Instrumentation and Control Theory Section

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Instrument control theory section 1. Into which two main categories are temperature measuring instruments classified based on their measurement method? Based on the measurement method, they are divided into two categories: contact and non-contact. 2. What are the categories of contact-type temperature measuring instruments? They are divided into four categories: expansion type, pressure type, thermocouple, and thermal resistor. 3. What are the categories of contactless temperature measuring instruments? Non-contact temperature measuring instruments are divided into two main categories: radiant and infrared types. 4. What are the common types of thermocouples? The common types of thermocouples are: (1) Platinirhodium 30–Platinirhodium 6 thermocouple (B); (2) Platinirhodium 10–Platinirhodium thermocouple (S); (3) Nichrome–Nickelsilicon (Nichrome–Nickelalumium) thermocouple (K); (4) Nichrome–Copper thermocouple (XK). 5. What components does a thermocouple consist of? It consists of five parts: the hot end, heat electrode, insulating tube, protective sleeve, and junction box. 6. What are the requirements for installing temperature sensing elements? ⑴ Insert it in the pipe in counterflow or co-current direction. ⑵ The temperature sensing point should be located at the point in the pipeline where the flow velocity is highest. ⑶ There should be sufficient insertion depth. ⑷ If the process pipe diameter is small, an expansion pipe needs to be installed. ⑸ The face of the junction box should face upward. ⑹ It should be installed at pipes or equipment equipped with insulation. ⑺ In the negative pressure tube, its sealing performance must be ensured. 7. What is a thermocouple thermometer? A thermocouple thermometer is a temperature measuring instrument based on the thermoelectric effect. 8. What is the principle of temperature measurement in thermocouple thermometers? The temperature measurement principle of a thermocouple thermometer is based on the thermoelectric effect of the thermocouple. The potential difference generated by a thermocouple changes as the temperature being measured changes; therefore, a thermocouple can be used as a temperature sensing element. 9. What is a thermistor thermometer? A thermistor thermometer is a thermometer that uses a thermistor as the temperature-sensing element. 10. What is the principle of temperature measurement in a thermistor thermometer? Thermistor thermometers utilize the property that the resistance value of metal conductors changes with temperature to measure temperature. 11. What material are the high-temperature and preheating thermocouples in the vaporization furnace? What is the temperature measurement range? The material of the high-temperature thermocouple is platinum-rhodium 10-platinum-rhodium, with a temperature measurement range of 0–1700°C; the material of the preheating thermocouple is nickel-chromium-nickel-silicon, with a temperature measurement range of 0–1000°C. 12. What is traffic? What are the various ways of expressing it? What is the relationship between them? Flow rate refers to the amount of fluid that passes through a cross-section of a pipe per unit of time, and it is divided into volume flow rate and mass flow rate. Mass flow rate = Volume flow rate × Fluid density. 13. What are the components used to measure flow rate? Elements used to measure flow rate include: orifice plates, nozzles, Venturi tubes, rotors, electrically transmitted rotors, epicyclic gears, turbines, and electromagnets. 14. How many types of standard throttling devices are used in this device? What are their respective features? There are two types: orifice plates and venturis. The pressure loss of orifice plates is relatively high, while that of Venturi tubes is lower. For the same differential pressure, the pressure loss of an orifice plate is 3 to 5 times greater than that of a Venturi tube. When the pipe diameter and differential pressure are the same, the Venturi tube has a higher flow coefficient and thus a larger flow rate, making it suitable for measuring fluid flow rates in large-diameter pipes. 15. There are the corner tapping method, flange tapping method, theoretical tapping method, radius tapping method, and pipe tapping method. 16. What are the basic components of a differential pressure flow meter? What is its measurement principle? A differential pressure flow meter consists of a throttling device and a differential pressure gauge. The function of the throttling device is to convert the flow rate being measured into a differential pressure signal ； The function of a differential pressure gauge is to convert the differential pressure signal into the corresponding flow rate value. A differential pressure flow meter is based on the throttling principle of fluid flow; it measures flow rate by utilizing the pressure difference that occurs when fluid passes through a throttling device. 17. What factors affect the measurement of differential pressure flow meters? ⑴ Changes in the operating conditions of the fluid under test ； ⑵ The throttle device is not installed correctly ； ⑶ Wear at the edge of the orifice plate opening ； ⑷ The pressure guide tube is installed incorrectly, or there are blockages or leaks ； ⑸ The differential pressure gauge was installed or used incorrectly. 18. What is the measurement principle of a rotor flowmeter? A rotameter is a flow meter with a constant pressure difference. Flow rate is measured based on the rising height of the rotor inside the conical tube; it utilizes the pressure difference generated as fluid passes through the gap between the rotor and the tube wall to balance the weight of the rotor. The greater the flow rate, the higher the rotor is lifted; the larger the cross-sectional area of the flow channel, but the pressure difference remains unchanged. 19. What is the measurement principle of electromagnetic flowmeters? The principle involves using a conductive liquid that, when passing through a magnetic field, cuts through the magnetic field lines to generate an induced electromotive force. Choosing the appropriate electrode material makes it suitable for flow measurement in high-temperature, highly corrosive environments with suspensions as the medium, and it features low pressure loss. 20. Why is the orifice plate installed in front of the control valve? In the theoretical basis for measuring flow rate using throttling devices, two assumptions are made: that the fluid flow is continuous, and that the law of conservation of energy applies during the flow process. This tells us that orifice plates should not be installed in areas where gas and liquid coexist, as this could increase measurement errors. Sometimes, after the liquid pressure is reduced by a control valve, part of it may vaporize, resulting in a two-phase mixture; therefore, the orifice plate for measuring flow rate should be installed as close as possible to the control valve.