Questions for Advanced Instrument Technician (Short Answer Questions)

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Short answer questions: 1. Why does the electro-hydraulic slide valve always move in one direction? Answer: When the spool moves slowly in the open or closed direction, the servo valve coil is disconnected ； When the spool moves rapidly in the open or closed direction, one side of the servo valve 2. What determines the ideal flow characteristic of the control valve? What determines the characteristics of work traffic? Answer: The ideal flow characteristic depends on the valve core shape. The working flow characteristics depend on the valve core shape and piping conditions. 3. How does the output of the locator change when the feedback rod connecting the locator to the actuator stem comes loose? Answer: When the feedback rod that connects the positioner to the valve stem of the control valve comes loose, the positioner loses its feedback and becomes a pneumatic amplifier with a high amplification factor. If the actuator is positive-acting, that is, as the signal increases, the output also increases, the valve stem comes loose and the output reaches its maximum value. If it is a reaction, it runs to zero. 4. Explain the measurement principle of differential pressure flow meters. Answer: A differential pressure flow meter is based on the throttling principle of fluid flow; it measures flow rate by utilizing the pressure difference generated when fluid passes through a throttling device. 5. What is the meaning of the proportional control law? Answer: Proportional control operates based on the ‘magnitude of the deviation’ ； Its output deviation is proportional to the size of the input deviation. 6. What is the meaning of the integral control law? Answer: Integral control operates based on whether a ‘deviation exists’ ； Its output is proportional to the time integral of the deviation. 7. What is the meaning of the differential regulation law? Answer: Differential regulation operates based on the ‘rate of change of the deviation’ ； Its output is proportional to the rate of change of the input deviation. 8. What is the regulation of the production process? Answer: To maintain the specified values of the parameters, it is necessary to control the manufacturing process in order to eliminate such deviations and bring the parameters back to their specified values. This process of control is known as regulation of the production process. 9. What should be considered when selecting the installation location for the measuring element? Answer: The installation location of the measuring element should be chosen carefully to minimize the time of pure lag as much as possible. 10. What is the control point of a PI regulator? What does deviation of control points mean? Answer: The control point is the point where the measured value coincides with the given value, that is, when the deviation is zero; this coinciding point is the control point of the proportional-integral regulator. Control point deviation refers to the difference between the measured value and the set value when the regulator output remains stable. 11. Explain the meaning of the legend symbols in the following relay wiring diagram? TDR ① ──┤├── ② ──┤├── Answer: ① normally open contact; ② normally open contact with a timing function. 12. What are the components of the shaft monitoring system? Answer: A conventional instrument status monitoring system generally consists of sensors (probes) fixed near the rotating shaft, extension cables, preamplifiers, signal cables, and a status monitor. 13. What are the common methods for installing probes? Answer: There are two. One is a mechanical gap measurement installation method, and the other is an electrical gap measurement installation method. 14. What are the characteristics of a cascade control system? Answer: (1) The system has two regulators connected in series, a primary regulator and a secondary regulator; the setpoint of the secondary regulator is the output value of the primary regulator ； ⑵The system has two control loops, a primary one and a secondary one, and possesses strong interference resistance. 15. Explain the meaning of the following symbols in microcomputer control systems. ①CPU ②CRT ③ROM ④CB Answer: ①CPU – Central Processing Unit; ②CRT – Cathode Ray Tube display; ③ROM – Read-Only Memory; ④CB – Control Bus. 16. Select the appropriate on-air and off-air type for the control valve. ①Flow control valve for reactor feed ② Catalyst feeding control valve ③ Control valve for the cooling fluid in the heat exchanger (water) ④ Pressure control valve at the top of the distillation tower. Answers: ① Air-operated open type ② Air-operated open type ③ Air-operated closed type ④ Air-operated closed type. 17. What is split-range control? Answer: Cascade control refers to a controller that simultaneously controls two or more control valves, with each valve operating within a certain signal range provided by the controller, in accordance with the requirements of the process. 