches, are designed to operate in a vacuum (negative pressure) range. They work on the same principle as standard pressure switches but are calibrated to actuate at specific vacuum levels.
20. Why might a new pressure switch not work after installation?
Common reasons for a new pressure switch failing to operate correctly include:
- Incorrect Wiring: The switch may be wired to the wrong terminals (NO instead of NC, or vice versa).
- Improper Settings: The cut-in and cut-out pressures may be set outside the system’s operating range.
- Blocked Inlet: The inlet port or the pipe leading to it might be blocked.
- Defective Switch: Although less common with new units, the switch itself could be faulty from the factory.
- System Issues: The problem may not be with the switch but with another component in the system (e.g., a faulty pump, a leak).