Basic knowledge of pressure differential measurement

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1. Why is it necessary to choose a location with a relatively constant ambient temperature when installing flange transmitters? Answer: A flanged transmitter is different from a regular transmitter; its capillary and flanged diaphragm form a sealed system, which functions like a large thermowell. When the surrounding environment changes, the fluid inside the system expands or contracts, thereby causing pressure changes in the system. These pressure changes act on the sensitive elements of the transmitter, resulting in additional errors in the instrument readings. In ordinary transmitters, the pressure lead is not a sealed system; pressure changes due to temperature variations, and the medium can diffuse into the process flow, thus not affecting the instrument’s output. When installing flanged transmitters, make sure that the transmitter and the flanged diaphragm system are not exposed to direct sunlight, to avoid sharp changes in ambient temperature caused by exposure to the sun. Additionally, the two capillaries of the differential pressure transmitter should be at the same ambient temperature, so that temperature changes within a certain range can cancel each other out. 2. Why are flange transmitters for high temperature and high vacuum particularly expensive? Answer: The flange diaphragm of a flange transmitter is in direct contact with the medium, so flange transmitters can easily operate under high-vacuum conditions. When the flange transmitter operates in a vacuum environment, an outward pulling force acts on the isolation diaphragm; as a result, the diaphragm bulges outward, increasing the volume of the transmitter’s sealed system. This leads to a decrease in pressure within the filling fluid, creating a vacuum condition. At this time, external gases may penetrate into the diaphragm box through the welds and joints, causing gas to be present in the filling fluid and thereby affecting the performance of the instrument. In a flange transmitter, the boiling point of the filling liquid decreases as pressure drops. If the flange transmitter operates under negative pressure, a vacuum will form within the sealed system, which in turn lowers the boiling point of the filling liquid and causes it to vaporize. If the temperature of the medium drops below the boiling point of the filling fluid, vaporization occurs. If the medium temperature is much higher than the boiling point of the filling fluid, the vapor pressure of the filling fluid will be very high. This not only exerts pressure on the sensor element of the transmitter, causing measurement errors, but also causes the diaphragm to bulge outward, leading to its permanent deformation. When the transmitter operates under conditions of high temperature and high vacuum, the filling fluid is in a condition where it vaporizes very easily. To this end, the manufacturer must treat such transmitters in a special manner: the filling fluid used in the diaphragm box must be high-temperature silicone oil, and the amount of filling must be controlled precisely, neither too much nor too little. Any gas present in the silicone oil must be completely removed before filling, and the diaphragm box must be evacuated to an absolute vacuum with no residual pressure left. The welding of the diaphragm box must also be absolutely secure. All of these increase the processing workload and cost of the transmitter, which is why flanged transmitters for high temperature and high vacuum environments are particularly expensive. 3. What are the differences between Rosemount 3051C, Honeywell ST3000, Yokogawa EJA, and Fuji FCX-A/C sensors? Answer: The sensor of the Rosemount 3051C smart transmitter is a silicon capacitive type; it converts the parameter being measured into a change in capacitance, and the differential pressure or pressure being measured is then determined by measuring this capacitance change. The sensor in Honeywell’s ST3000 series of intelligent transmitters is of the composite semiconductor type; it converts the parameter being measured into changes in the resistance value of a silicon chip, and the measured differential pressure or pressure is then determined by measuring this resistance. The sensor of the Yokogawa EJA intelligent transmitter is of the silicon resonator type; it converts the parameter to be measured into the vibration frequency of a silicon beam, and the measured differential pressure or pressure value is obtained by detecting this frequency. The sensor of the Fuji FCX-A/C intelligent transmitter is a silicon microcapacitive type; it converts the parameter to be measured into electrostatic capacitance, and the measured differential pressure or pressure value is then obtained by measuring this silicon capacitance. 4. A transmitter with a flange has a rated liquid contact temperature of -15 to 250°C. If the medium being measured exceeds this range for a short period of time, will the instrument get damaged? Answer: The liquid-contacting temperature of a flanged transmitter is primarily determined by the properties of the filling fluid inside the flanged diaphragm box. When the temperature of the medium being measured exceeds the upper limit of the instrument’s rated liquid contact temperature, two situations can occur. 1) The filling fluid expands in volume due to heat, and may even evaporate. If the expansion is excessive and the pressure exceeds the allowable value, the diaphragm will suffer permanent deformation, and the filling fluid will be forced out through the connections and welds. 2) The filling fluid is an oily substance that contains more or less gas. Furthermore, the sealed system before filling cannot be reduced to an absolute vacuum either; there is still gas present. Although their quantity is small, their high coefficient of thermal expansion means that excessive temperatures can also cause permanent deformation of the isolation diaphragm. In summary, if the temperature of the medium being measured exceeds the upper limit of the instrument’s rated range, it may lead to a decline in the instrument’s performance in mild cases, or even damage to the instrument in severe cases. Therefore, when selecting an instrument, it is essential to ensure that its allowable contact temperature is not lower than the highest temperature of the medium being measured. If the temperature of the medium being measured is below the lower limit of the instrument’s rated temperature, the filling fluid will freeze and become immobile. In this way, when the pressure of the medium under test changes, the output of the instrument will change less significantly, or even remain unchanged. However, the instrument generally does not get damaged; once the temperature of the medium being measured returns to normal, the instrument will function again properly. 