The Omnitrol Model 621 single stage controls and the Model 721 two stage controls are “special purpose” top mounted level switches of the fixed differential type. The feature which sets these level devices apart from other top mounted controls is the lead displacers that are suspended on a stainless steel beaded chain for the purpose of determining floating roof tank top alarm positions.
As the level in a floating roof tank or internal floating pan rises and falls so does the roof. A Model 621 unit may be mounted above the top of the tank (bracket mounted) with the displacer weight adjusted to the desired level of actuation.
As the roof rises to the Hi level position and picks up the weight, the control senses the release of the weight and opens or closes an electric or pneumatic contact switch.
The beaded suspension chain is used to prevent coiling and kinking that is often associated with cable. The 721 works in the same manner as the 621 except that it offers two stage operation for such applications as Hi and Hi-Hi level alarms. Both the 621 and 721 may be mounted via the standard 2-1/2” NPT mountings or by any standard flange sized 3” or larger. For open roof tanks, an external mounting bracket which folds flat for shipping, can be supplied.