Five, two, one. That is the whole numerical content of a reduced pressure assembly test, and you will have it memorized before the first coffee break of your certification class. Nobody fails a tester exam on those three numbers. What people fail on — and what gets a real assembly signed off wrong in a real vault — is the forty minutes of sequence wrapped around them: which cock, which hose, which valve closes before which other valve, and when the needle you are staring at is actually telling you something about the check valve instead of something about your own hose. Worse, the sequence is not even settled. Two different authorities will walk you through the same assembly in opposite directions, and both of them are right.
So this is a post about order. We will cover what each test cock sees, what the gauge is really comparing, and the ten-step spine that sits underneath every one of these procedures — and then the places where the two authorities part company, including the one assembly where they run the tests in exactly reverse order from each other.
Where Everything Is, and What Each Cock Sees
Every instruction in every procedure is phrased in terms of test cocks and shutoff valves, so the geography has to be automatic before the sequence will make any sense at all. A reduced pressure assembly — an RP, or RPZ — is two independently acting check valves in a single body, with a resilient-seated shutoff valve at each end, four test cocks, and a differential relief valve hanging off the middle.
Two things on that drawing earn their keep. The first is the zone. A reduced pressure assembly is named for it: the first check holds back most of the supply pressure, so the space between the checks sits permanently lower than the main. That is the pressure differential doing the protecting, and it is why the relief valve senses the zone rather than the outlet or the building piping. If protection starts to fail, it fails there first.
The second is that the test cocks are only useful if you read them as positions rather than numbers. The ordering above — inlet, supply side, zone, downstream — is the convention the USC 10th Edition procedures use on an RP, and it is why the high hose goes on #2, the low hose on #3, and the bypass hose on #4. But a PVB has two cocks, not four, and numbering does vary between assembly types and manufacturers. Testers who last a while learn to identify a cock by what it sees and confirm it against the body in front of them, because that habit survives walking up to an assembly you have never met.
What the Gauge Is Actually Comparing
A differential gauge has two ports and reads the difference between them. It is not measuring pressure; it is measuring disagreement between two places in the assembly. That sounds like a technicality until it changes what you do with your hands.
Running a one-hose procedure, the low side is simply open to air. So the gauge is comparing the test cock you are hosed to against atmosphere — at the gauge's own centerline. Which means the height you hold the kit at is part of the measurement. Water surrenders about 1 psi for every 2.31 feet of elevation:
Hold the kit eighteen inches above the cock you are reading and you have moved the number about 0.65 psi in the direction that flatters the assembly. On a second check valve with a 1.0 psid floor, that is most of the pass. This is not a rounding concern — it is enough to sign off a leaking check valve, which is the one outcome the entire certification exists to prevent. The USC 10th Edition takes it seriously enough that maintaining kit elevation appears in the DC, PVB and SVB procedures as a numbered step rather than as advice.
Level the gauge with whatever component is under test, and keep it there while you read.
And know what the reference is, because it moves as you work through an assembly: a sight-tube water column, test cock #2, the vent bleed screw, the air inlet itself. Whatever is open to atmosphere is the reference. Level to that.
The Ten-Step Spine Every Procedure Shares
Here is the part that makes eight procedures feel like one. Every ASSE one-hose test — RP, DC, PVB, SVB — runs the same spine. The assemblies differ in what they contain, so the procedures differ in what they stop to measure, but the skeleton does not move:
| # | Step | What it is for |
|---|---|---|
| 1 | Declare | Notify the customer, identify the assembly against the paperwork, inspect for violations and hazards |
| 2 | Flush | Cycle the test cocks to clear line debris before it lands on a seat and fakes a failure |
| 3 | Isolate downstream | Close shutoff valve #2 — the premises comes off the assembly |
| 4 | Connect | All five gauge valves closed, high hose straight onto the first cock |
| 5 | Purge and soft-land | Run it until the water is solid, then open the cock fully and let the needle rise and rest smoothly |
| 6 | Isolate upstream | Close shutoff valve #1 — now the assembly holds static |
| 7 | Level the gauge | Set it with the component under test, and leave it there |
| 8 | Vent the downstream side | Open the next cock, or back out the vent screw — this creates the differential |
| 9 | Read at zero flow | The moment the draining stops and the needle settles. Not before |
| 10 | Move on | Re-hose for the next component, and re-pressurise if that test needs it |
Look at what steps 3 through 9 are doing. Not one of them is a measurement. They are six consecutive moves whose entire job is to make step 9's number mean something. Isolate, so the only thing holding pressure is the component. Purge, so you are not reading compressed air. Level, so you are not reading elevation. Vent, so a differential exists to read. Wait, so you are reading a held pressure instead of a flow.