5. The technical specifications of a certain flanged transmitter specify that the maximum liquid contact temperature of the instrument is 300°C, and the minimum static pressure is 0.13 KPa abs. Can this table operate under conditions where the operating temperature of the medium being tested is 300°C and the operating pressure is 0.13 Kpa abs? Answer: No, if a flange transmitter operates in high-temperature conditions, the volume of the filling liquid inside the flange diaphragm box will expand. If air is mixed into the filling liquid during the filling process, the expansion coefficient of air is much greater than that of the filling liquid, resulting in more severe volume expansion. This causes the diaphragm to bulge outward, and in severe cases, permanent deformation may occur. However, if the pressure of the medium being measured is very high, it acts on the diaphragm, preventing it from bulging outward; as a result, even if the temperature of the medium is high, it has no effect on the instrument. Therefore, the parameters of pressure and temperature are related to each other; high temperature combined with a high vacuum poses the greatest threat to instruments, otherwise their effects can partially offset one another. The two parameters mentioned in this question, 0.13 KPA abs and 300°C, cannot coexist. If the instrument is to operate at 300°C, the operating pressure cannot be negative; it must be above atmospheric pressure. If the instrument is to operate in a high vacuum of 0.13 KPa abs, then the temperature must be well below 300°C. 6. A Bourdon tube pressure gauge is used to measure pressure; its reading is P in atmospheric conditions. If it is moved into a vacuum, how does the gauge’s reading change? Why? Answer: Because in the atmosphere, P_gauge = P_absolute – P0 (atmospheric pressure). If the gauge is moved to a vacuum, it measures absolute pressure; thus P_absolute = P_gauge + P0, and as a result the reading on the gauge increases. 7. What are the operation steps for the initial startup of a differential pressure transmitter used to measure steam flow rate after it has been installed? Answer: The operation should be carried out according to the following steps: 1) Check whether each valve, pressure guide tube, union, etc. is securely connected ； 2) Check whether the secondary cutting and drainage valves are closed, and whether the balance valve is open ； 3) Open the valve slightly once, then check the pressure guide tube, valve gate, union fittings, etc.; if there is no leakage, open the valve fully ； 4) Open the drain valves separately, discharge the waste fluid, and then close the drain valves ； 5) Loosen the screw plug in the differential pressure chamber to release the air inside ； 6) The differential pressure transmitter can be activated only after the pressure guiding pipe is filled with condensed water ； 7) To start the differential pressure transmitter, the positive pressure valve should be opened, the balance valve should be closed, and the negative pressure valve should be opened. 8. What are zero adjustment and zero shift of pressure or differential pressure transmitters? Answer: Zero-point adjustment refers to the adjustment of the zero point when the input pressure or differential pressure is zero, but the output is not zero ； Zero-point migration: refers to the adjustment in which the output is set to zero when the input pressure or differential pressure is not zero. 9. Pressure can be divided into static pressure, dynamic pressure, and total pressure. What is the relationship between them? What pressure does a pressure transducer measure? Answer: Total pressure = dynamic pressure + static pressure, and the pressure transducer measures the static pressure. 10. Describe the correct pressure-taking locations for instruments used to measure liquid, gas, and steam pressures? Answer: When measuring gases: To allow any small amount of condensate present in the gas to flow back smoothly into the process pipeline, without entering the measurement lines and instruments, the pressure tapping point should be located in the upper part of the pipeline ； When measuring liquids: To allow the small amount of gas that forms within the liquid to return smoothly to the process pipeline, without entering the measurement pipeline or the instruments, it is best to place the pressure tapping at an angle of 0–45° below the horizontal center line of the pipeline. When measuring steam: It is necessary to maintain a steady supply of condensate within the measurement pipeline, while also preventing solid particles from the bottom of the process pipeline from entering the measurement pipeline or the instruments; in this case, it is best to place the pressure tapping at an angle of 0–45° above the horizontal center line of the pipeline. 11. Explain the meanings and interrelationships between 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. Gauge pressure is a pressure expressed relative to atmospheric pressure; therefore, it differs from absolute pressure by exactly one atmosphere, which is 0.1 MPa. 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. Absolute pressure = atmospheric pressure + gauge pressure. 12. What are the features of intelligent transmitters? Answer: 1) Temperature characteristics and static pressure characteristics have been significantly improved ； 2) High precision ； 3) Large adjustable range ratio ； 4) Has remote control setting function ； 5) It has a self-diagnosis function. 