7.5 Reading Agreement, Out-of-Range Steps, and Invalid Results

Reading Agreement: The 0.25 % Rule

A single rolling cycle rarely releases all the air, so C173 builds in a stability check. After the first roll and settle, the level is read; the meter is rolled again, settled, and re-read. The two readings must agree within 0.25 %. If they do, the test is essentially complete; the second reading governs. If they differ by more than 0.25 %, air is still coming out, so a third 1-minute roll is performed and a third reading taken.

All readings and the reported air content are taken to the nearest 0.25 %. The exam often gives two numbers and asks whether they "agree" — apply the 0.25 % window.

Work an example. First reading 5.50 %, second 5.75 % → they differ by exactly 0.25 %, which is within the window, so the result is stable and 5.75 % governs. Change the second to 6.25 % → a 0.75 % difference, so a third roll is required; if the third reading lands at 6.25 % (within 0.25 % of the second), it is accepted. If the third still drifts more than 0.25 %, the test is discarded and rerun with more alcohol. This iterative agreement check, not the completion of any single roll, is what makes a C173 result final.

Off-Scale (Out-of-Range) Readings

If the air content is high, the liquid level can drop below the bottom of the graduated neck, leaving no meniscus to read. The procedure is not to estimate beyond the scale. , full units of the scale) to bring the meniscus back onto the scale, then add that known amount of water back to the meter reading to get the true air content. For example, if you add water equal to 5 % of scale to return the meniscus to a readable point, the actual air is the on-scale reading plus 5 %. This keeps high-air mixes measurable without guessing past the end of the neck.

The water is added in known scale units so the bookkeeping is exact, and the addition is recorded so the final calculation can add it back. Lightweight mixes, which often carry high design air, are the usual reason a level runs off scale, which is one more way C173 and lightweight concrete go together.

Conditions That Invalidate the Test

The Grade I exam expects the candidate to recite the validity failures while performing the test. A test is invalid when:

• Persistent foam obscures the neck and will not clear even with added alcohol — restart with more alcohol from the beginning.
• Leakage from the seal or cap during inversion/rolling — any lost liquid voids the result.
• The liquid level never stabilizes (no agreement within 0.25 %) after the third roll, or fails to stabilize within about 6 minutes.
• Material was lost at any point — spilled concrete or water changes the closed-system balance.
• Concrete remains stuck/caked in the bowl when the meter is opened at the end; trapped concrete means trapped air that never reached the neck, so the reading is low and the test is invalid.

The final-bowl examination is a real exam scenario: even a textbook-looking reading is invalid if, on opening the meter, a wad of concrete is found clinging to the base.

A Decision Flow for the Reading Phase

Candidates do best when they carry a single decision flow for what to do with each reading:

1. Foam in the neck? If it will not clear with alcohol, the test is invalid — restart with more alcohol.
2. Level off the bottom of the scale? Add measured water back, record it, and add it to the result.
3. Two readings within 0.25 %? Yes → accept. No → roll a third minute and re-read.
4. Third reading still off by >0.25 %, or no stability within ~6 min? Discard and rerun.
5. Open the bowl: any concrete caked on the base? If yes → invalid, regardless of how clean the number looked.

Why These Checks Exist

Each validity rule maps to a physical failure that would otherwise hide in a plausible-looking number. Leakage removes liquid the calculation assumes is present. 25 % precision is meaningless. Instability past 6 minutes usually means the concrete is stiffening or air is still escaping — the moving target cannot be reported. Concrete stuck in the bowl means some air never reached the neck, so the reading understates the true value. The Grade I exam rewards the technician who can name the failure behind each rule, not merely memorize the rule, because the field throws combinations these checks are designed to catch.

Reading-agreement quick recap

• Successive readings must agree within 0.25 % air before the result is accepted.
• If they do not agree, roll the meter again; discard the test if it will not stabilize within about 6 minutes.
• Foam, leaks, lost material, or caked aggregate in the bowl invalidate the test and require a fresh sample.