That is the honest answer to why the order matters, and it is worth being precise about it: a reading taken before those conditions are true is not a wrong number. The gauge is fine. The number is real. It is just a faithful measurement of something other than the check valve — your hose, your air pocket, your elbow height — and it goes on the report as though it were the check valve.
Isolate, Then Vent
One sequencing rule deserves pulling out on its own, because it is where order stops being an exam concern and becomes a wet-boots-and-a-conversation-with-your-supervisor concern.
Close the shutoff first, then open a test cock to atmosphere. Never the reverse.
Open a cock wide with supply still behind it and you do not get a drain that finishes — you get a jet that does not stop, in a vault, usually at face height. And the reading you are setting up for is taken at zero flow, so there was never anything to be gained by having supply behind it.
Read the published procedures with that rule in hand and the pattern is consistent almost everywhere. The RP phases hose onto a cock, purge, close shutoff #1, and then vent the next cock to atmosphere. The PVB and SVB setup phases close shutoff #2, then shutoff #1, and only then pull the vent screw. The SVB procedure even states the reasoning in its own words — crack the high bleed briefly first to let a little bulk pressure off, so water does not blast out at you.
You will find the occasional numbered step that reverses it, and if you are new this is the kind of thing that makes you doubt yourself. Don't. The direction of travel across the body of both authorities' procedures is clear, the physics of the reading does not require pressure behind the drain, and the people who do this for a living isolate first.
The RP: Three Tests, Two Legal Orders
Now the interesting part. An RP has three things to measure, and the two authorities measure them in nearly opposite order.
That last point is the one worth carrying out of this post, because it turns an arbitrary-looking disagreement into something you can reason about instead of memorize.
USC proves check valve #2 with backpressure. It pushes pressure onto the downstream side of that check and watches whether it holds. Pushing pressure requires somewhere to push from, which is the third hose — the yellow bypass line on cock #4 — and it requires supply to stay available, which is why shutoff valve #1 stays open for the entire test.
ASSE proves the same check by isolating and venting. Close everything off, vent downstream of check #2, and read what the check holds back. Nothing has to be pushed anywhere, so one hose does the whole job. Less kit to carry into a vault, which is not a small consideration when the vault is half full of mud.
Two legitimate ways to ask one check valve the same question. Once you have that, the sequences stop being two lists to memorize and become two consequences of a single decision.
The three minimums, and why they are not three independent numbers
| Component | Passing | What the number is protecting |
|---|---|---|
| Check valve #1 | at least 5.0 psid | The margin above the relief's opening point |
| Relief valve | opens by 2.0 psid | That it dumps the zone to air before the first check fails |
| Check valve #2 | tight, at least 1.0 psid | That premises water cannot enter the zone |
New testers almost always read that table as three unrelated specs and wonder why the first check has to work five times harder than the second. It doesn't — the 5 and the 2 are one idea written twice.
A reduced pressure assembly is designed to fail safe by dumping the zone to atmosphere, and for that to work the relief valve has to open before the first check loses its seal. The relief opens at 2.0. So the first check's floor is set deliberately above it, leaving roughly 3 psid of daylight between "the relief starts protecting you" and "the first check stops holding." A first check reading 3.5 psid is still holding water perfectly well — and has eaten most of the buffer the assembly's whole safety argument rests on. That is why it fails at a number that would pass comfortably as a second check.
