Reduced Pressure
Detector Assembly.

Why detector?

A fire service line is unmetered. Water moves through it only when the sprinkler system fires, the standpipe is exercised, or someone — without authorization — opens a connection somewhere on the building side of the assembly. The detector variant adds a parallel 5/8″ meter sized for low flow (typically 0.5–10 gpm) so the utility can see any small, sustained flow that wouldn't register on the main fire-flow meter. A glycol leak, a coffee maker mistakenly tapped to a fire-only riser, an irrigation contractor mis-routed to the wrong meter — all show up on the detector read.¹

Wisconsin uses RPDAs (rather than DCDAs) on fire systems that contain antifreeze, foam, or any chemistry — most pre-2000 commercial fire systems in Milwaukee, where the heating contractor added glycol to keep sprinklers in an unheated stairwell from freezing. SPS 382.41(11) Table 41 prescribes the RPDA for any "fire system containing additives." We file the AHJ submittal on the day of test, separately to Milwaukee Water Works for the meter read and to the local fire marshal's office for the system inspection.

Construction

The main path is a full-size RP. The bypass is a smaller RP (typically 3/4″ Watts 909 or Wilkins 375) in parallel, with a low-flow detector meter in series with the bypass. At any flow below the cracking pressure of the main check, water diverts through the bypass and the meter logs it. At fire-flow rates the main checks open and the bypass is bypassed.

Test sequence

The annual test is an RP test on the main path, an RP test on the bypass path, and a meter accuracy check on the bypass meter. All three pass / fail conditions must be met for the assembly to pass. We file three separate test forms to the AHJ and one consolidated report to the customer.

Fire-service rules

• Coordinate with FD inspection.

  When the fire department conducts its annual sprinkler-system inspection, we align our backflow test to the same morning whenever possible. One shutdown.

• Notify monitoring station.

  The customer's alarm monitoring company must be on-test before we close any valve on a fire-line assembly. We supply the standard letter MWW expects.

• Re-test after firefighting use.

  If the sprinkler system fired between annual cycles, the assembly is re-tested before being put back into service. Any RPDA can foul during high-flow.

USC FCCCHR-approved RPDA models

Mfr / Model	Sizes	USC list #
Wilkins 350ADA / 375ADA	2.5″ – 10″	47-005
Watts 957RPDA	2.5″ – 10″	47-008
Ames C500	4″ – 10″	47-011
Conbraco Apollo 4ADA	2.5″ – 8″	47-014
Hersey FRDC	4″ – 10″	47-002

References