7.3 Bleeding, Hookup Sequence, and Air Elimination

Quick Answer

Reliable PSID readings require a disciplined sequence: attach correctly → open test cocks carefully → bleed air from hoses and gauge paths → equalize/zero as trained → set valves for the static differential → wait for a stable needle → record. Air left in the lines compresses and expands, so the gauge jumps or hunts. A partially open bleed can steal pressure from one side and create a false low differential. On the practical exam, speed comes from rehearsed order, not from skipping bleeds.

Why air destroys accuracy

Water is nearly incompressible at field pressures; air is not. A pocket of air in a hose or gauge chamber acts like a soft spring:

If the needle is not stable, you do not yet have a reportable static differential. Waiting forever without bleeding is not a strategy—vent the air, then stabilize.

Stepwise mental model (use every time)

Exact valve names differ by 3-valve vs 5-valve and by school procedure, but the logic is shared:

1. Pre-check the site and assembly

• Confirm you are on the correct assembly, orientation is valid, and shutoffs/test cocks are operable.
• Note hazards: hot water, steam, chemicals, confined space, traffic, electrical panels near spray paths.
• Wear eye protection; gloves as needed; know where discharge will go.

2. Identify gauge valves and hoses

• Map high, low, equalize, bleeds.
• Confirm hose identity (tags/colors).
• Start with gauge valves in the safe starting positions your school teaches (often equalize open or bleeds set so you do not shock the element—follow the wet-lab script).

3. Attach hoses to the correct test cocks

• Connect high hose → high cock for this step, low hose → low cock for this step.
• Snug fittings; support hose weight.
• Point free ends and future bleed streams away from people and away from electrical equipment.

4. Open test cocks carefully

• Open slowly. High static can whip a loose hose.
• Listen for hiss of air vs solid water.
• Never stand with your face over a cock or bleed.

5. Bleed air from hoses and gauge

• Use bleed valves (5-valve) or the approved bleed sequence (3-valve) until a steady solid stream (or manufacturer-specified clear of bubbles) appears and the path feels hydraulically “hard.”
• Bleed both high and low paths. Bleeding only one side leaves the other soft.
• Catch or direct water appropriately—some sites require capture, not floor flooding.

6. Equalize / zero as required

• Open bypass/equalize per procedure so both sides see common pressure; verify the differential reads zero (or within the kit’s zero check tolerance).
• If zero is off after proper equalizing, do not invent a correction factor—see Section 7.4.

6a. Hold the kit at the correct elevation

Water in the hose is heavy, and the gauge reads whatever the water column hands it. About 2.31 feet of water equals 1.0 psi, so roughly every 5 inches you raise or lower the kit shifts the reading about 0.18 psi. On criteria measured in whole psid — a DC check at 1.0, a PVB air inlet at 1.0 — that is the difference between a defensible pass and a manufactured one.

The USC 10th Edition made this explicit. Where the 9th Edition carried a general preliminary note that “the gage must be held at the same level as the assembly,” the 10th Edition writes elevation into the numbered steps for the DC, PVB, and SVB: maintain the field test kit at the proper elevation while the No. 1 shutoff is closed. AWWA M14 states the same reference points:

Field habits that protect the reading:

• Set the kit down deliberately, on a hook, strap, or bracket at the reference height — do not let it swing from a hose while you read.
• Do not raise the kit to see the dial better mid-reading; move your head, not the gauge.
• On an SVB, resist the instinct to lower the kit between the two tests. USC 10th keeps it elevated until the air-inlet opening point is recorded; you lower it only for the final fully-open observation.
• If you had to re-hang the kit, re-stabilize before recording.

A proctor who watches you drop the gauge two feet and then write down 1.1 psid has seen an action that could adversely affect the outcome — the standard the CCC Guidelines use to score practical errors.

7. Establish the static test condition

• Close the downstream shutoff (or perform the isolation step the USC/school sequence requires for that assembly and step).
• Set high/low/bypass needles to the positions that isolate the intended differential.
• Allow the needle to settle. Do not record the first twitch.

8. Record, then proceed or restore

• Write the stable PSID (or note fail-mode behavior such as relief discharge timing).
• Move to the next step with re-bleed if you broke into air again.
• When finished, restore shutoffs/test cocks/caps per procedure; verify no continuous spray hazards remain.

Partial bleed = false low differential

A cracked-open bleed or weeping vent on one side continuously relieves that side toward atmosphere (or toward a lower pressure). Effects:

• The affected side’s pressure sags.
• The differential shrinks if you are bleeding the high side, or inflates/distorts depending on which side leaks.
• A tight check can look weak (false low hold).
• An examiner may see the drip and mark instrument technique, not just the number.

Rule: During the actual differential reading, bleeds that should be closed must be closed. If you need continuous venting to “make the needle behave,” you still have air, a leak, or a wrong valve state—fix the setup, do not live with a permanent bleed.

Order-of-operations under practical exam pressure

Time limits and fail-mode assemblies raise stress. Protect yourself with habits:

1. Say the step silently — “RP check #1: high TC__, low TC__.”
2. Hands follow the script — attach, open, bleed, equalize, isolate, read.
3. Stability first — a 10-second stable needle beats a 1-second guess.
4. Nonsense check — if PSID is absurd vs system statics, recheck hoses/valves/air before calling the assembly.
5. One change at a time — do not spin three needles and a cock simultaneously when recovering from a mistake.

Candidates fail practicals as often on rushed hookup and air as on forgotten criteria numbers.

Safety: pressure, heat, chemicals, eyes, spray

Differential testing is a pressurized activity. Treat every test cock like a loaded port until proven otherwise.

Troubleshooting unstable readings (instrument side first)

Before condemning a check:

Document true assembly behavior only after the instrument path is solid.

Field vs school lab differences

• Labs often have catch buckets, good lighting, and known statics.
• Field sites may have vibrating pipes, intermittent demand, and inaccessible cocks. Still apply the same bleed-and-static logic; if you cannot achieve a valid static isolation, do not invent a number—note the limitation per local reporting rules.

Bridge

Once hookup and air elimination are automatic, calibration and accuracy (Section 7.4) decide whether the number you carefully captured is legally and technically trustworthy. Perfect bleeding on an out-of-calibration gauge still produces wrong pass/fail risk.