ASSE 5120 common mistakes
ASSE 5120 · Understand the traps and how to avoid them.
Treating any cross-connection as automatically requiring a reduced-pressure principle assembly, instead of first classifying the hazard as high or low and the connection as direct or indirect.
Candidates memorize device names without linking them to the hazard classification logic, so they default to the most protective assembly for every scenario.
What to do instead
Before selecting a device, write two labels: hazard class (high/low) and connection type (direct/indirect). Then apply the rule that high-hazard direct connections need an RP assembly, low-hazard connections may use a double check or PVB depending on conditions. Practice with 20 short scenarios until the two-step classification is automatic.
Confusing backsiphonage with backpressure when diagnosing a described scenario, and therefore choosing the wrong protective device.
Both are backflow conditions, and candidates focus on the visible plumbing rather than on which side of the system has the higher pressure.
What to do instead
Use a pressure-comparison test: ask whether the supply pressure dropped below atmospheric (backsiphonage) or the downstream pressure rose above supply (backpressure). Backpressure cannot be stopped by a vacuum breaker or PVB, which is the key discriminator to memorize.
Mixing up gauge pressure and absolute pressure, or ignoring elevation changes, when calculating pressure at a point in a system.
Formulas are memorized without units, so candidates plug in gauge readings where absolute pressure is required, or forget that 1 psi equals about 2.31 feet of water column.
What to do instead
Always write units next to every number. Convert elevation with the 2.31 ft per psi relationship and add atmospheric pressure (about 14.7 psi) only when the problem explicitly asks for absolute pressure. Work three practice problems aloud, narrating each unit conversion.
Assuming flow rate stays constant when pipe diameter changes, rather than applying continuity so velocity rises as area falls.
Everyday intuition suggests smaller pipes carry less water, and candidates conflate pressure with flow.
What to do instead
Anchor on continuity (area times velocity is constant) and Bernoulli for pressure changes. Sketch the pipe with diameters labeled and compute velocity ratios before answering. Remember that velocity increase corresponds to a pressure decrease in a converging section.
Misidentifying an assembly from a verbal description, such as calling a double check valve assembly an RP or confusing a PVB with an atmospheric vacuum breaker.
The devices share components (check valves, test cocks), and candidates rely on superficial feature matching instead of the defining testable elements.
What to do instead
Build a comparison table with columns for number of check valves, presence of a relief valve, test cock count and location, and whether the device may be under continuous pressure. Recite the distinguishing feature for each device from memory before reading answer choices.
Forgetting that atmospheric vacuum breakers cannot be installed under continuous pressure or downstream of a shutoff valve.
Candidates focus on the anti-siphon function and overlook the installation restrictions that make the device legal or illegal.
What to do instead
Memorize the AVB restriction as a hard rule and pair it with the PVB alternative when downstream shutoff is needed. When a question describes a shutoff valve downstream of an AVB, flag it immediately as a code violation.
Recording test results without verifying gauge calibration, or reading the gauge at the wrong moment in the test sequence.
Candidates rush to the numeric answer and skip the pretest checks that determine whether the reading is valid at all.
What to do instead
Adopt a fixed pretest checklist: confirm gauge calibration date, verify test cock operation, and bleed lines before recording. Practice the sequence in order so the reading step is never separated from the setup step.
Reporting a differential pressure reading without accounting for gauge resolution or applying the correct pass/fail threshold for that specific assembly.
Thresholds differ by device and jurisdiction, and candidates apply a single memorized number to every assembly.
What to do instead
Keep a reference card listing the applicable threshold for each assembly type and note that local rules may be stricter. State the threshold explicitly in your reasoning before comparing it to the measured value.
Citing a general standard when the question turns on a local ordinance or utility requirement, or vice versa.
Candidates treat codes and standards as interchangeable and do not distinguish model codes from adopted local rules.
What to do instead
Classify each regulatory question first: is it asking about a model standard, a state adoption, or a local water purveyor requirement? Answer at the level asked. Build a short hierarchy note showing how model codes flow into local adoption.
Leaving test reports incomplete, such as missing the assembly serial number, tester certification number, or date, or failing to tag the assembly after testing.
Candidates treat paperwork as an afterthought and focus only on the mechanical procedure.
What to do instead
Use a standard report template with required fields and a post-test tagging step built into the workflow. Before closing any test scenario, verify all mandatory fields are populated and the tag reflects the current status.