Pressure Transmitter – Top 25 Commonly asked Interview Questions and Answers

Top 25 Commonly Asked Pressure Transmitter Interview Questions and Answers

For anyone preparing for a role in instrumentation and control, a thorough understanding of pressure transmitters is crucial. These devices are fundamental to process control in a vast array of industries. Here is a comprehensive list of the top 25 most frequently asked interview questions about pressure transmitters, complete with detailed answers to help you ace your next technical interview.

1. What is a pressure transmitter and what is its primary function?

A pressure transmitter is an instrument that measures the pressure of a fluid (liquid or gas) in a process and converts it into a standardized electrical signal that can be understood by a control system. Its primary function is to provide a continuous and reliable indication of the process pressure to a control room, PLC (Programmable Logic Controller), or DCS (Distributed Control System) for monitoring, control, and safety purposes.

2. What are the different types of pressure measured by transmitters?

There are three main types of pressure measurements:

• Gauge Pressure (psig): This is the pressure measured relative to the local atmospheric pressure. It is the most common type of pressure measurement.
• Absolute Pressure (psia): This is the pressure measured relative to a perfect vacuum (absolute zero pressure). It is used in applications where atmospheric pressure variations would affect the process, such as in vacuum applications or at high altitudes.
• Differential Pressure (psid): This is the difference in pressure between two separate points. It is widely used for measuring flow, level, and filter differential pressure.

3. Explain the working principle of a pressure transmitter.

A pressure transmitter works by using a pressure-sensitive element to detect the process pressure and a transducer to convert this physical measurement into an electrical signal. The process pressure exerts a force on a diaphragm or sensor. This force causes a physical change in the sensing element, such as a change in resistance (strain gauge), capacitance, or resonant frequency. This change is then converted by the transmitter’s electronics into a proportional electrical output, typically a 4-20 mA analog signal or a digital signal (like HART or Foundation Fieldbus).

4. What are the common output signals of a pressure transmitter?

The most common output signal is a 4-20 mA analog current loop. This is a robust and widely used standard in the industry for several reasons:

• It is less susceptible to noise than voltage signals over long distances.
• A “live zero” of 4 mA allows for easy fault detection (a 0 mA signal indicates a loop break or transmitter failure).
• It can power the transmitter itself (in a 2-wire configuration).

Other common output signals include 1-5 VDC or digital protocols like HART (Highway Addressable Remote Transducer), Foundation Fieldbus, and Profibus PA.

5. What is a 2-wire, 3-wire, and 4-wire transmitter?

• 2-wire transmitter: The most common type. It is loop-powered, meaning the same two wires are used for both the power supply and the 4-20 mA output signal. This simplifies wiring and reduces costs.
• 3-wire transmitter: Uses one wire for power, one for the signal output, and a common wire for both.
• 4-wire transmitter: Uses two separate wires for the power supply and two separate wires for the signal output. This is typically used for transmitters with higher power consumption or those that provide a voltage output.

6. What are the different sensing technologies used in pressure transmitters?

Several technologies are used to sense pressure. Some of the most common include:

• Piezoresistive (Strain Gauge): A change in pressure deforms a diaphragm, which in turn changes the resistance of a strain gauge bonded to it. This is a very common and reliable technology.
• Capacitive: Pressure applied to a diaphragm changes the distance between two capacitor plates, thus altering the capacitance. These are known for their high accuracy and overpressure protection.
• Piezoelectric: Certain crystalline materials generate an electrical charge when subjected to mechanical stress. The amount of charge is proportional to the applied pressure. This type is suitable for measuring dynamic pressures.
• Resonant Frequency: The pressure is applied to a resonating element, and the change in pressure alters the resonant frequency. This is a highly accurate and stable method.

7. What do the terms LRV and URV stand for?

• LRV (Lower Range Value): The lowest pressure value that the transmitter is calibrated to measure, which corresponds to the 4 mA output signal.
• URV (Upper Range Value): The highest pressure value that the transmitter is calibrated to measure, which corresponds to the 20 mA output signal.

The difference between the URV and LRV is the span of the transmitter.