18. What are the usual components of an interlocking circuit? Answer: An interlocking circuit usually consists of an input section, an output section, and a logic section. 19. What are the advantages of DCS compared to conventional instruments? Answer: 1. Rich functionality 2. High system reliability 3. Centralized monitoring, operation, and management 4. Good cost-performance ratio 5. Low installation effort 6. Modular hardware design 7. Easy to modify control schemes. 20. For the 1751 dual-flange transmitter, what do the symbols “Z” and “R” mean? How to calibrate its zero point? Answer: “Z” represents zero point, and “R” represents the range. Close the positive and negative pressure tapping valves, and open the vent valve to perform zero-point adjustment. 21. What happens when the target plate of an electric target flow meter is stuck in the maximum position? Answer: The flow meter indicates maximum. 22. When adjusting a differential pressure transmitter, do the zero point and range affect each other? Answer: When adjusting the range, the force balance of the instrument is altered, which affects the zero point. When adjusting the zero point, it generally does not affect the range. 23. The feedback rod of the control valve positioner (reaction type) is broken; what is the output? Answer: The output is minimized. 24. Thermal resistors are used to measure temperature; what happens to the readings on the instrument when the thermal resistor is short-circuited? What happens when the thermal resistor is open circuit? Answer: When the thermal resistor is short-circuited, the display instrument shows a negative value. When the thermal resistor is open-circuited, the display instrument shows infinity. 25. When adjusting a differential pressure transmitter, do the zero point and range affect each other? Answer: When adjusting the range, the force balance of the instrument is altered, which affects the zero point. When adjusting the zero point, it generally does not affect the range. 26. What is the working principle of a thermistor thermometer? What materials are commonly used to make thermal resistors? Answer: A thermistor thermometer utilizes the property that the resistance value of a conductor or semiconductor changes with temperature. Commonly used thermal resistors include platinum resistors and copper resistors. 27. When calibrating instruments, how many calibration points should be selected? Answer: 0%, 25%, 50%, 75%, and 100% of the range should be selected. 28. How to calibrate the zero point of a differential pressure transmitter for measuring liquid levels? Answer: First, close the positive and negative pressure tapping valves, fill the positive and negative pressure tubes with isolation fluid, and adjust the zero point to meet the requirements. 29. What are the operating conditions for a standard throttling device? Answer: 1. The medium under test should fill the entire pipe cross-section and flow continuously ； ⒉The flow pattern inside the pipe should be stable ； ⒊The medium under test should not undergo phase change when passing through the throttling device ； ⒋There should be sufficiently long straight sections of pipe before and after the throttling device. 30. What is the correct sequence for starting a caisson-type instrument? Why? Answer: First open the negative pressure valve, then open the positive pressure valve. Avoid excessive one-way pressure, which can wash away the sinker and prevent measurement. 31. What are the differences between smart meters and conventional meters? Answer: 1. High precision. 2. Digital transmission of the signal, providing strong resistance to interference. 3. Wide measurement range. 4. Self-diagnosis function. 32. When a thermistor is short-circuited, what will happen to the readings displayed by the instrument? What happens when the thermal resistor is open circuit? Answer: When the thermal resistor is short-circuited, the display instrument shows a negative value. When the thermal resistor is open-circuited, the display instrument shows infinity. 33. What is the working principle of a thermistor thermometer? What materials are commonly used to make thermal resistors? Answer: A thermistor thermometer utilizes the property that the resistance value of a conductor or semiconductor changes with temperature. Commonly used thermal resistors include platinum resistors and copper resistors. 34. For the 1751 type double-flange transmitter, what do the symbols “Z, R” mean? How to calibrate its zero point? Answer: “Z” represents zero point, and “R” represents the range. Close the positive and negative pressure tapping valves, and open the vent valve to perform zero-point adjustment. 35. What happens when the target disc of an electric target flow meter is stuck in the maximum position? Answer: The flow meter indicates maximum. 36. When adjusting a differential pressure transmitter, do the zero point and range affect each other? Answer: When adjusting the range, the force balance of the instrument is altered, which affects the zero point. When adjusting the zero point, it generally does not affect the range. 