Common Mistakes and Errors in Pressure Measurement: Top 25 Interview Q&A
Navigating the nuances of pressure measurement is a critical skill in numerous industries, from process control and manufacturing to aerospace and pharmaceuticals. A thorough understanding of the potential pitfalls and how to avoid them is often a key focus in technical interviews. Here are the top 25 commonly asked interview questions and answers regarding mistakes and errors in pressure measurement.
Category 1: Installation and Setup Errors
1. Q: What is one of the most common installation mistakes when fitting a pressure transmitter?
A: Overtightening the transmitter into the process connection is a frequent and costly error. This can cause zero shifts, linearity problems, or even permanent damage to the diaphragm. Always follow the manufacturer’s recommended torque specifications.
2. Q: How can the orientation of a pressure transmitter lead to measurement errors?
A: In liquid applications, if a transmitter is mounted above the pressure tap, gas bubbles can get trapped in the impulse piping, leading to inaccurate low readings. Conversely, in gas applications, mounting the transmitter below the tap can allow liquids to accumulate, causing erroneous high readings. Proper mounting orientation is crucial for accurate and stable measurements.
3. Q: What is the purpose of impulse lines and what are common errors associated with them?
A: Impulse lines are the pipes that connect the process to the pressure instrument. Common errors include using lines that are too long or have a small diameter, which can cause delays in pressure response. Leaks in the impulse lines are also a major source of error, leading to consistently low-pressure readings.
4. Q: Why is proper sealing important and what mistakes are common?
A: Proper sealing with gaskets or thread sealant prevents process fluid from leaking. A common mistake is using incompatible sealing materials that can be corroded by the process media, leading to leaks and measurement failure. Another error is improper application of Teflon tape, which can clog the pressure port if not wrapped correctly.
5. Q: What is the significance of the “wet leg” vs. “dry leg” in differential pressure measurement, and what errors can occur?
A: In differential pressure (DP) level measurement, a “wet leg” is an impulse line filled with a reference fluid, while a “dry leg” is filled with the process vapor. A common error in wet leg systems is a change in the reference fluid’s density due to temperature fluctuations, which directly impacts the measurement accuracy. In dry leg systems, condensation in the line can lead to significant errors.
Category 2: Incorrect Equipment Selection
6. Q: How does selecting a pressure transmitter with the wrong range affect measurement?
A: Choosing a transmitter with a range that is too wide for the application will result in poor accuracy and resolution. Conversely, a range that is too narrow can lead to over-pressurization, which can damage the sensor and cause a permanent shift in its output. As a rule of thumb, the normal operating pressure should be between 25% and 75% of the sensor’s range.
7. Q: What is the consequence of ignoring the process media’s chemical compatibility with the sensor?
A: This is a critical and dangerous oversight. If the wetted parts of the pressure sensor are not chemically compatible with the process fluid, it can lead to corrosion, sensor failure, and a potential loss of containment, posing significant safety and environmental risks.
8. Q: When is it a mistake to use a gauge pressure transmitter instead of an absolute pressure transmitter?
A: It’s a mistake to use a gauge pressure transmitter for applications sensitive to atmospheric pressure changes, such as in low-pressure measurements or processes that are close to a vacuum. Gauge pressure references the local atmospheric pressure, so any change in weather can introduce errors. Absolute pressure transmitters reference a full vacuum, providing a more stable reading in such scenarios.
9. Q: Can you explain the difference between static and dynamic pressure and why choosing the right sensor is important?
A: Static pressure is the pressure of a fluid at rest, while dynamic pressure is the pressure exerted by a fluid in motion. A common mistake is using a standard pressure sensor, designed for static or slowly changing pressures, in an application with rapid pressure fluctuations or pulsations. This can lead to premature sensor failure. For dynamic applications, a sensor with a higher frequency response and robust construction is necessary.
10. Q: What problems can arise from not considering the process temperature?
A: Every pressure sensor has a specified operating temperature range. Exceeding this range can cause zero and span shifts, leading to inaccurate readings. In extreme cases, high temperatures can damage the sensor’s electronics or fill fluid. Using a diaphragm seal with a capillary can isolate the transmitter from high-temperature processes.
Category 3: Environmental and Process-Related Errors
11. Q: How can vibration affect pressure measurement?
A: High vibration environments can cause the internal components of a pressure transmitter to resonate, leading to noisy and inaccurate output signals. Over time, it can also cause mechanical fatigue and failure. Remote mounting of the sensor or using vibration dampeners can mitigate this issue.
12. Q: What is the effect of temperature changes on the measurement accuracy, and how is it compensated?
A: Temperature fluctuations can cause the sensor’s diaphragm and the fill fluid to expand or contract, leading to zero and span shifts. Modern “smart” transmitters have built-in temperature sensors that actively compensate for these effects to maintain accuracy over a wide temperature range.
