Dual-flange differential pressure transmitter
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I would like to ask the experts how to accurately calculate the upper and lower measurement ranges of a double-flange transmitter. The liquid level gauge reads 1.7 meters. The medium is crude oil with a density of 0.8. The lower range limit is set by installing the transmitter with double flanges, allowing the flanges to be exposed to air; at this point, zero shift is performed. Is it correct to set the zero point this way? ? ? The upper limit of the measurement range is the product of the liquid density, gravity acceleration, and liquid level height. The density of crude oil is 0.8x10^3 kg/m3, gravity is taken as 10, and the liquid level height is 1.7 meters. Therefore, the pressure at a height of 1.7 meters is 0.8x10^3 x 10 x 1.7 = 13,600 Pa, which is equivalent to 13.6 kPa. Then is the upper limit of the range obtained by subtracting the lower limit from the upper limit? ? ?
When calculating the range of a double-flange differential pressure transmitter, your method is basically correct; however, there are some details to be mindful of to ensure accuracy. ### Setting of the lower range limit: The lower range limit is usually set at the output of the system under its lowest operating conditions. For level measurement, when the liquid level is 0 (i.e., there is no liquid present), the transmitter should output the lowest possible pressure value. If your installation involves the transmitter being in direct contact with air through the double flanges, it is indeed necessary to perform a zero point adjustment to ensure that the transmitter’s output is zero in the absence of liquid. Such a zero calibration is correct. ### Calculation of the upper limit of the measurement range: The upper limit of the measurement range you calculated is basically correct; it involves the density of the liquid, the acceleration due to gravity, and the liquid level height. The calculation formula is: \ Where: - \( \rho \) is the density of the liquid (for crude oil, it’s \( 0.8 \times 10^3 \, \text{kg/m}^3 \)) - \( g \) is the acceleration due to gravity (approximately \( 10 \, \text{m/s}^2 \)) - \( h \) is the height of the liquid level (1.7 meters) According to your calculations: \ That is, 13.6 kPa. ### Determination of the measurement range: The determination of the measurement range should cover the entire span from the lower limit to the upper limit. In your case, if the lower range limit is set to 0 kPa (after zero shift), and the upper range limit is 13.6 kPa, then the transmitter’s range should be from 0 to 13.6 kPa. ### Summary: Your calculations and setup methods are correct; just ensure that the zero-point shift is carried out properly, and that all calculation parameters are accurate. In this way, you can obtain an accurate range setting, thereby ensuring the accuracy and reliability of the transmitter in actual applications. .
I understand what you mean. You’re right. Your method is also the most practical and commonly used method on-site. The second method involves knowing the density of the silicone oil in the double-flange capillary tube; after performing the calculations, a negative shift is applied. This method is suitable for cases where a complete set of equipment is supplied by a vendor who has already installed the double-flange level gauge. In such cases, parameters like the medium’s density are provided to the instrument manufacturer in advance, and after theoretical calculations, the manufacturer adjusts the measurement range accordingly. The third method is: when conditions permit on site, after isolation, disconnect the flange, drain the contents, then perform zero calibration. After that, fill the system with the medium until it reaches the overflow mark within the measurement range before putting it into service. But it’s rather difficult.
Also, you can post your question regarding this topic in the Instrumentation & Control section; you’ll get a very clear answer there
Thank you for your reply. If the root valve cannot be closed and it is not possible to isolate the double flanges for venting, how can the zero point and range be calculated? The density of silicone oil is 0.9
Thank you for your reply. If the root valve cannot be closed and it is not possible to isolate the double flanges for venting, how can the zero point and range be calculated? The density of silicone oil is 0.9
Negative drift: the zero point and range calculated using the medium density are each reduced by the pressure of the silicone oil; thus, the final zero point after drift = -silicone oil density * g * double flange height. The range is equal to = (medium density – silicone oil density) * g * double flange height.
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