37. What is a voltage relay? Answer: A relay that operates in response to changes in coil voltage is called a voltage relay. 38. The feedback rod of the control valve positioner (reactive type) is broken; what is the output? Answer: The output is minimized. 39. Thermal resistors are used to measure temperature; what happens to the readings on the instrument when the thermal resistor is short-circuited? What happens when the thermal resistor is open circuit? Answer: When the thermal resistor is short-circuited, the display instrument shows a negative value. When the thermal resistor is open-circuited, the display instrument shows infinity. 40. When adjusting a differential pressure transmitter, do the zero point and range affect each other? Answer: When adjusting the range, the force balance of the instrument is altered, which affects the zero point. When adjusting the zero point, it generally does not affect the range. 41. What is the working principle of a thermistor thermometer? What materials are commonly used to make thermal resistors? Answer: A thermistor thermometer utilizes the property that the resistance value of a conductor or semiconductor changes with temperature. Commonly used thermal resistors include platinum resistors and copper resistors. 42. When calibrating instruments, how many calibration points should be selected? Answer: 0%, 25%, 50%, 75%, and 100% of the range should be selected. 43. How to calibrate the zero point of a differential pressure transmitter for measuring liquid levels? Answer: First, close the positive and negative pressure tapping valves, fill the positive and negative pressure tubes with isolation fluid, and adjust the zero point to meet the requirements. 44. In DDZ—III type instruments, why is a 4–20mA current signal used for communication between the field and the control room, while a 1–5V voltage signal is used in the control room? Answer: (1) The 4–20mA current signal is suitable for long-distance transmission between the field and the control room, as it helps to avoid errors caused by voltage drop losses in the transmission wires. ⑵1–5V voltage signals are suitable for short-distance communication within a control room; instruments can be connected in parallel, which facilitates their installation and maintenance. 45. What are the methods for tuning regulator parameters? Answer: The experienced method, the attenuation curve method, and the critical proportionality method. 46. What are the operating conditions for a standard throttling device? Answer: 1. The medium under test should fill the entire pipe cross-section and flow continuously ； ⒉The flow pattern inside the pipe should be stable ； ⒊The medium under test should not undergo phase change when passing through the throttling device ； ⒋There should be sufficiently long straight sections of pipe before and after the throttling device. 47. What is the correct sequence for starting a caisson-type instrument? Why? Answer: First open the negative pressure valve, then open the positive pressure valve. Avoid excessive one-way pressure, which can wash away the sinker and prevent measurement. 48. What are the differences between smart meters and conventional meters? Answer: 1. High precision. 2. Digital transmission of signals, providing strong resistance to interference. 3. Wide measurement range. 4. Self-diagnosis function. 5. Easy calibration. 49. What is PV tracking? Answer: PV tracking refers to the SP value changing in accordance with the PV value in manual control mode. 50. How are the reactions at the control points defined? Answer: When the input deviation signal of the control point increases, the output of the control point also increases, which is referred to as positive action ； Conversely, a situation in which the output signal of the control point decreases as the deviation increases is called feedback. V. Essay Questions: 1. The output of the thermocouple is unstable; list at least three possible reasons. Answer: ① Whether the thermoelectric electrode is broken or not. ② Poor contact between the thermocouple terminals and the thermoelectric electrode. ③ Intermittent short circuits or grounding in the thermocouple measurement circuit. ④ External interference (such as electromagnetic induction). 2. Why is cold-junction compensation necessary when using a thermocouple for temperature measurement? Answer: ① The magnitude of the thermocouple’s electromotive force is related to the temperatures at its two ends, and its temperature-electromotive force curve is calibrated when the temperature at the cold end is 0°C. ②In practical applications, since the cold end of the thermocouple is exposed to the environment, it is affected by the surrounding temperature; therefore, the temperature of the cold end cannot be kept at 0°C or fixed at a constant value. The thermocouple potential depends both on the temperature of the hot end and that of the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors. To eliminate this error, cold-end temperature compensation must be performed. 