A level controller used for interface service should be able to respond over a wide range of adjustments of specific gravity differences. The chemical and petroleum industries especially see applications where interface control is more important than measurement. However, for feedback control, you need to provide measurement. If you control the water distillate interface level in a separator at some point around the midlevel value, the water can draw off through a valve whose position is a function of the interface level. The main concern in such an application is the valve should be closed when the distillate level is low to prevent losing product, and open when the level is high to permit removing water from the distillate. Some field-mounted liquid-level controllers can see use for liquid interface applications.
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