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A differential pressure transmitter is a device that measures the difference in pressure between two ends, and it outputs a standard signal (such as 4~20mA, 1~5V). Unlike ordinary pressure transmitters, differential pressure transmitters have two pressure interfaces. They are generally divided into a positive pressure side and a negative pressure side; under normal circumstances, the pressure on the positive pressure side of the differential pressure transmitter must be higher than that on the negative pressure side in order to enable measurement. Generally, pressure transmitters come in two types: piezoresistive and capacitive. II. Application of differential pressure transmitters in oil depot metering 1. Introduction In current designs for measuring the liquid level in oil depot tanks, differential pressure transmitters are often used alongside radar level gauges, as well as float, buoy, and steel-band type level gauges. Although radar level gauges offer high precision, they are also expensive, while level gauges such as floats and balls are more troublesome to install and maintain. Differential pressure level gauges are widely used in enclosed vessels such as boiler drums, but their measurement results do not represent the actual liquid level; therefore, they are rarely used in the design of oil tank level measurement systems. In fact, the exact liquid level in oil storage tanks is not that important; what users really need to know is not the level itself, but rather the actual amount of oil in the tank (in terms of tons), so as to prevent overflow. Based on this analysis, using the differential pressure method to measure the liquid level (actually the amount in tons) is also a good option. Since the use of differential pressure transmitters is now highly mature, transmitters such as 1151, 3051, and EJA feature advanced technology, achieving an accuracy level of 0.075. Moreover, their prices have dropped significantly, resulting in a favorable performance-to-price ratio. 2 Design principle of differential pressure transmitters As the name implies, what a differential pressure transmitter measures is the pressure difference, namely △P=ρg△h. Since oil tanks are usually cylindrical, the area S of their circular cross-section remains constant. Therefore, the weight G = △P·S = ρg△h·S; with S being constant, G is directly proportional to △P. In other words, as long as the △P value is detected accurately – and it is inversely proportional to the height △h – then even when the temperature changes and the volume of the oil expands or contracts, causing the actual liquid level to rise or fall, the measured pressure remains constant. If the user needs to display the actual liquid level, medium temperature compensation can also be introduced to address this issue. 3 Practical Applications of Differential Pressure Transmitters In the Wenzhou New Century Oil Depot project, the author applied this approach to the actual design. Design conditions: 2000 m3 oil tank, diameter d=14.5 m; the height allows the actual inventory volume G of the oil to be determined. The formula also shows that its density ρh is 14 m. Primary gauge: The BTL-1151LT flanged flameproof differential pressure transmitter from Wenzhou Bethel Instrument Co., Ltd. was selected; the flanged design is intended to prevent dirt at the bottom of the tank from accumulating and blocking the pressure transduction tubes. The transmitter’s range is 0–140 kPa. Secondary display: The WP series intelligent light-column alarm meter is used, featuring universal signal input that allows for arbitrary range adjustment; the liquid level is displayed via light columns, while the amount of oil in tons is shown numerically. Taking Tank No. 6 as an example, S=π×r2=3.14×7.252=165 m2, with a height of 14 m. On top of the oil tank, a differential pressure transmitter is used to install a level alarm system as a backup measure to prevent the oil from overflowing. In the application, since the measured value is given directly in tons, regardless of the type of oil stored in the tank, the value displayed on the secondary meter represents the tons of oil present in the tank, eliminating the need to determine the density for conversion. In general, when oil is transferred in and out of storage, pumps are used for transportation and elliptical gear flow meters are employed for measurement. Due to the limited accuracy of these flow meters, which is at most 0.2 grade, differential pressure transmitters are also needed to calculate density; as a result, there can be some discrepancies, leading to measurement disputes. Currently, since the measurement results for oil tanks are in tons and can achieve an accuracy of 0.2 level or even 00.1 level, the measurement results obtained using differential pressure transmitters are more accurate compared to those from volumetric flowmeters. Although when a small quantity of oil is received or dispatched, the absolute error in the measurement is relatively large due to resolution limitations, for large quantities of oil, the differential pressure transmitter offers higher precision and lower relative errors, making it incomparable to other measurement methods; it is particularly suitable for monthly, quarterly, and annual inventory checks. Practice has shown that its main advantages are: ① Simple and convenient installation and maintenance; ② Intuitive, direct, and clear readings, allowing for the immediate determination of the oil inventory level; ③ Elimination of the need for density measurement and conversion. 4 Points to Note for Differential Pressure Transmitters (1) During design and installation, it should be considered to place the pressure tapping holes at the bottom of the oil tank as low as possible, in order to eliminate errors caused by temperature changes; temperature compensation should be employed if necessary. (2) When the horizontal cross-section of the oil tank is not uniform (e.g., smaller at the top and larger at the bottom), compensation measures need to be considered. For secondary meters, the WP-H80 series level-volume controller is selected. (3) To achieve a certain level of accuracy, when a breather valve is installed on the top of an oil tank, a differential pressure transmitter must be used instead of a pressure transmitter. For open-top oil tanks or when high precision is not required, a pressure transmitter can be used directly to facilitate installation. (4) Smart meters should be preferred for secondary meters, as they facilitate range adjustment and enable temperature compensation, among other functions.