8. What is the difference between turndown ratio and rangeability?

• Turndown Ratio: It is the ratio of the maximum span to the minimum span for which the transmitter can be calibrated while maintaining its specified accuracy. For example, a transmitter with a maximum span of 100 bar and a turndown ratio of 100:1 can be accurately calibrated for a minimum span of 1 bar.
• Rangeability: This term is often used interchangeably with turndown ratio but can also refer to the ratio of the maximum to the minimum flow that a differential pressure flowmeter can accurately measure.

9. Why is calibration of a pressure transmitter important?

Calibration is essential to ensure the accuracy and reliability of the pressure transmitter. Over time, factors like temperature fluctuations, vibration, and aging of components can cause the transmitter’s output to drift from its calibrated values. Regular calibration against a known standard ensures that the transmitter provides accurate readings, which is critical for process control, safety, and product quality.

10. How do you perform a 3-point calibration on a pressure transmitter?

A 3-point calibration checks the transmitter’s accuracy at 0%, 50%, and 100% of its calibrated range. The steps are as follows:

1. Isolate the transmitter from the process and depressurize it.
2. Connect a pressure calibrator and a multimeter (to measure the mA output) to the transmitter.
3. Apply the LRV (0%) pressure and adjust the “Zero” setting until the output is 4 mA.
4. Apply the URV (100%) pressure and adjust the “Span” setting until the output is 20 mA.
5. Apply the 50% pressure and check if the output is 12 mA. If not, minor adjustments to zero and span may be needed.
6. Repeat the process to ensure linearity and accuracy across the range.

11. What is a HART communicator and how is it used with a pressure transmitter?

A HART (Highway Addressable Remote Transducer) communicator is a handheld device used to configure, calibrate, and diagnose smart transmitters that use the HART protocol. It communicates with the transmitter over the same 4-20 mA wiring without interfering with the analog signal. With a HART communicator, a technician can:

• Set the LRV, URV, and other configuration parameters.
• Perform zero and span adjustments.
• Read diagnostic information about the transmitter’s health.
• View the digital process variable alongside the analog output.

12. Explain the concept of Zero Suppression and Zero Elevation.

These terms are relevant when using a differential pressure transmitter for level measurement in a closed tank.

• Zero Suppression: This is used when the transmitter is mounted below the bottom tap of the vessel. The liquid in the high-pressure impulse line creates a hydrostatic head that needs to be “suppressed” or compensated for so that the transmitter reads zero when the tank is empty.
• Zero Elevation: This is used in applications with a wet leg on the low-pressure side (e.g., measuring the level of a liquid that can vaporize). The hydrostatic pressure of the liquid in the wet leg needs to be compensated for to ensure an accurate level reading.

13. How would you select a pressure transmitter for a specific application?

The selection of a pressure transmitter depends on several factors:

• Process Fluid: Its chemical compatibility with the wetted parts of the transmitter.
• Pressure Range: The normal operating pressure and the maximum possible pressure.
• Temperature: The operating temperature of the process fluid and the ambient temperature.
• Accuracy Requirement: The level of precision needed for the measurement.
• Environment: Whether the area is hazardous (requiring explosion-proof or intrinsically safe certification).
• Output Signal: Compatibility with the existing control system.
• Process Connection: The type and size of the connection to the process piping.

14. What is a diaphragm seal and when is it used?

A diaphragm seal (or remote seal) is a flexible membrane that separates the pressure transmitter from the process fluid. It is used in applications where:

• The process fluid is corrosive, highly viscous, or contains solids that could clog the transmitter’s impulse lines.
• The process temperature is too high or too low for the transmitter’s operating limits.
• The process fluid is prone to freezing or polymerization in the impulse lines.
• Sanitary connections are required, such as in the food and beverage industry.

The pressure is transferred from the process fluid through the diaphragm and a fill fluid in a capillary tube to the transmitter’s sensor.

15. How can a differential pressure (DP) transmitter be used to measure liquid level?

A DP transmitter measures the hydrostatic pressure exerted by a column of liquid to determine the level.

• Open Tank: The high-pressure (HP) side of the transmitter is connected to the bottom of the tank, and the low-pressure (LP) side is left open to the atmosphere. The measured pressure is directly proportional to the liquid level.
• Closed Tank: The HP side is connected to the bottom of the tank, and the LP side is connected to the top of the tank to compensate for the pressure of the gas or vapor above the liquid.

