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RPM4™ OPERATION AND MAINTENANCE MANUAL
© 2004-2007 DH Instruments, a Fluke Company Page 44
PRINCIPLE
SDS is a self defense system to protect RPM4 Q-RPTs from overpressure. SDS CLOSES
automatically to protect a Q-RPT that is not active. In a two Q-RPT RPM4, activating one
Q-RPT automatically CLOSES SDS on the other Q-RPT. Powering down the RPM4 or
energizing it also CLOSES SDS on all Q-RPTs.
When a Q-RPT’s module’s SDS is CLOSED, the Q-RPT is isolated from the TEST(+) port
and opened to the VENT port. SDS must be OPENED for the pressure applied to the
TEST(+) port to be measured. The [SDS] key is used to OPEN SDS.
The SDS function can also be used to CLOSE SDS as might be desired when leaving RPM4
at rest or when a possible overpressure situation is anticipated. Pressing [SDS] always
CLOSES any SDS systems that are OPEN.
See Section 3.2.7 for general information on SDS and its principles and Section 5.6,
Figure 8 for Q-RPT module schematics.
SDS closed is indicated by <SDS CLOSED> flashing in the current measured pressure
display field. When SDS is closed, the Q-RPT is isolated from the TEST(+) port and
connected to the VENT port.
OPERATION
Though the SDS self defense system includes features to protect Q-RPTs against
overpressure, SDS should NOT be considered a fail-safe overpressure protection system.
SDS cannot guarantee that overpressure damage will NOT occur. SDS closing automatically
may protect an RPT in case of accidental overpressure but conventional measures for
overpressure protection should always be used. Damage to Q-RPTs due to overpressure
is NOT covered by the RPM4 product warranty even when SDS is present and ON.
The maximum pressure that should be applied to an RPM4 TEST(+) port when SDS is
CLOSED is 10 MPa (1 500 psi).
When SDS is CLOSED with a pressure applied, a valve shuts isolating the Q-RPT from the
TEST(+) port. Another valve vents the Q-RPT to atmosphere through the VENT port.
If the pressure applied at the time SDS is CLOSED is less than 1 700 kPa (250 psi),
the VENT valve will open causing sudden release of pressure through the VENT port and
returning the Q-RPT to atmospheric pressure. If the pressure applied at the time SDS is
CLOSED is greater than 1 700 kPa, the VENT valve may NOT open. In this case, to vent
pressure from the Q-RPT module, remove the pressure from the TEST(+) port. This will
cause the SDS TEST port isolation valve to open partially, reducing pressure under
1 700 kPa at which time the SDS VENT valve will open (see Section 5.6, Figure 8)
Pressing [SDS] always CLOSES any SDS system that is OPEN. In a dual Q-RPT RPM4,
even if the active Q-RPT has NO SDS, pressing [SDS] CLOSES SDS for the inactive Q-RPT
if SDS is present.