Answer / s vedpathak

In closed vessals, the vessal pressure may vary.this can
cause erratic level feedback.But if DP LT is used it take
into account this vessal pressure as it acts on both LP &
HP side. So LT feedback is purely based on the head due to
vessal fluid.Wet leg & dry leg method can be used depending
on the application.

Answer / vaishnavi

In the case of closed tank der will pressure acting above the liq also.So in this case inorder to get accurate measurement we need to measure both pressure so we use dpt.But in the case of an open tank there will atmospheric pressure above the liq surface.Hence we need not measure pressure above liq.moreover as radar involves reflection principle this cannot be used to measure both the pressures in the closed tank.

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