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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.
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