The second thing that table hides: these are three separate findings, and none of them implies either of the others. Two immaculate check valves can sit in front of a relief valve seized with scale, and the assembly will pass both check tests and still be unable to protect anything. The relief test asks whether it opens at its design point; the check tests measure tightness. Neither proves the other.
The DC Is the RP Minus the Relief Test
A double check assembly is two independently acting checks, two resilient-seated shutoffs, and test cocks — and no relief valve. So the procedure is the RP's with the middle test deleted, and both checks pass at 1.0 psid. There is no 5 psid first check here, because there is no relief valve for it to keep margin above. The number was never about the check valve's strength; it was about the relationship.
The USC route on a DC introduces a wrinkle worth knowing about before you meet it, because it looks like a malfunction: it uses a bleed-off valve tee and a sight tube, and the water column in that tube falls while you are reading it. That is the test working, not failing. The column is your atmospheric reference, the reference is dropping, so the gauge gets re-levelled to the settled line rather than to where the column started. Read the gauge against where the water ends up.
The Vacuum Breakers, Where the Order Flips
A pressure vacuum breaker is a spring-loaded check valve plus a separate spring-loaded air inlet. It stops back-siphonage only — it offers nothing against backpressure, which is why finding a booster pump downstream of one is a finding and not a curiosity. There are two things to test: the air inlet must open by 1.0 psid and open fully, and the check valve must hold at least 1.0 psid.
Two tests, two possible orders, and this is where the authorities genuinely cross over each other.
Sit with that for a second. If you learned PVBs under ASSE, you lead with the check valve. If you learned them under USC, you lead with the air inlet. And if you then reason "right, USC leads with the air inlet" and carry it to an SVB — you are wrong, because USC's SVB procedure leads with the check valve, exactly like ASSE's.
There is a further detail in the USC PVB procedure that catches people honestly, and it is a good illustration of why reading a procedure is not the same as running one: the bleed-off valve tee gets fitted during setup, in Phase 1, even though the first test never uses it. It is there for the second test. Fit it at the start and you are not breaking down a hose connection mid-procedure on a pressurised assembly.
The Pattern That Lets You Reconstruct Any of Them
Eight procedures is a lot to hold. The good news is that nobody actually holds eight. Underneath all of them is one four-beat move, repeated once per component:
Isolate the component so it is the only thing holding pressure. Establish something open to atmosphere as your reference, and level the gauge to it. Vent the downstream side so a differential exists. Then wait for the flow to stop and read what is left.
Every difference between the eight procedures is a variation on one of those four beats. The reference changes — a test cock on an RP, a sight-tube column on a DC, a vent bleed screw or the air inlet itself on an SVB. The isolation changes — ASSE closes shutoff #1, USC leaves it open and brings a third hose instead. The order changes, because the method you chose for one component dictates what state the assembly has to be in for the next one. The four beats do not change.
Learn the beats and the specifics have somewhere to attach. Learn eight step lists cold and you are carrying 178 facts with nothing holding them together — which works right up until you are standing in a vault, two steps out of sequence, trying to work out whether the needle is lying to you.
Walk All Eight Procedures, Step by Step
Reading a procedure teaches you the list. It does not teach you the order, because you can nod along to a list in any order and still have no idea what the assembly is doing at step 14. The Cross-Connection Control & Backflow Testing product includes a Step Trainer: all eight procedures — RP, DC, PVB and SVB, each under ASSE and under USC 10th Edition / ABPA — broken into 178 decision points. At every step you pick the next action from four plausible options, and an animated assembly and test kit show you exactly what your choice did: valves move, water drains, the needle swings hard over and settles. Every wrong option is a mistake a real tester actually makes, with an explanation of why it is wrong at that point in the procedure. It ships alongside a 152-question practice exam, 110 flash cards across 9 decks, and the field procedures themselves.