13. Q: What are pressure pulsations and how can they lead to errors?
A: Pressure pulsations are rapid increases and decreases in pressure. They can cause the instrument’s pointer or digital display to fluctuate wildly, making it difficult to get a stable reading. More importantly, they can cause excessive wear and premature failure of the sensing element. A pressure snubber or damper can be installed to smoothen out these pulsations.
14. Q: Explain the “water hammer” effect and its impact on pressure sensors.
A: Water hammer is a pressure surge caused when a fluid in motion is forced to stop or change direction suddenly. This can generate a pressure spike that is many times the normal operating pressure, leading to catastrophic failure of the pressure sensor. Proper valve sequencing and the use of surge arrestors can prevent water hammer.
15. Q: How can electromagnetic interference (EMI) or radio frequency interference (RFI) affect pressure measurement?
A: EMI/RFI from nearby motors, VFDs, or radio transmitters can induce noise in the sensor’s electrical signal, leading to erroneous readings or a complete loss of signal. Proper grounding, shielded cables, and the use of transmitters with good EMI/RFI immunity are essential in such environments.
Category 4: Calibration and Maintenance Errors
16. Q: What is a common mistake made during the calibration of a pressure transmitter?
A: A frequent error is not exercising the sensor before calibration. This involves cycling the pressure from zero to the full-scale value and back a few times. This helps to stabilize the output and ensures a more accurate calibration. Another mistake is performing the calibration at a different temperature than the process operating temperature without considering the temperature effects.
17. Q: Why is it a mistake to only perform a zero-point calibration?
A: A zero-point calibration only corrects for errors at the zero-pressure point. It does not account for span errors, which are inaccuracies in the slope of the output signal. A full two-point calibration (zero and span) is necessary to ensure accuracy across the entire measurement range.
18. Q: What are the consequences of not having a regular calibration schedule?
A: Over time, all pressure instruments can experience drift in their output due to factors like aging, temperature cycling, and mechanical stress. Without regular calibration, these small drifts can accumulate, leading to significant measurement errors, which can impact product quality, process efficiency, and safety.
19. Q: What is meant by “turndown ratio” and how can it be misused?
A: The turndown ratio indicates how much the calibrated range of a transmitter can be reduced from its maximum range. For example, a 100:1 turndown on a 1000 psi transmitter means it can be accurately calibrated to a range of 10 psi. A common mistake is using an excessive turndown, which can significantly reduce the accuracy of the measurement. It’s always best to choose a transmitter with a range that is as close as possible to the required measurement range.
20. Q: What is a common error when troubleshooting a pressure transmitter that is reading incorrectly?
A: A common pitfall is immediately assuming the transmitter is faulty. The issue could be with the process itself, the impulse piping (leaks or blockages), the power supply, or the receiving device. A systematic approach to troubleshooting, starting from the process and moving towards the instrument, is crucial.
Category 5: Fundamental Knowledge and Best Practices
21. Q: What is the difference between accuracy and precision in the context of pressure measurement?
A: This is a fundamental concept often misunderstood. Accuracy refers to how close a measurement is to the true value. Precision refers to the reproducibility of the measurement, i.e., how close multiple measurements are to each other. A sensor can be precise but not accurate. The goal is to have both.
22. Q: Why is it a mistake to ignore the ingress protection (IP) rating of a pressure transmitter?
A: The IP rating indicates the level of protection an enclosure provides against the ingress of solid objects (like dust) and liquids (like water). Using a transmitter with an inadequate IP rating in a wet or dusty environment will lead to premature failure of the electronics.
23. Q: What is the importance of “zeroing” a transmitter in the field, and when should it be done?
A: Zeroing the transmitter after installation corrects for any mounting position effects or static head pressure. It is a critical step to ensure accurate readings, especially in low-pressure applications. It should be done with the process at a known (usually atmospheric) pressure.
24. Q: Can you describe a scenario where a non-contacting pressure measurement method would be preferable and why using a contacting type would be a mistake?
A: In applications involving highly corrosive, viscous, or abrasive fluids, using a traditional contacting pressure sensor would be a mistake as it would quickly get damaged or clogged. In such cases, a non-contacting method, like using a diaphragm seal with an appropriate fill fluid, is the correct approach to isolate the sensor from the harsh process media.
25. Q: What is a common mistake when interpreting pressure readings in a flowing system?
A: A common error is not understanding the difference between static pressure and total pressure (static + dynamic pressure). A pressure sensor installed perpendicular to the flow will measure static pressure. If the sensor opening faces the flow (like in a Pitot tube), it will measure total pressure. Misinterpreting which pressure is being measured can lead to significant errors in process control or flow calculations.