3. The output value of the regulator increases as the measured value increases, which is positive action; otherwise, it is negative action. Is this statement correct, and explain why. Answer: This statement is incorrect. The output of the regulator increases as the positive deviation increases, which is a positive action, whereas the output of the regulator decreases as the positive deviation increases, which is a negative action. 4. There is an air-operated valve that is in operation and fails to close properly; list at least five possible reasons. Answer: 1. Severe wear between the valve core and the valve seat ； ⒉Foreign objects are stuck between the valve core and the valve seat ； ⒊Leakage in the diaphragm head of the control valve ； ⒋The preload force of the zero-point spring is too high ； ⒌The valve stem is too short ； ⒍The pressure difference before and after the control valve is too large ； ⒎The locator output does not reach its maximum value. 5. Why is differential control generally added in temperature control systems? Answer: For systems with large lag, adding a differential action can improve the quality of the control system. In this case, when the regulator detects a deviation, the disturbance has actually been present for some time; therefore, increasing the output slightly will help to overcome the effects of that disturbance. It helps to prevent the continued increase of deviations. For temperature control systems, the lag is large, so an appropriate differential action should be added. 6. How many ways are there to represent PLC language? Answer: 1. Statement Table (STL) represents a program using mnemonics for functional design. 2. Control System Flow Chart (CSF), which represents a program using the logic function diagram of semiconductor logic circuits. 3. Ladder Diagram (LAD), which represents the program in the form of a ladder of contacts similar to a relay circuit diagram. 7. List the reasons for malfunctioning of the signal alarm and interlock systems. Answer: ① The power supply loses power or the voltage fluctuation range exceeds the specified value ； ②Fault in the transmitting element ③ Damage to the logic element or relay failure ④ Actuator failure ⑤ Short circuit or open circuit in the connection wires. 8. What is DCS? Answer: DCS, also known as a distributed control system, is a control system that utilizes computer technology, control technology, communication technology, and graphic display technology to achieve process control and process management. 9. What are absolute pressure, atmospheric pressure, gauge pressure, and vacuum? Answer: The pressure in a perfect vacuum is called absolute zero pressure, and the pressure measured relative to absolute zero pressure is known as absolute pressure. The reading on a pressure gauge used to measure fluid pressure is called gauge pressure, which is the difference between the absolute pressure of the fluid and the atmospheric pressure at that location. If the absolute pressure of the fluid being measured is lower than atmospheric pressure, the pressure indicated by the gauge is negative, and this value is referred to as vacuum level. 10. What is the static pressure error of a differential pressure transmitter? Answer: When the same pressure is applied to both the positive and negative chambers of a differential pressure transmitter, the output zero point of the transmitter shifts. The magnitude of this shift changes as the static pressure increases; this error caused by static pressure is known as static pressure error. 11. What are the characteristics of intrinsically safe instruments? Answer: ① Inherently safe instruments are also known as spark-safe instruments ； ②Its feature is that, whether in normal or faulty conditions, the sparks generated by the circuits and systems, as well as the temperatures reached, will not cause an explosion in the explosive mixture. 12. What is traffic? What are those several ways of expressing it? What is the relationship between them? Answer: Flow rate refers to the amount of fluid that passes through a cross-section of a pipe per unit of time. Flow rate is divided into volumetric flow rate and mass flow rate. The volume of fluid that passes through in a unit of time is known as volumetric flow rate, commonly denoted by Q. The mass of fluid that passes through in a unit of time is called mass flow rate, commonly denoted by M. The relationship between them is: M=Q×P or Q=M/ρ, where ρ is the density of the fluid. 13. Why are differential pressure flowmeters generally not suitable for use at levels below 30% of their full scale? What should be done if this happens? Answer: In flow measurement, **the standard specifies that the flow ratio applicable to throttling devices is 30%. This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow rate is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, resulting in unstable flow coefficient and inaccurate flow measurement. When the flow rate is below 30%, the following actions can be taken: reduce it to the maximum extent permitted by the process, and ensure that the Reynolds number is high enough to allow the issue to be resolved through the orifice plate or differential pressure. Secondly, it can be measured using other types of flowmeters, such as turbine flowmeters. 