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Differential pressure transmitters are the most widely used instruments for remote signal-based level measurement. It calculates the height of the liquid surface by measuring the static pressure generated by the liquid column inside the container. 1. Features and advantages 1) Wide range of applications, suitable for high-temperature, high-pressure, high-viscosity, and highly corrosive environments. 2) Large measurement range. Select the appropriate type of transmitter based on the different ranges, ensuring no blind spots across the entire range. 3) High reliability, good stability, long service life, and low maintenance costs. 4) High measurement accuracy. The accuracy of imported differential pressure transmitters can reach (+0.075%F.S.), while that of traditional domestic differential pressure transmitters is (+0.25%F.S.); in some cases it can reach as high as (+0.1%F.S.). 5) It features intelligent self-diagnosis and remote setup capabilities. 6) Diverse signal outputs. It can output standard 4mA-20mA current signals, pulse signals, and fieldbus communication signals for various protocols. 2. Key points for selection: 1) For ranges (differential pressure) < 5 KPa, differential pressure level transmitters are not suitable when the density change of the measured medium exceeds 5% of the design value. 2) For liquids that are flammable, explosive, toxic, corrosive, viscous, contain suspended particles, are easy to vaporize, or have a vapor phase that condenses easily at ambient temperature, these designs are suitable; they include both double-flange and single-flange versions. In the double-flange version, pressure is transmitted through capillaries, and the lengths of the two capillaries should be equal. This design is appropriate for containers operating at atmospheric pressure. 3) For liquids that are prone to crystallization, precipitation, high viscosity, coking, or polymerization, an insert-type diaphragm-sealed differential pressure level transmitter is advisable. 4) In situations where, at ambient temperature, the vapor phase may condense and the liquid phase may vaporize, resulting in high temperature and pressure inside the container, when using a conventional differential pressure level transmitter to measure the liquid level, it is necessary to install condensers, isolators, balance vessels, etc., depending on the operating conditions. 5) Differential pressure level transmitters used in practical applications generally require range migration. Therefore, the transmitter must have a range shift function, with the shift amount being at least 100% of the upper limit of the range. The amount of migration should be taken into consideration during the design and selection phase, especially for media with high density, as the migration amount can be significant; the appropriate transmitter range should be chosen based on this amount of migration. The installation and use of conventional differential pressure level transmitters are relatively complex; process connections require the use of pressure transfer pipes, and venting and drainage systems must be designed into the connection pipelines, or three-valve or five-valve assemblies should be used. In many cases, it is also necessary to provide heat tracing for the pressure transfer pipes and the transmitter itself. Choosing a diaphragm-sealed differential pressure level transmitter can simplify these issues, as its installation and maintenance are relatively straightforward. Therefore, the author recommends giving priority to diaphragm-sealed differential pressure level transmitters when the conditions permit. 3. Diaphragm-sealed differential pressure level transmitters also have limitations regarding their operating conditions. 1) Process temperature: These transmitters transmit pressure through the filling fluid inside the sealing components. Common filling fluids include 200 silicone oil, 704 silicone oil, chlorinated hydrocarbons, and mixtures of glycerin and water. Each filling fluid has a specific temperature range in which it can be used; therefore, the appropriate filling fluid should be selected based on the chemical properties of the medium being measured and the process temperature. Overall, when the process temperature exceeds 200°C, care should be taken when choosing a diaphragm-sealed transmitter; if such a transmitter is to be used, an extended sealing system or a thermal optimizer must be employed, and this should be confirmed with the transmitter manufacturer. 2) Ambient temperature: Attention should be paid to the ambient operating temperature of the filling fluid; when the ambient temperature is lower than this temperature, the capillary tube needs to be heated. In EOEG units, since EO is flammable, explosive, and prone to polymerization, diaphragm-sealed differential pressure level transmitters are used for measuring the liquid level of EO-containing media. Since carbonate solutions tend to crystallize, an insert-type diaphragm sealing system is used for level measurement in such solutions. The inserted part is flush with the inner wall of the equipment, and its outer diameter and length are determined based on the specifications of the equipment’s ports. In the boiler drum of the device, where the operating temperature is above 250°C, ordinary pressure guide tubes are used; a condensation tank is installed at the upper end of these tubes, and the pressure guide tubes are also heated. There is also a new type of differential pressure level measurement system—the ERS (Electronic Remote Sensor) system. This system consists of two ordinary pressure transmitters connected via a cable (using CAN communication) to form a differential pressure measurement system. The calculation of the differential pressure is carried out by the transmitters located at the site, while the main gauge directly outputs the differential pressure signal. Only a 4mA–20mA circuit is required to connect to the main gauge; the auxiliary gauges are powered by the main gauge. By utilizing patented synchronization technology, the differential pressure zero point is adjusted directly on the main gauge, ensuring reliable synchronization of the differential pressure signals. The typical installation of the ERS system is shown in Figure 1. ERS technology serves as a complement to traditional double-flange level measurement methods, and it has its own characteristics and advantages. When using a double-flange diaphragm transmitter to measure liquid levels, if the pressure tapping distance is too large or the length of the capillary is too long, a series of problems arise. For example, when the capillary is too long, temperature changes have a significant impact on measurement accuracy, the response speed slows down, and high-temperature oils require heating when the ambient temperature is too low. ERS technology can just compensate for the shortcomings of double-flange level measurement in such situations. 4. The ERS system is suitable for the following operating conditions: 1) Tall towers such as distillation towers and fermentation towers with a large pressure difference between the top and bottom, i.e., greater than 6 m. The ERS system can effectively eliminate measurement errors in long capillaries and reduce response time. Time, to improve overall measurement performance. 2) For ERS with a large pressure difference and low static pressure, the pressure difference can be determined by measuring the pressure. To ensure the accuracy of the differential pressure signal, the ratio of static pressure SP to differential pressure DP must not exceed the transmitter’s minimum accuracy requirement, that is, (SP+DP):DP