Level DetectorsDENSITY COMPENSATIONDENSITY COMPENSATIONIf a vapor with a significant density exists above the liquid, the hydrostaticpressure added needs to be considered if accurate transmitter output is required.EO 1.2 EXPLAIN the process of density compensation in leveldetection systems to include:a. Why neededb. How accomplishedSpecificVolumeBefore examining an example which shows the effects of density, the unit "specific volume" mustbe defined. Specific volume is defined as volume per unit mass as shown in Equation 3-1.Specific Volume = Volume/Mass (3-1)Specific volume is the reciprocal of density as shown in Equation 3-2.(3-2)Specific Volume1densitySpecific volume is the standard unit used when working with vapors and steam that have lowvalues of density.For the applications that involve water and steam, specific volume can be found using "SaturatedSteam Tables," which list the specific volumes for water and saturated steam at differentpressures and temperatures.The density of steam (or vapor) above the liquid level will have an effect on the weight of thesteam or vapor bubble and the hydrostatic head pressure. As the density of the steam or vaporincreases, the weight increases and causes an increase in hydrostatic head even though the actuallevel of the tank has not changed. The larger the steam bubble, the greater the change inhydrostatic head pressure.Rev. 0 Page 11 IC-03
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