16. How can a DP transmitter be used to measure flow?

A DP transmitter can measure flow by being installed across a primary flow element, such as an orifice plate, venturi tube, or flow nozzle. These elements create a pressure drop that is proportional to the square of the flow rate. The DP transmitter measures this pressure difference, and the flow rate can be calculated using Bernoulli’s equation.

17. What are some common causes of pressure transmitter failure or inaccurate readings?

• Clogging of impulse lines: This is a common issue in processes with slurries or particulate matter.
• Leaks in impulse lines: This will lead to incorrect differential pressure readings.
• Incorrect calibration: The transmitter is not set up correctly for the desired range.
• Overpressure damage: Exposing the transmitter to a pressure beyond its limit can damage the sensor.
• Moisture or corrosion in the electronic housing.
• Power supply issues.
• Grounding problems leading to electrical noise.

18. What is the purpose of a 3-valve or 5-valve manifold?

A valve manifold is used to isolate, bleed, and equalize the pressure to the transmitter for maintenance and calibration without shutting down the process.

• 3-Valve Manifold: Has two block valves (one for the high-pressure side and one for the low-pressure side) and one equalizing valve.
• 5-Valve Manifold: In addition to the three valves of a 3-valve manifold, it has two more valves for bleeding or venting the trapped pressure.

19. How would you troubleshoot a pressure transmitter that is reading a constant high or low value?

1. Check for process issues: Is the actual process pressure indeed high or low?
2. Isolate the transmitter: Use the manifold to isolate the transmitter from the process.
3. Check for trapped pressure: Bleed the pressure from the transmitter.
4. Perform a zero check: With no pressure applied, the output should be 4 mA. If it’s not, the transmitter may need recalibration or could be faulty.
5. Inspect impulse lines: Check for any blockages or leaks.
6. Verify power supply and wiring.
7. If the issue persists, the transmitter may need to be replaced.

20. What is meant by the “wetted parts” of a pressure transmitter?

The wetted parts are all the components of the transmitter that come into direct contact with the process fluid. This typically includes the diaphragm and the process connection. It is crucial to select materials for the wetted parts that are chemically compatible with the process fluid to prevent corrosion and ensure the longevity of the transmitter. Common materials include stainless steel, Hastelloy C, Monel, and Tantalum.

21. What is the difference between accuracy and repeatability?

• Accuracy: How close a measured value is to the true or actual value.
• Repeatability: The ability of a transmitter to produce the same output for repeated applications of the same input pressure under the same conditions.

A transmitter can be repeatable but not accurate. For example, it might consistently read 10.5 psi for a true pressure of 10 psi.

22. What is the purpose of damping in a pressure transmitter?

Damping is a feature that allows the user to filter out unwanted noise or rapid fluctuations in the pressure signal. By increasing the damping value, the transmitter’s response time is slowed down, resulting in a more stable output reading. This is useful in applications with pulsating flow or pressure spikes.

23. What are Intrinsically Safe (IS) and Explosion-Proof (XP) pressure transmitters?

These are two methods of protecting equipment for use in hazardous areas where flammable gases or dust may be present.

• Intrinsically Safe (IS): This approach limits the electrical and thermal energy that the transmitter can produce to a level below that which can cause ignition of a hazardous atmosphere. This is achieved through the use of an IS barrier.
• Explosion-Proof (XP) or Flameproof: This method involves enclosing the transmitter in a housing that is robust enough to contain an internal explosion and prevent it from igniting the surrounding hazardous atmosphere.

24. What is a multivariable transmitter?

A multivariable transmitter is a single device that can measure multiple process variables simultaneously. A common example is a multivariable DP transmitter that measures differential pressure, static pressure, and process temperature. This allows for a more accurate calculation of mass flow or compensated gas flow in a single instrument.

25. What is the significance of the ingress protection (IP) rating of a pressure transmitter?

The IP rating (Ingress Protection rating) classifies the degree of protection provided by the transmitter’s enclosure against the intrusion of solid objects (like dust) and liquids (like water). The first digit indicates protection against solids, and the second digit indicates protection against liquids. A higher number signifies a greater level of protection. For example, an IP67 rating means the transmitter is completely dust-tight and can be submerged in water up to 1 meter deep for a limited time. This is an important consideration for transmitters installed in harsh outdoor or wash-down environments.