Maintenance & Reliability · SOP

Spray Booth Air Velocity Measurement At Face

Standardised procedure for measuring face air velocity in industrial spray booths using a 16-point grid traverse method. Ensures compliance with NFPA 33, OSHA 1910.94, and applicable Canadian provincial fire codes (target 100 fpm ±10% at the face opening).

Estimated Time

45 min

Difficulty

Intermediate

Regulation

NFPA 33 / CSA Z432

Version

1.0

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Purpose

To establish a repeatable, verifiable method for measuring the face air velocity of industrial paint spray booths. Adequate face velocity (typically 100 fpm ±10% for cross-draft and downdraft booths per NFPA 33) is critical for operator health, explosion safety, and finish quality. This procedure ensures that ventilation performance meets regulatory standards and that corrective action is taken when velocity falls outside acceptable limits.

Scope

This SOP applies to all downdraft, cross-draft, and semi-downdraft industrial spray booths used in automotive refinishing, wood finishing, aerospace coating, and general industrial painting operations. It covers the face opening measurement (the plane of the booth intake or filter bank) only — duct velocity, plenum static pressure, and filter differential pressure are covered under separate PM procedures. Personnel performing this task must hold current confined-space and LOTO certification where applicable.

Safety Precautions

LOTOPerform lockout/tagout on all booth electrical systems before accessing the interior. Verify zero-energy state with a non-contact voltage tester on the exhaust fan disconnect and lighting circuits.
FLAMEBooth interior is a Class I Division 2 / Zone 2 location. No spark-producing tools, cell phones, or unrated electronics. Use only intrinsically safe or non-incendive measurement instruments.
PPEWear flame-retardant coveralls (NFPA 2112), N95 or half-face respirator with organic vapour cartridges, chemical-rated safety glasses, and nitrile gloves. If the booth has active overspray residue, upgrade to a full-face supplied-air respirator.
FALLUse a properly rated ladder or staging (Type IA, 300 lb capacity) when reaching the upper grid points. Maintain three points of contact at all times. Do not over-reach — reposition the ladder.

Required Tools & Materials

Hot-wire anemometer or rotating vane anemometer (0–500 fpm range, ±3% accuracy, NIST-traceable calibration within 12 months)
Tripod or telescoping extension pole with camera-mount adaptor for probe positioning
Masking tape (1 in wide) and permanent marker for grid layout
Measuring tape (25 ft / 7.5 m minimum)
Digital manometer with Pitot tube (optional, for verification of velocity pressure)
Data collection sheet or tablet with pre-formatted spreadsheet
PPE as listed in Safety Precautions: flame-retardant coveralls, N95/half-face respirator, safety glasses, nitrile gloves
Non-contact voltage tester for LOTO verification
Booth operational log and PM work order form

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Procedure: Step-by-Step

1 Lockout/Tagout and Pre-Measurement Checks. Perform LOTO on booth electrical and ventilation systems per facility policy. Verify that the exhaust fan is operational (visual inspection of belt, pulleys, and motor tags) and that the booth interior is free of combustibles, overspray accumulation, and loose residue. Check that all filters are in place and not visibly loaded.

2 Don PPE and Establish Safe Access. Put on all required PPE before entering the booth. Position a Type IA ladder or staging at the booth face opening. Ensure the ladder is on stable, level ground and within its safe working load. Verify that the ladder extends at least 3 ft above the landing point.

3 Grid Layout on Booth Face Opening. Measure the booth face opening width and height. Divide the opening into a 16-point grid (4 columns × 4 rows). Use masking tape to mark each measurement point at equal spacing — typical cell size is 12 in × 12 in for a standard 8 ft × 8 ft opening. Label each point A1 through D4 with a marker on the tape.

4 Anemometer Setup and Positioning. Mount the anemometer probe on the tripod or extension pole. Position the probe face perpendicular to the airflow direction, with the sensing element located exactly at the centre of each grid cell and 12 in ± 0.5 in from the plane of the booth face/filter bank. Orient the probe so that the airflow arrow (if marked) points in the direction of flow. Allow the instrument to stabilize for 10–15 seconds at each position before recording.

5 16-Point Traverse Measurement. Measure and record air velocity in feet per minute (fpm) at each of the 16 grid points in a systematic pattern (e.g., left-to-right, bottom-to-top). At each point, note any observable turbulence, dead zones, or flow reversals. If the reading fluctuates more than ±5 fpm over 10 seconds, record the average of a 15-second sampling window.

6 Calculate Average Face Velocity. Calculate the arithmetic mean of the 16 recorded readings. The average face velocity must be ≥100 fpm and ≤125 fpm for a compliant cross-draft or downdraft spray booth per NFPA 33. No individual reading should fall below 80 fpm or exceed 140 fpm. Document all raw readings, the calculated average, and the pass/fail determination on the data sheet.

7 Pass/Fail Determination and Corrective Action. If the average is below 100 fpm: inspect fan belt tension, duct obstructions, filter loading, and damper positions. If the average exceeds 125 fpm: check for open dampers, oversized fan sheave, or missing filters. Perform corrections, re-establish safe conditions, and repeat steps 3–6. Record all corrective actions and re-test results on the PM log.

8 Restore Equipment and Document. Remove all tape and markings from the booth face. Verify that the booth interior is clean and free of debris. Restore power per LOTO removal protocol. Complete the PM log entry — include all 16 individual readings, the calculated average, pass/fail status, any corrective actions taken, anemometer serial number and calibration date, technician name, and date/time. Attach the digital log to the CMMS work order.

Spray Booth Air Velocity Measurement At Face

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