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Selection and application of transmitters in several special operating conditions 1 High-temperature conditions: When working in high-temperature environments, attention should be paid to the choice of the sensor filling fluid and the capillary filling fluid. The silicone oils commonly used as filling fluids for capillaries are generally 200 silicone oil and high-temperature 704 silicone oil. At atmospheric pressure, 200 silicone oil can be used in the range of -45 to 205°C, while high-temperature 704 silicone oil can be used in the range of 0 to 305°C. In actual selection, in addition to knowing the operating temperature, the operating pressure is also an important parameter. As the temperature rises, the saturated vapor pressure of the medium also decreases accordingly. When making a selection, it is necessary to refer to the manufacturer’s curve showing saturated vapor temperature and pressure; it is recommended to use high-temperature silicone oil for temperatures above 130°C. In situations where the temperature of the medium to be measured is high, and the process temperature exceeds the normal operating range of the transmitter, and it is not possible to bring the temperature within the acceptable limits using pressure conduits or directly mounted diaphragms, a remote-type transmitter can be used. This type of transmitter utilizes capillary tubes to transfer pressure, thereby preventing the effects of high temperatures on the electronic components of the transmitter. 2. In conditions of high viscosity that facilitate crystallization: When the fluid leaves the process equipment, crystallization may occur, or if the viscosity of the fluid is too high, the use of pressure guide tubes can lead to blockages. Such situations require frequent cleaning and unclogging, and they also cause instability in measurements; therefore, isolated transmitters are generally used in such cases. For conditions with severe crystallization, a diaphragm-type transmitter with a protruding diaphragm can be used; the pressure-sensing diaphragm is inserted directly into the device, allowing it to sense the temperature of the medium directly. This prevents crystallization at the diaphragm chamber, thereby ensuring more stable and reliable measurements. Crystallization conditions often require heat tracing and insulation; for such conditions, high-temperature silicone oil is suitable as the isolation fluid. In situations where the medium tends to condense and deposit at the flange connections, it is recommended to install a flushing ring between the process flange and the diaphragm seal flange. Water or similar fluids can be passed through the holes on the side of this flushing ring to wash away the deposits and keep the diaphragm clean, thereby ensuring proper pressure sensing by the diaphragm box. 3 Corrosive conditions Corrosive conditions include environmental conditions as well as the corrosivity of the application environment. In applications where the pressure guide tube is made of carbon steel or stainless steel, ordinary pressure and differential pressure transmitters should be selected ； When carbon steel or stainless steel cannot meet the operational requirements, an isolated transmitter should be selected. Special attention must be paid to material selection in corrosive environments. The materials of a transmitter include those used for the diaphragm and liquid-contacting parts, the flange material, the housing material, the materials for exhaust/vent valves, the filling fluid for the sensor, the filling fluid for the capillary, as well as the materials for bolts and nuts. When selecting a transmitter, the most important factors to consider are the materials for the diaphragm and liquid-contacting parts, the flange material, and the housing material. 3.1 Diaphragm and liquid-contacting materials are selected based on the chemical properties of the medium; the corrosivity of the same medium at different concentrations also varies with temperature. For diaphragm-sealed transmitters, the inner diaphragm box is usually made of stainless steel, while the outer diaphragm box used in contact with the fluid is selected based on the properties of that fluid. The following are the diaphragm material options for several common media in our company. For other materials, refer to Table 1-5-18 in the Instrument Technician’s Manual, which provides a table of the corrosion resistance of common metal materials used in instruments, or section 8.1 on Common Corrosion-Resistant Materials for Instruments in the Common Data Manual for Instruments. 3.2 Flange material: For transmitters connected via flanges, the flange material is generally available in three options: carbon steel, 304SST, and 316SST. The choice of flange material depends on environmental conditions and the properties of the medium. Since the flange is not the part that comes into direct contact with the liquid, carbon steel can be used when environmental conditions are favorable and cost considerations are important. However, for critical monitoring points or safety and environmental control points, to prevent rapid corrosion of the flanges due to membrane corrosion, it is recommended to use 316SST. 3.3 Shell Material The material of the transmitter’s shell is determined based on environmental conditions and cost considerations. Common materials for the shell include aluminum alloy and stainless steel; in environments with low corrosion levels, using aluminum alloy is more cost-effective. 