13. Is it relevant whether the input signal of the intelligent transmitter is read using a communicator or not, to the set range of the transmitter? The static pressure error of the transmitter can be detected using the input signal; is it possible to observe trends outside the measurement range? Answer: It’s okay ； Sure. 14. What is the HART communication protocol? What is the DE communication protocol? Answer: HART protocol: A protocol in which digital signals are superimposed on analog signals, allowing both to be transmitted simultaneously. DE protocol: A protocol in which digital signals and analog signals are transmitted separately, with the analog signal needing to be interrupted when digital signals are being transmitted. 15. What is the damping of a transmitter? Answer: It refers to the response speed of the transmitter’s output as the parameter being measured changes. 16. Why is the accuracy of flanged transmitters lower than that of ordinary transmitters? Answer: Because flanged transmitters have an additional remote flanged diaphragm box and one more conversion step compared to ordinary transmitters; as a result, the error is increased by the conversion error associated with the flanged diaphragm box, on top of the original error. 17. What are the two types of contacts in pressure switches? Answer: Normally open and normally closed. 18. What is the difference between flanged transmitters and regular transmitters? In ordinary transmitters, the medium to be measured enters the transmitter’s measurement chamber through pressure transfer tubes; if the medium is a viscous liquid, it can easily clog these tubes. Now, by using a capillary in place of a pressure guide tube, blockage will not occur. Since the capillary is sealed, the filling liquid inside it is carefully selected for its stable chemical and physical properties; it does not freeze or vaporize easily like the medium in pressure transfer tubes. As a result, there is no need to fill it with an isolation fluid or install auxiliary devices such as isolators and condensers, as is required in ordinary transmitters. In this way, the measurement accuracy can be improved, and the maintenance workload can be reduced. 19. What is the composition and working principle of an intelligent transmitter? Answer: An intelligent transmitter consists of two main components: a composite sensor and a microprocessor. A composite sensor integrates three sensors for differential pressure, temperature, and static pressure onto a single piece of monocrystalline silicon using integrated circuit diffusion techniques. The detection signals from these three sensors are read into a microprocessor, where they are processed through comprehensive calculations to enable accurate determination of pressure and differential pressure values ； It is then converted via D/A to output a 4–20 mADC DC signal. 20. What is the range ratio of a transmitter? The range ratio of a transmitter is: maximum measurement range / minimum measurement range. 21. What properties should the filling fluid inside the transmitter diaphragm box have? What is the difference between silicone oil and fluorinated oil? Answer: The interior of the diaphragm box needs to be filled with a filling fluid in order to transmit pressure. The requirements for this filling fluid are that it should have a low coefficient of thermal expansion, not freeze at low temperatures, and not evaporate or vaporize at high temperatures; moreover, its viscosity should not change significantly with temperature changes, so as to ensure stable performance of the instrument. Typically, the diaphragm box is filled with silicone oil or fluorine oil. Silicone oils are divided into low-temperature silicone oils and high-temperature silicone oils; low-temperature silicone oils can operate at temperatures as low as -40°C, while high-temperature silicone oils can handle temperatures up to 315°C. Depending on the operating temperature of the transmitter, different grades of silicone oil are selected. Fluorine oil is an inert liquid with low chemical reactivity; it is therefore used in transmitters for oxygen or chlorine measurement. Its operating temperature range is from -45 to 250°C, it has a low viscosity, and its price is higher than that of silicone oil. 22. What is degreasing? Answer: During the processing of the transmitter’s components, and while filling the diaphragm box, it is inevitable that substances such as grease come into contact with them. The degreasing process involves removing the grease from the diaphragm box components and the high and low pressure measurement chambers, so as to prevent them from contaminating the medium being measured. Degreasing is commonly used in transmitters for oxygen or chlorine measurement, as oxygen can explode when in contact with oil, and chlorine is also a reactive chemical element; moreover, beverages and similar products must not be contaminated, so the diaphragm box needs to be cleaned thoroughly. 23. Can flange transmitters for carbon steel flanges be used to measure corrosive media? Why? Answer: The flange of a flanged transmitter does not come into contact with the medium; only the diaphragm is in contact with the liquid. Therefore, as long as there are no corrosive gases in the surrounding environment, carbon steel flanges can also be used to measure corrosive media. 24. In the static pressure specifications of flanged transmitters, are there limits set for both the upper and lower ranges of operating pressure? Why? Answer: The operating pressure of a flanged transmitter has limits for both the upper and lower ranges. But generally, people only pay attention to the upper limit of the operating pressure and ignore the lower limit, thinking that any value is acceptable; in fact, this is incorrect. There are also specified limits for the minimum operating pressure of flange transmitters; they generally cannot operate under negative pressure, especially in high-vacuum conditions. At high vacuum, the isolation diaphragm bulges outward and is very prone to damage; therefore, restrictions are necessary. 25. Difference between gauge pressure transmitters and absolute pressure transmitters. An absolute pressure transmitter measures the absolute pressure of the medium inside a device; it has no relation to atmospheric pressure. In contrast, a gauge pressure transmitter measures the pressure relative to atmospheric pressure, and it is related to atmospheric pressure. Structurally: on the other side of the pressure-sensing element in an absolute pressure transmitter, there is absolute vacuum, whereas on the other side of the same element in a gauge pressure transmitter, there is the atmosphere.