Design principles of differential pressure transmitters and issues to consider in oil depot metering
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As the name implies, a differential pressure transmitter measures the pressure difference, that is, △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 detected pressure remains constant. If the user needs to display the actual liquid level, medium temperature compensation can also be introduced to address this issue. Issues to consider in the use of differential pressure transmitters for oil tank measurement: (1) During design and installation, it should be ensured that the pressure sampling openings at the bottom of the oil tanks are placed 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, if a breather valve is installed on the top of the oil tank, a differential pressure transmitter must be used instead of a pressure transmitter. For open-top oil tanks or situations where high precision is not required, a pressure transmitter can be used directly to facilitate installation. (4) Where possible, use smart meters for secondary instruments, as they facilitate range adjustments and enable temperature compensation, among other functions.
In my opinion, just because the weight remains unchanged does not mean that the pressure exerted also remains unchanged. Our plant has a 10-meter floating roof tank; the dual-flange level indication and differential pressure values change with temperature (around -1 degree at night and about 10 degrees at noon), with a variation of roughly 0.9%. After consulting Yokogawa’s after-sales team, it was determined that temperature is the factor affecting the instruments.
This post was last edited by 1111111 on 2018-2-26 08:35. 1. Since the quality of the liquid remains constant, the pressure generated by the liquid column does not change with temperature; it is a fixed value, and this requires no discussion. 2. The differential pressure gauge made by this crappy company has an excessively large temperature coefficient; it’s complete junk. You can’t misrepresent the stench of dog poop as an unreliable physical law. 3. For pressure gauges with standard ranges, if the temperature coefficient exceeds 0.02%FS/℃, it’s embarrassing enough to not be able to face anyone; if it exceeds 0.05%FS/℃, it’s like getting drowned in a toilet. 4. :P
I didn’t express it clearly; it is the temperature that affects the liquid filling in the capillary, causing an indication error. Yokogawa’s meter quality is pretty good. Is it that the differential pressure transmitter is not suitable for measurement? I’ve seen Honeywell’s servo level gauges, and they don’t have this problem at all.
This post was last edited by 1111111 on 2018-2-24 at 19:18. 1. You expressed yourself very clearly; there isn’t the slightest problem. 2. The pressure of the liquid itself does not change with temperature, but the reading of the differential pressure gauge does change, and by a significant amount – this indicates that the gauge has poor temperature performance. 3. Temperature compensation refers to the mechanism by which a gauge tracks and corrects the errors in readings caused by temperature effects on the liquid filling the capillaries. When compensation is effective, the overall temperature coefficient of the instrument is low, and the readings are less affected by temperature changes; if compensation is insufficient, the instrument’s temperature coefficient is high, and the readings are more affected by temperature changes. 4. Well, I think I’ve explained this concept clearly now. Do you understand? The capillaries weren’t installed by you, nor was the oil filled by you, right? If you still think that \"Yokogawa’s watches are of decent quality,\" then it’s up to you. . . Anyway, my brother thinks it’s nothing but a pile of stinky dog shit, just like Japanese dogs that love to eat shit. Hehehe, it’s possible you really don’t know how good Japanese dogs are at eating shit. Go search on Baidu for “Japanese people eating shit” and you’ll gain a lot of knowledge. Lying to your brother is like being a dog {:2_32:}
5. Differential pressure level gauges actually measure not the liquid surface but rather the volume of liquid. Servo level gauges, as well as other level gauges that use floats, actually measure the liquid surface. These are two types of measurements with different characteristics; refer to this post: http://bbs.hcbbs.com/thread-1391891-1-1.html