The Foxboro Co.; King Engineering Corp.; L&J Engineering Inc.; Sarasota M&C Inc.; Texas Instruments Inc.; Varec, a Rosemount Div.
The level measurement device used most often on slurry and sludge services is the extended-diaphragm-type differ ential pressure transmitter (see Figure 7.6.22). The diaphragm extension eliminates the dead-ended cavity in the nozzle, where materials accumulate, and brings the sensing diaphragm flush with the inside surface of the tank. The sensing diaphragm is sometimes coated with Teflon to further minimize material buildup. One of the best methods of keeping the low-pressure side of the d/p cell clean is to insert another extended-diaphragm device in the upper nozzle. This device can be a pressure repeater, which can repeat both vacuums and pressures within the range of the available vacuum and plant or instrument air supply pressures. When air or vacuum is unavailable at the process pressures, extended-diaphragm-type chemical seals can be used (see Figure 7.6.23) if properly compensated for ambient temperature variations and sun exposure.
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