The level in the reboiler sump is shown via a sight glass at the base of the column. The upper tapping point is just above the windows in the reboiler baffle. The sight glass (custom made, 3/8 in. diameter borosilicate glass) can be isolated, if necessary, and is protected by a perspex shield. The same tapping points are connected to a differential pressure (dP) transmitter so that the level can be controlled automatically via the bottoms draw rate. The transmitter signal is dependent on the liquid density (estimated at 700 kg/m3 at normal operating temperature) which varies with temperature so that some discrepancy with the sight glass indication is possible.
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