4. If the installation location makes maintenance difficult, it is advisable to use pressure guide tubes for connection; in general, a standard transmitter is chosen. When cost considerations permit, a remote-type transmitter can also be used, with the transmitter installed in a location that facilitates regular maintenance. At the same time, a capillary of appropriate length should be selected based on the operating conditions; the response time varies depending on the length of the capillary, with common lengths ranging from 3 to 5 meters. The price of remote-type transmitters is generally more than twice that of those for direct installation; when selecting one, it is necessary to consider both the actual application requirements and cost-effectiveness. Selection and application of transmitters in several special operating conditions 1 High-temperature conditions: When working in high-temperature environments, attention should be paid to the choice of the sensor filling fluid and the capillary filling fluid. The silicone oils commonly used as filling fluids for capillaries are generally 200 silicone oil and high-temperature 704 silicone oil. At atmospheric pressure, 200 silicone oil can be used in the range of -45 to 205°C, while high-temperature 704 silicone oil can be used in the range of 0 to 305°C. In actual selection, in addition to knowing the operating temperature, the operating pressure is also an important parameter. As the temperature rises, the saturated vapor pressure of the medium also decreases accordingly. When making a selection, it is necessary to refer to the manufacturer’s curve showing saturated vapor temperature and pressure; it is recommended to use high-temperature silicone oil for temperatures above 130°C. In situations where the temperature of the medium to be measured is high, and the process temperature exceeds the normal operating range of the transmitter, and it is not possible to bring the temperature within the acceptable limits using pressure conduits or directly mounted diaphragms, a remote-type transmitter can be used. This type of transmitter utilizes capillary tubes to transfer pressure, thereby preventing the effects of high temperatures on the electronic components of the transmitter. 2. In conditions of high viscosity that facilitate crystallization: When the fluid leaves the process equipment, crystallization may occur, or if the viscosity of the fluid is too high, the use of pressure guide tubes can lead to blockages. Such situations require frequent cleaning and unclogging, and they also cause instability in measurements; therefore, isolated transmitters are generally used in such cases. For conditions with severe crystallization, a diaphragm-type transmitter with a protruding diaphragm can be used; the pressure-sensing diaphragm is inserted directly into the device, allowing it to sense the temperature of the medium directly. This prevents crystallization at the diaphragm chamber, thereby ensuring more stable and reliable measurements. Crystallization conditions often require heat tracing and insulation; for such conditions, high-temperature silicone oil is suitable as the isolation fluid. In situations where the medium tends to condense and deposit at the flange connections, it is recommended to install a flushing ring between the process flange and the diaphragm seal flange. Water or similar fluids can be passed through the holes on the side of this flushing ring to wash away the deposits and keep the diaphragm clean, thereby ensuring proper pressure sensing by the diaphragm box. 3 Corrosive conditions Corrosive conditions include environmental conditions as well as the corrosivity of the application environment. In applications where the pressure guide tube is made of carbon steel or stainless steel, ordinary pressure and differential pressure transmitters should be selected ； When carbon steel or stainless steel cannot meet the operational requirements, an isolated transmitter should be selected. Special attention must be paid to material selection in corrosive environments. The materials of a transmitter include those used for the diaphragm and liquid-contacting parts, the flange material, the housing material, the materials for exhaust/vent valves, the filling fluid for the sensor, the filling fluid for the capillary, as well as the materials for bolts and nuts. When selecting a transmitter, the most important factors to consider are the materials for the diaphragm and liquid-contacting parts, the flange material, and the housing material. 3.1 Diaphragm and liquid-contacting materials are selected based on the chemical properties of the medium; the corrosivity of the same medium at different concentrations also varies with temperature. For diaphragm-sealed transmitters, the inner diaphragm box is usually made of stainless steel, while the outer diaphragm box used in contact with the fluid is selected based on the properties of that fluid. The following are the diaphragm material options for several common media in our company. For other materials, refer to Table 1-5-18 in the Instrument Technician’s Manual, which provides a table of the corrosion resistance of common metal materials used in instruments, or section 8.1 on Common Corrosion-Resistant Materials for Instruments in the Common Data Manual for Instruments. 3.2 Flange material: For transmitters connected via flanges, the flange material is generally available in three options: carbon steel, 304SST, and 316SST. The choice of flange material depends on environmental conditions and the properties of the medium. Since the flange is not the part that comes into direct contact with the liquid, carbon steel can be used when environmental conditions are favorable and cost considerations are important. However, for critical monitoring points or safety and environmental control points, to prevent rapid corrosion of the flanges due to membrane corrosion, it is recommended to use 316SST. 3.3 Shell Material The material of the transmitter’s shell is determined based on environmental conditions and cost considerations. Common materials for the shell include aluminum alloy and stainless steel; in environments with low corrosion levels, using aluminum alloy is more cost-effective. .4 If the installation location makes maintenance difficult, a pressure guide tube can be used for connection; in such cases, a standard transmitter is generally chosen. When cost considerations permit, a remote-type transmitter can also be used, with the transmitter installed in a location that facilitates regular maintenance. At the same time, a capillary of appropriate length should be selected based on the operating conditions; the response time varies depending on the length of the capillary, with common lengths ranging from 3 to 5 meters. The price of remote-type transmitters is generally more than twice that of those for direct installation; when selecting one, it is necessary to consider both the actual application requirements and cost-effectiveness. The information was collected and provided by Huaheng, a domestic transmitter manufacturer