Answer / venbha123

If the LP leg of the transmitter is kept dry, steam may get condensed in the LP leg because of which the level indication may be wrong. Hence We are using wet leg for LP side of the transmitter, which is initially filled with pure water. When the drum is at zero level, the pressure difference is only the head pressure acting on the LP side - that is 100% level head at LP side. Normally in DP transmitter the HP side pressure will be more than the LP side. In this case since the pressure in the LP side is more than the pressure in HP side the range is mentioned as negative. At 100% level both the head pressures at HP & LP are the same, the DP becomes zero. Therefore the range is mentioned as: from negative value to zero.

Answer / ayaz

IN CLOSE VESSEL/DRUM
we use wet leg mean(low impulse line fill with same liquid)
we know that our measurement is when right then dp is creat
and when we use without low side filling  and hp side only pressure the steam create pressure on lp side so dp not create and show wrong indication
so we use fill lp side pressure and address in negative and when hp side posative or actual pressure create a dp in right indication will show

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