Figure 13 shows a typical pressurizer level system. Pressurizer temperature is held fairly constant during normal operation. The DP detector for level is calibrated with the pressurizer hot, and the effects of density changes do not occur. The pressurizer will not always be hot. It may be cooled down for non-operating maintenance conditions, in which case a second DP detector, calibrated for level measurement at low temperatures, replaces the normal DP detector. The density has not really been compensated for; it has actually been aligned out of the instrument by calibration. Density compensation may also be accomplished through electronic circuitry. Some systems compensate for density changes automatically through the design of the level detection circuitry. Other applications compensate for density by manually adjusting inputs to the circuit as the pressurizer cools down and depressurizes, or during heatup and pressurization. Calibration charts are also available to correct indications for changes in reference leg temperature.
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