14. For differential pressure flow meters used for filling with isolation fluid, what should be noted when opening and closing the balance valve? Answer: For differential pressure flowmeters filled with isolation fluid, before activation, that is, before opening the orifice pressure-taking valve, the balance valve must be closed first to prevent the isolation fluid from being flushed away. When shutting down, the pressure-taking valve must be closed first, after which the balance valve can be opened to bring the gauge into a balanced state. 15. Is there any specification regarding the length of the instrument pressure lead piping, and what length is considered optimal? Answer: To ensure the measurement accuracy of the instrument and reduce lag, the piping should be as short as possible. However, for high-temperature and high-pressure steam, in order to enable its complete condensation, the measurement pipeline is required to have a certain length ； For media with large pulsations, it is also necessary for the measurement pipeline to have a certain length in order to stabilize the fluctuations. But it should not be longer than 50m. 16. Why is it reasonable to use an orifice plate to measure flow rate, with the orifice plate installed in front of the control valve? Answer: ① When using a throttling device to measure flow rate, there are two theoretical requirements: the fluid flow must be continuous, and the law of conservation of energy must hold true during the fluid flow. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, in order to avoid increasing measurement errors. ②Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable. 17. Describe the process of putting a differential pressure transmitter into operation. Answer: ① Check whether the positive and negative pressure valves of the differential pressure transmitter are closed, and whether the balance valve is open ； ②Open the drain valve. Slowly open a valve to flush the conduit; after flushing, close the drain valve. Generally, flushing should be carried out at least twice ； ③Only after the conduit has cooled down can the instrument be started. If there is a drain valve in the pipeline, it should be opened first to remove any air before the instrument can be started ； ④Gradually open the positive pressure door of the transmitter. When the medium being measured is steam or a liquid, wait until the condensate water or liquid fills the area; then loosen the drain screws for the positive and negative measurement chambers of the transmitter. Once the medium has escaped and all air has been removed, tighten the screws again. After that, check for any leaks and verify the zero point of the instrument ； ⑤Close the balance valve and gradually open the negative pressure valve. 18. On a horizontal pipeline where the process medium is liquid, a differential pressure flow measurement system is installed vertically. The differential pressure transmitter is installed after being calibrated; however, once the instrument was put into use, the indicated values turned out to be low. After verifying that there were no issues with the process operation conditions or the design and manufacturing data of the orifice plate, analyze the possible causes and suggest solutions. Give at least three reasons. Answer: ① The orifice plate is installed in the wrong direction – in this case, the distance between the flow constriction and the orifice plate is greater than when it is properly installed, which results in an increased reading at the negative pressure port of the orifice plate and a decrease in the pressure difference; therefore, it needs to be reinstalled correctly. ②The presence of gas in the positive-pressure pressure tapping line, or the fact that the highest point of the negative-pressure pressure tapping line is higher than that of the positive-pressure line, results in an additional static pressure error, causing the instrument reading to be lower than the actual value. ③If the gas in the positive pressure chamber of the differential pressure transmitter has not been completely removed, the exhaust valve in that chamber can be opened; once liquid begins to flow out, it should be tightened again. ④The balance valve in the three-valve assembly is not closed properly, or although it has been tightened, internal leakage still occurs; it needs to be closed tightly or replaced. 19. What are the calibration methods for float level gauges? Answer: There are mainly two methods: the irrigation method and the weight-hanging method. ①On-site, the liquid calibration method is generally used; if the liquid to be measured is not water or alcohol, it can be calibrated using water or alcohol through density conversion. ②During maintenance, the weight-hanging calibration method can be used. During verification, remove the float, attach a weight plate, determine the values at the starting, middle, and ending points of the float, and conduct verification by adding weights. 