Free Tool — Backflow Tester
Backflow PSID Calculator
Enter field test readings for any backflow prevention assembly — DC, RP, PVB, or SVB — and get instant pass/fail results per USC and AWWA M14 differential pressure criteria.
Assembly Type
Two independently acting check valves. Used for low-to-moderate hazard cross-connections.
Gauge reading with No. 2 shutoff closed — differential across first check valve
Gauge reading with No. 1 bypass opened — differential across second check valve
Pass/Fail Criteria — All Assembly Types
| Assembly | Check 1 | Relief / Air Inlet | Check 2 / Poppet |
|---|---|---|---|
| DC / DCVA | ≥ 1.0 psi | — | ≥ 1.0 psi |
| RP / RPBA | ≥ 5.0 psi | ≥ 2.0 psi & < Check 1 | ≥ 1.0 psi |
| PVB / SPVB | — | Air inlet ≥ 1.0 psi | Poppet ≥ 1.0 psi |
| SVB | — | Air inlet ≥ 1.0 psi | Poppet ≥ 1.0 psi |
Source: USC Foundation for Cross-Connection Control, AWWA M14. Verify against your state or local authority having jurisdiction (AHJ) requirements.
Backflow PSID Testing — What You Need to Know
Differential pressure (PSID) is the core measurement in backflow prevention testing. A properly functioning backflow assembly maintains positive pressure differential across its check valves — meaning the inlet side always holds more pressure than the outlet side, preventing contaminated water from flowing back into the potable supply. A differential pressure gauge (test kit) directly measures this pressure difference across each component during a field test.
The four main assembly types each have distinct test procedures and minimum PSID thresholds. Double Check Valve (DC) assemblies are the simplest — both checks must hold at least 1.0 psi. Reduced Pressure Zone (RP) assemblies are more complex: the No. 1 check must hold at least 5.0 psi, the relief valve must open between 2.0 psi and the Check 1 reading, and the No. 2 check must hold at least 1.0 psi. Pressure Vacuum Breakers (PVB) and Spill-Resistant Vacuum Breakers (SVB) protect only against back-siphonage and use an air inlet mechanism — both the air inlet and poppet must test at ≥ 1.0 psi.
For backflow certification exam prep, PSID calculation questions are among the most common. Know the thresholds for each assembly type cold, understand when an assembly fails (and why), and be familiar with the repair and retest requirements that follow a failed test. The certification exam also tests your knowledge of which assembly type is appropriate for different hazard levels and installation conditions.
Backflow Assembly Types — When to Use Each
Per USC Foundation for Cross-Connection Control and AWWA M14. Verify against your state or local AHJ requirements.
| Assembly | Hazard Level | Protection Type | Min. PSID |
|---|---|---|---|
| DC / DCVA | Low-to-moderate | Backpressure + back-siphonage | 1.0 psi each check |
| RP / RPBA | High (health hazard) | Backpressure + back-siphonage | Check 1 ≥ 5.0, Check 2 ≥ 1.0 |
| PVB / SPVB | Moderate | Back-siphonage only | Air inlet + poppet ≥ 1.0 |
| SVB | Moderate (indoor) | Back-siphonage only | Air inlet + poppet ≥ 1.0 |
Note: Continuously pressurized assemblies (DC, RP) may be used in both intermittent and continuous service. PVBs cannot be used in continuously pressurized applications or below the highest downstream outlet.
Frequently Asked Questions
What does PSID mean in backflow testing?
PSID stands for Pounds per Square Inch Differential — the pressure difference measured across a check valve or assembly component during a field test. A differential pressure gauge (backflow test kit) connects to the test cocks on the assembly and measures the PSID across each component. This reading is compared to the minimum threshold to determine a pass or fail.
What PSID is needed to pass a Double Check Valve (DC) assembly?
A Double Check Valve assembly passes when both check valves hold a differential pressure of at least 1.0 psi. If either check 1 or check 2 reads below 1.0 psi, the assembly fails and requires repair or replacement before being returned to service.
What are the RP assembly pass criteria?
A Reduced Pressure Zone (RP/RPBA) assembly passes when: (1) No. 1 check holds ≥ 5.0 psi differential, (2) the relief valve opens at a differential ≥ 2.0 psi and below the No. 1 check differential, and (3) No. 2 check holds ≥ 1.0 psi differential. All three criteria must be met. If the relief valve opens at less than 2.0 psi, or below the zone pressure instead of above it, the assembly fails.
How do I test a PVB assembly in the field?
To field-test a Pressure Vacuum Breaker (PVB), connect the test gauge to the two test cocks. Slowly close the No. 2 shutoff and open the No. 1 test cock. Record the differential across the air inlet check — it must open at ≥ 1.0 psi. Then record the poppet/check differential — it must also be ≥ 1.0 psi. PVBs only protect against back-siphonage, not backpressure, so installation must be above the highest downstream outlet.
What is the difference between a PVB and SVB?
A Pressure Vacuum Breaker (PVB) vents water to atmosphere when the air inlet opens, which can cause spilling indoors. A Spill-Resistant Vacuum Breaker (SVB) uses a design that prevents water from spilling during the air inlet opening — making it suitable for indoor installations. Both use the same pass/fail PSID criteria: air inlet ≥ 1.0 psi and poppet/check ≥ 1.0 psi. SVBs cannot be used in continuously pressurized applications.
What happens if an assembly fails the PSID test?
A failed assembly must be taken out of service, repaired or replaced, and retested before it can be returned to service. The tester completes a failed test report which is typically submitted to the local water purveyor or authority having jurisdiction (AHJ). Repair must be performed by a licensed plumber or backflow tester (depending on jurisdiction). After repair, a passing retest report must be filed within the AHJ's required timeframe.
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