1. What is the difference between absolute, gauge, and differential pressure?

• Absolute Pressure: Measured relative to a perfect vacuum (zero pressure).
• Gauge Pressure: Measured relative to the local atmospheric pressure. It can be positive or negative (vacuum).
• Differential Pressure: The difference in pressure between two separate points.

2. What is the working principle of a Bourdon tube pressure gauge?

A Bourdon tube is a C-shaped, sealed tube that is flattened on one side. When pressure is applied to the inside of the tube, it tries to straighten out. This movement is mechanically amplified through a system of levers and gears to move a pointer on a calibrated scale, indicating the pressure.

3. What is a diaphragm seal and why is it used?

A diaphragm seal (or chemical seal) is a flexible membrane that isolates the pressure measuring instrument from the process fluid. It is used to protect the instrument from corrosive, viscous, or very hot/cold fluids. The pressure is transferred through a non-compressible fill fluid to the instrument.

4. What is a pressure transducer?

A pressure transducer is a device that converts applied pressure into an electrical signal. The output signal is typically a voltage (e.g., 0-5V) or current (e.g., millivolts) that is directly proportional to the pressure. It does not typically have integrated signal conditioning.

5. What is a pressure transmitter?

A pressure transmitter is a more complex device that includes a pressure transducer and signal conditioning electronics. It converts the pressure into a standardized industrial signal, most commonly a 4-20 mA current loop, which is robust against electrical noise and can be transmitted over long distances.

6. What is a Dead Weight Tester and what is its purpose?

A Dead Weight Tester (DWT) is a primary calibration standard for pressure instruments. It works on the principle of P = F/A (Pressure = Force/Area). A known mass (force) is applied to a piston of a known area to generate a very precise pressure, which is then used to calibrate other pressure devices like gauges and transmitters.

7. How do you select a pressure gauge for an application?

Key factors include:

• Pressure Range: The gauge's range should be about twice the normal operating pressure.
• Accuracy: The required precision for the measurement.
• Process Fluid Compatibility: The wetted parts must resist corrosion from the fluid.
• Operating Temperature: The gauge must be able to withstand the process temperature.
• Environment: Consider vibration, humidity, and dust.

8. What is a pulsation dampener (or snubber)?

A pulsation dampener is a device installed before a pressure gauge to smooth out rapid pressure fluctuations or pulses, such as those from a reciprocating pump. It protects the gauge's delicate internal mechanism from damage and makes the pointer easier to read by averaging out the pressure spikes.

9. What are common causes of pressure gauge failure?

Common causes include overpressure, vibration, extreme temperatures, corrosion of the sensing element, and clogging of the pressure port. Mechanical shock (like dropping the gauge) can also cause immediate failure or loss of calibration.

10. Explain the principle of a piezoelectric pressure sensor.

A piezoelectric sensor uses a crystal (like quartz) that generates an electrical charge when it is mechanically stressed. When pressure is applied to the crystal, it deforms slightly and produces a voltage proportional to the applied pressure. These sensors are excellent for measuring dynamic or rapidly changing pressures.

11. What are the main types of pressure sensing elements?

Besides the Bourdon tube, other common types are:

• Diaphragm: A flexible, circular membrane that deflects under pressure.
• Bellows: A thin-walled, corrugated metal tube that expands and contracts with pressure changes.
• Piezoresistive/Strain Gauge: A material whose electrical resistance changes when it is stretched or compressed by pressure.

12. Why is a 4-20 mA signal preferred for pressure transmitters?

The 4-20 mA current loop is preferred because it's highly resistant to electrical noise, can be transmitted over long distances without signal loss, and is intrinsically safe. The "live zero" of 4 mA allows the system to distinguish between a true zero pressure reading (4 mA) and a broken wire or fault condition (0 mA).

13. What does "turndown ratio" mean for a pressure transmitter?

Turndown ratio (or rangeability) is the ratio of the maximum pressure the transmitter can be configured to measure (Upper Range Limit) to the minimum pressure span it can be set to. A high turndown ratio (e.g., 100:1) means a single transmitter can be accurately used for a wide variety of pressure ranges, reducing inventory needs.

14. How is a pressure gauge typically calibrated?

Calibration involves comparing the gauge's reading to a known, accurate pressure source (like a dead weight tester or a high-precision digital calibrator). The gauge is checked at several points, typically 0%, 25%, 50%, 75%, and 100% of its range, both increasing and decreasing. Adjustments are made to the gauge's mechanism to minimize any errors found.

15. What is the purpose of filling a pressure gauge case with liquid (like glycerin)?

Liquid filling serves two main purposes:

• It dampens the effect of mechanical vibration, protecting the internal components and making the pointer steady.
• It lubricates the moving parts, reducing wear and extending the life of the gauge.

16. What is a vacuum pressure measurement?

Vacuum is pressure that is below the local atmospheric pressure. It is often measured as a negative gauge pressure (e.g., -5 psig) or as an absolute pressure (e.g., 10 psia). A perfect vacuum is zero absolute pressure.

17. What is a compound gauge?

A compound gauge is a type of gauge that can measure both positive pressure and vacuum (negative pressure) on the same scale. The scale typically has zero in the middle or at the standard atmospheric pressure point.

18. What is a smart pressure transmitter?

A smart transmitter has an onboard microprocessor that allows for two-way communication with a control system or handheld communicator (using protocols like HART or Foundation Fieldbus). This enables remote diagnostics, configuration, and calibration, and generally provides higher accuracy and stability.

19. What is overpressure protection?

Overpressure protection refers to methods used to prevent a pressure instrument from being exposed to pressure exceeding its maximum limit. This can be achieved with pressure relief valves, rupture discs, or by selecting a gauge with a much higher range than the expected maximum pressure.

20. What safety precautions are necessary when working with high-pressure systems?

Key precautions include:

• Always depressurize the system before installing or removing any instrument (LOTO).
• Use correctly rated fittings and tubing for the pressure involved.
• Never stand in front of a pressure gauge when pressurizing a system for the first time.
• Wear appropriate PPE, including safety glasses or a face shield.