Testing, Adjusting and Balancing (TAB): Measurement, Proportional Balance, and Documentation
The TAB domain is the technical core of the NEBB Certified Professional exam. It tests field procedure rather than theory: which instrument you choose, where you place a traverse, how you correct for air density, how you sequence a proportional balance, and what the final report must contain. NEBB Procedural Standards and ASHRAE 111 are the reference documents behind most items, so answer from procedure, not from rules of thumb.
Measurement fundamentals
- Pitot-static traverse: V (fpm) = 4005 × √Vp, with Vp in in. w.c. at standard air (0.075 lb/ft³). At other densities use V = 1096 × √(Vp / ρ).
- Airflow: cfm = fpm × free area in ft².
- Locate the traverse where the profile is stable - roughly 7.5–8.5 diameters downstream and 1.5–3 diameters upstream of an elbow, transition, damper, or takeoff. Short runs need more points and a note in the report.
- Space points at log-Tchebycheff centers so each reading represents an equal area; a simple average is only valid then.
Balancing logic
Proportional balancing: read every terminal with all balancing devices wide open, convert each reading to percent of design, and treat the lowest percentage as the index circuit. Leave the index unthrottled and throttle the others until each matches the index percentage. Then raise total flow - fan speed, sheave change, or pump trim - until the index reaches 100 percent.
Worked example
A 24 in × 18 in duct is 3.0 ft². An eight-point traverse averages Vp = 0.36 in. w.c. √0.36 = 0.600, so V = 4005 × 0.600 = 2,403 fpm and cfm = 2,403 × 3.0 = 7,209, reported as 7,210 cfm. At 95 °F instead of 70 °F the density falls, the same Vp corresponds to a higher velocity, and the correction must be applied before comparing to design.
Traps that cost marks
- Applying a capture hood without that diffuser's factory correction factor.
- Averaging velocities from unevenly spaced points, or skipping the density correction entirely.
- Signing off before re-reading every terminal after the last adjustment; systems interact.