20. What are the reasons for malfunction alarms in combustible gas detectors? List at least 2. Answer: (1) Circuit short circuit; (2) Circuit open circuit; (3) Sensor failure or output current less than 2mA. 21. What aspects are usually checked to assess the condition of an instrument? Answer: 1) Parts are complete and meet technical requirements (A) Brand is clearly visible and unambiguous (B) Parts are intact, complete, and standardized (C) Fasteners are not loose (D) Connectors have good contact (E) Terminal connections should be secure (F) Seals show no leakage. 2) The equipment and the environment are neat, clean, and meet the work requirements. (A) The entire unit should be clean, free from corrosion; its paint coating should be even, shiny, and free from peeling. (B) Instrument pipelines and wiring should be neatly arranged and securely fixed in place. (C) There should be permanent markings on obvious parts of the instrument enclosure indicating the direction of flow. (D) Pipeline and wiring labels should be complete, clear, and accurate. 22. Why is the sensing element of a differential pressure transmitter designed in the form of a diaphragm box? Is a single diaphragm sufficient? Answer: ① The sensing elements of differential pressure transmitters generally come in two types: single diaphragm and diaphragm box assemblies. A single diaphragm is easy to manufacture and offers high sensitivity, but it cannot withstand one-way overloads. ②The sensing element in the vast majority of differential pressure transmitters is a diaphragm box assembly. This is because such assemblies exhibit excellent sensitivity and linearity within the range of differential pressures for which they are designed to operate. Even when the differential pressure exceeds this operating range, that is, in cases of one-way overload, the diaphragm box neither gets damaged nor is its performance significantly affected – this is a characteristic of the diaphragm box structure. ③Since the diaphragm box is filled with silicone oil, in addition to transmitting pressure it also provides damping, resulting in a stable instrument output. 23. How to perform zero point migration for the 1151 transmitter? Answer: The steps for zero migration of 1151 are as follows: (1) First, adjust the range to the desired value before performing the migration ； (2) Perform zero adjustment using the lower limit value of measurement; enter the pressure corresponding to this lower limit, and adjust it with the zero-setting potentiometer so that the output is 4mA ； (3) Recheck at full scale, and make fine adjustments if necessary ； (4) If the migration amount is large, first switch the transmitter’s migration switch (connector) to the positive or negative migration position (determined by the direction of migration), then apply the lower measurement limit pressure and use the zero adjustment potentiometer to set the output at 4mA ； (5) Repeat step (3). 24. How to select gaskets when installing a pressure gauge? Answer: ① When the medium under test is below 80°C and 2 MPa, rubber or elastic gaskets can be used ； When the temperature is below 450°C and the pressure is below 5 MPa, asbestos or lead gaskets can be used ； At higher temperatures and pressures, annealed copper or aluminum gaskets can be used. ②When measuring oxygen pressure, oil-impregnated gaskets or organic compound gaskets must not be used ； Copper gaskets must not be used when measuring acetylene pressure. Because they all pose a risk of exploding. 25. Is there any specification regarding the length of the instrument pressure lead pipes, and what length is considered optimal? Answer: To ensure the measurement accuracy of the instrument and reduce lag, the piping should be as short as possible. However, for high-temperature and high-pressure steam, in order to enable its complete condensation, the measurement pipeline is required to have a certain length ； For media with large pulsations, it is also necessary for the measurement pipeline to have a certain length in order to stabilize the fluctuations. But it should not be longer than 50m. 26. A external floating ball level gauge is installed on a horizontal high-pressure hydrogenation tank in a certain factory; the pressure tapping hole at the lower part is located at valve 1. After some time, the level gauge began to respond slowly and eventually stopped working. What is the main reason for this? How to modify it? Answer: ① The main reason is that the pressure sampling points of the valves are located at the very bottom of the dewatering tank; debris in the tank can easily settle into the connection pipes of the sedimentation chamber, preventing the sedimentation chamber from functioning properly. ②Solution: Reinstall it, with the pressure-taking hole at the bottom located at valve 2 or valve 3, so that the pressure measurement point is about 200 mm away from the bottom wall of the tank (alternatively, it is also possible to extend the original pressure conduit into the tank to a position 200 mm below the tank bottom)