Flow Hood Reading Correction and Diffuser Airflow

A hood is a resistance in series with the diffuser, and adding resistance to a system reduces the flow through it -- so a hood commonly reads LOW, and how much depends on how much authority the diffuser had to begin with. On a stiff system with plenty of pressure available the effect is small; on a soft one, a long flex run, a nearly closed damper, or a fan riding a flat part of its curve, it can be large. The correction factor is therefore not a property of the hood alone, which is why entering a reference traverse reading here DERIVES the factor from the system in front of you rather than taking one from a table. That is the better practice: traverse three or four representative branches, compare against the sum of the hood readings on their outlets, and apply the ratio to the rest. One exercise calibrates the whole report. The direction of the correction is computed rather than assumed, because it can run either way: a factor above one means the hood was reading low and the correction raises the number, below one means the reverse. What matters more than the direction is that the error is SYSTEMATIC -- it is the same sign on every reading -- so it does not average out across a report, and an uncorrected total misstates the whole system by the same proportion it misstates one outlet. The placement failures are simpler and larger than any correction factor: a hood that does not seal to the ceiling leaks and reads low, and one used on a linear slot or a perforated face without the right adapter has an unrepresentative velocity profile across its sensor grid. Both are worth checking before any factor is applied, and neither is captured here. This does not measure anything, select a hood or adapter, or model the back pressure from the hood's own resistance and the diffuser's pressure-flow curve. A duct traverse is the reference measurement, the hood manufacturer's data and AABC or NEBB procedure govern, and the balancer's own judgment decides when a reading is not usable at all.

corrected flow = hood reading x correction factor; where a reference traverse is entered the factor is DERIVED as traverse / reading; the report total corrects by the same factor, and the difference is the systematic error the uncorrected report carried.

The balancing-hood correction as AABC and NEBB field practice states it, with the factor best established on the actual system by comparing hood readings against a duct traverse rather than taken from a table. It does not model the back pressure from the hood's own resistance, select a hood or adapter, or capture placement failures such as an unsealed hood or a linear slot without the right adapter.

One multiplication and one ratio; no manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: hood_reading_cfm, correction_factor, reference_traverse_cfm, design_cfm, system_reading_total_cfm, corrected_cfm, pct_of_design, uncorrected_pct_of_design, system_corrected_cfm, system_error_cfm

• The factor is not a property of the hood alone it depends on the diffuser and the system's available pressurea duct traverse on the same system
• Placement failures are not captured an unsealed hood or a missing slot adapter is larger than any factorthe balancer's own inspection
• It measures nothing this corrects a reading you tookAABC or NEBB procedure