An alternative to using a compensating stand to reduce the gas pressure inside an enclosed chamber is to use a second pressure transmitter that electronically subtracts two pressures in a computing device:
This approach has the obvious advantage of avoiding potentially wet compensating feet, but suffers from additional cost advantages and more error due to the possible calibration scroll of two transmitters rather than just one. Such a system is also impractical in applications where the gas pressure is significant compared to the hydrostatic pressure (altitude head). If we add a third pressure transmitter to this system, which is located at a certain distance (x) above the lower transmitter, we have all the necessary components for what is called an expert tank system. These systems are used in large storage tanks operating at or near atmospheric pressure and have the ability to measure the height of the inferred fluid, the density of the liquid, the total volume of the liquids and the total mass of the liquid stored in the tank:
The pressure difference between the lower and middle transmitters changes only if the liquid density changes (Note 1), because the two transmitters are separated by a known and constant height difference. Tip 1: Assume that the liquid level is equal to or greater than x. Otherwise, the pressure difference between Pbottom and Pmiddle depends on the liquid density and height of the liquid. However, this situation is easily verifiable: the surface computer simply checks whether Pmiddle and Ptop are unequal. In this case, the computer knows that the liquid level is greater than x and the density calculation is safe. If this is not the case and Pimidel records like a Ptop, the computer knows that these two transmitters record both gas pressures, and knows that stopping the density calculation. Algebraic manipulation shows us how computer-measured pressures (LY) can be used to continuously calculate liquid density (γ):
When the computer knows the value of γ, it may calculate the height of the liquid in the tank very accurately based on the pressure measurements made by the low transmitters:
Here we see a clear example of how several measurements can be deduced from just a few actual process measurements (in this case pressure). Three pressure measurements in this tank allow us to calculate four inferential variables: fluid density, liquid height, liquid volume, and liquid mass. Accurate measurement of liquids in storage tanks is not only useful for process operations but also for trade. Whether the liquid represents raw materials purchased from a supplier or a processed product ready to be shipped to the customer, both parties are interested in knowing the exact amount of liquid purchased or sold. Measurement programs like this are known as custody transfers because they represent the transfer of custody (ownership) of a substance that is exchanged in a commercial agreement. In some cases, the buyer and seller retain their custody instrument, while in others there is only one instrument whose calibration has been verified by a neutral party. A photo showing two low-pressure transmitters of a specialist tank system on a refrigerated butane storage tank can be seen here:
The upper and lower instruments are pressure transmitters, while the middle instruments are a temperature sensor used to report the temperature of refrigerated butane to the control system.
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