Double Check
Detector Assembly.

When DCDA, not RPDA

A DCDA is allowed when the fire system contains nothing but potable water — no glycol, no foam concentrate, no biocide, no firefighting additive. In Milwaukee that's roughly 60% of newer commercial fire systems (post-2000) and a smaller share of older ones. The cost difference is meaningful: a 4″ DCDA test is roughly $420, the equivalent RPDA is $520, and the install cost difference between the two assemblies at 4″ is approximately $1,400 in materials. When we are commissioning a new fire system we ask the sprinkler contractor in writing whether antifreeze is or will be added; the answer goes on the install record and the choice of detector follows.

Construction

Identical to the RPDA except the main and bypass paths use DC assemblies (no relief vent). The detector meter is identical to the RPDA's; the bypass DC is typically a 3/4″ Watts 007 or Wilkins 350.

Test sequence

DC test on the main path, DC test on the parallel bypass DC, and a meter accuracy check on the detector meter. Three sub-tests; one consolidated report. We submit to Milwaukee Water Works and to the fire marshal the same day; we copy the property manager. Each step of the sequence:

Step	Action	Acceptance
1	Close downstream SOV on main. Test CK-2 on main path via TC #3.	≥ 1.0 psid, tight
2	Test CK-1 on main path via TC #2.	≥ 1.0 psid, tight
3	Isolate the bypass loop with its valves. Test CK-2 of bypass DC.	≥ 1.0 psid, tight
4	Test CK-1 of bypass DC.	≥ 1.0 psid, tight
5	Run a calibrated flow (~2 gpm) through the bypass. Verify meter reading.	±2% of calibrated flow
6	Reopen the main path. Verify no continuous discharge.	tight

DCDA failure modes

Fire-service DCDAs sit unfilled with motion for months at a time, which is its own failure mode. Of the 47 DCDA tests we ran in 2025, the failures we recorded:

• Bypass meter drift · 32%

  The detector meter reads ±5% or more from its calibration baseline. Re-calibrate during the visit; replace the meter if drift exceeds 10%.

• CK-2 on main fouled with sediment · 28%

  Stagnant water in the sprinkler riser deposits scale and biofilm on the second check seat. Rebuild kit with new diaphragm and seat ring.

• SOV stuck open · 14%

  Brass ball valve seized after years without exercise. Free with penetrating oil during the visit; replace if the stem stripped.

• Bypass DC weak · 12%

  The smaller bypass assembly fatigues faster than the main because every detector flow passes through it. Rebuild the bypass on a five-year cycle.

• Tag missing or illegible · 8%

  Re-tag with current serial, install date, and last-test date.

• Alarm-monitoring miscommunication · 6%

  Not a device fail; a procedural one. A closed SOV during the test triggered the alarm because the monitoring company wasn't placed on-test in advance. We have written letters on file for every alarm company in the metro area to prevent this.

USC FCCCHR-approved DCDA models

Mfr / Model	Sizes	USC list #
Wilkins 350DA	2.5″ – 10″	48-002
Watts 709DCDA	2.5″ – 10″	48-006
Ames C400DCDA	4″ – 10″	48-009
Hersey FDC-II Detector	4″ – 10″	48-001
Apollo 4ADA-300	2.5″ – 6″	48-013

References