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Automatic
vs. Manual Reset
All
indicators with electric switches reset automatically
to their original position when the pressure across the
filter drops below trip pressure. This is true, also,
for the type B visual indicator. However, on the type
BM visual indicator with manual reset, the signal arm
extends once the trip pressure is exceeded and remains
that way until physically reset. The advantage is that
the indicator signals that the element is dirty even after
the system is shut down, thus, simplifying maintenance.
Thermal
Lockout
When mobile and other equipment is started in the
cold, the hydraulic or lube fluid is likely to be viscous
until it approaches normal operating temperature. The
high pressure drop created by a highly viscous fluid can
trip the indicator and falsely signify that the element
is clogged. An optional thermal lockout device, available
on many HYDAC electric indicators, prevents the indicator
from tripping until the fluid reaches a certain specified
temperature. The device consists of a switch in series
in the indicator circuit, which is caused to make or break
by a bi-metal strip that alters in shape according to
temperature.
The
thermal lockout feature may be chosen so that the indicator
is deactivated at a fluid temperature less than 70°
F ± 5° (called T70), or less than 100° F
± 5° (called T100).
Because
electric indicators automatically reset once the fluid
heats up, thermal lockout is necessary only when a false
signal of filter condition during cold start-up poses
a problem.
Single
Pole, Double Throw Switches
All of HYDAC's differential pressure and most
static pressure electrical indicators contain single-pole,
double-throw switches. This provides the choice of normally
open or normally closed contacts when the pressure differential
is below trip-point. It also allows for a warning arrangement
in which a green light represents a clean element and
a red light one that is dirty. Whether the contacts are
normally open (N/O) or normally closed (N/C) is determined
by the way in which the indicator is wired on site.
Magnetic
Coupling
Most
of HYDAC's indicators employ magnetic coupling, which
separates the fluid from the actuating device. The benefit
is that there is no need for a dynamic seal, therefore,
far less chance of leakage of fluid that may be under
high system pressure.
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