The eight procedures, and what each one costs you
| Assembly | Authority | Kit | Steps | Order of tests |
|---|---|---|---|---|
| RP | ASSE | one hose | 22 | check #1 → relief → check #2 |
| RP | USC / ABPA | three hoses | 26 | relief → check #2 → check #1 |
| DC | ASSE | one hose | 20 | check #1 → check #2 |
| DC | USC / ABPA | tee + sight tube | 20 | check #1 → check #2 |
| PVB | ASSE | one hose | 22 | check valve → air inlet |
| PVB | USC / ABPA | tee | 24 | air inlet → check valve |
| SVB | ASSE | one hose | 22 | check valve → air inlet |
| SVB | USC / ABPA | tee | 22 | check valve → air inlet |
The Bottom Line
The numbers are the part of backflow testing that fits on a flash card, and they are genuinely worth having cold: 5, 2 and 1 on an RP; 1 and 1 on a DC; 1 psid and opens fully on an air inlet. Learn them. They will take you an afternoon.
But a differential reading is not a thing you take. It is a thing you construct — out of six or seven preceding moves, every one of which exists to remove one more reason the number might be about something other than the component you are testing. The order is not procedural fussiness laid over the measurement. The order is the measurement.
Which is also the kindest thing about the job, once it clicks. You are not being asked to memorize eight lists. You are being asked to understand one four-beat move well enough to rebuild any of the eight from the assembly in front of you — and that is a thing you can actually carry down a ladder.
Common Questions About Backflow Assembly Testing
What are the three tests on a reduced pressure (RP) assembly?
Check valve #1, the differential relief valve, and check valve #2. Check #1 is measured for how much differential it holds, the relief valve is measured for the differential at which it starts to discharge, and check #2 is measured for tightness against reverse flow. They are three separate findings and none of them implies either of the others — both checks can be perfect behind a relief valve that is rusted shut.
What are the minimum passing readings for an RP backflow test?
Check valve #1 must hold at least 5.0 psid, the relief valve must open by 2.0 psid, and check valve #2 must hold at least 1.0 psid and be tight against reverse flow. On a double check assembly there is no relief valve and both checks pass at 1.0 psid. On a pressure vacuum breaker or spill-resistant vacuum breaker, the air inlet must open by 1.0 psid and open fully, and the check valve must hold at least 1.0 psid.
Why is check valve #1 on an RP required to hold 5 psid when check valve #2 only needs 1?
Because the two numbers are not independent. The relief valve opens at 2.0 psid, and the whole safety logic of a reduced pressure assembly is that the relief valve dumps the zone to atmosphere before the first check loses its seal. Check #1's 5.0 psid minimum sits deliberately above the relief's 2.0 psid so that margin exists. A first check reading 3 psid is still holding water, but it has eaten the buffer the assembly is designed around — which is why it fails at a number that would pass as the second check.
Why do ASSE and USC test the same backflow assembly in a different order?
Because they interrogate check valve #2 by different physics. USC 10th Edition puts backpressure on the downstream side of check #2 to prove it, which requires a third hose and means supply pressure has to stay available, so shutoff #1 stays open for the whole test. ASSE instead isolates the assembly and vents downstream of check #2, reading what the check holds without pushing pressure anywhere — which one hose can do. Both are legitimate ways to ask the same check valve the same question, and each one's sequence falls out of the method it chose.
Does the order of a backflow test actually matter?
Yes, because almost every step exists to establish the condition the next reading depends on. A differential reading is only meaningful if the component under test is isolated, the air is out of the hoses, the gauge is level with the reference, and the flow has stopped. Take the reading before any one of those is true and the number you write down is real — it is just a measurement of something other than the check valve.
Why does the height of the test kit change the reading?
Because with the low side open to atmosphere, the gauge reads against atmosphere at its own centerline, so any height difference between that centerline and the reference point lands directly on your number. Water gives up about 1 psi for every 2.31 feet of elevation, so holding the kit a foot and a half off the reference moves the reading most of a psi — enough to pass a failing check valve or fail a passing one. This is why USC 10th Edition wrote maintaining kit elevation into the procedure as a numbered step rather than leaving it as advice.
Does a passing backflow test mean the assembly is good?
It confirms the assembly's condition on test day, and nothing beyond that. A passing report is a dated snapshot, not a warranty. It is also worth knowing what a single finding does not cover: a first check holding tight proves only that supply water cannot pass into the zone between the checks, and says nothing about whether the second check holds against the premises side.