LAF · Calibration procedure
Air velocity
Measures the speed of the filtered air at a grid of points across the filter face or working plane, using a calibrated anemometer, to confirm uniform unidirectional flow.
PURPOSE
Why this is checked
Unidirectional flow protects the work only if the air moves fast enough and evenly enough to sweep particles away. Low or uneven velocity indicates a loaded filter, a fan problem or blockage.
Frequency
Typically every 6 months (some sites test HEPA integrity annually), and after filter replacement, relocation or major repair, as defined in your SOP.
Learn how the LAF worksYOU WILL NEED
Standards & tools
- Calibrated thermal (hot-wire) or vane anemometer, traceable to national standards
- Measuring grid template or tape
BEFORE YOU START
Prerequisites
- Unit switched on for at least the stabilisation time in your SOP (e.g. 15–30 minutes)
- Work zone empty; sash and panels in the normal operating position
- Pre-filter clean
PROCEDURE
Step by step
- 1Divide the filter face into a grid of equal areas (e.g. 3 × 3 for a 4 ft unit) and mark the centre of each.
- 2Hold the probe at the measuring distance in your SOP (commonly 15–30 cm from the filter face, or at the working height) and perpendicular to the flow.
- 3Record the velocity at each point after the reading stabilises (or average over a few seconds).
- 4Calculate the average velocity and check each individual reading.
- 5Optionally carry out a smoke study to show the airflow is unidirectional and sweeps away from the work.
RECORD
Example observation table
| Point | Velocity (m/s) |
|---|---|
| 1 | 0.46 |
| 2 | 0.44 |
| 3 | 0.48 |
| 4 | 0.43 |
| 5 | 0.45 |
| 6 | 0.47 |
| 7 | 0.42 |
| 8 | 0.44 |
| 9 | 0.46 |
CALCULATION
How to calculate the result
Average velocity = Σ readings ÷ number of points; % deviation of a point = (Reading − Average) ÷ Average × 100
- Sum of 9 readings = 4.05 m/s; average = 4.05 ÷ 9 = 0.45 m/s
- Lowest point 0.42 m/s: (0.42 − 0.45) ÷ 0.45 × 100 = −6.7%
- Highest point 0.48 m/s: (0.48 − 0.45) ÷ 0.45 × 100 = +6.7%
- All points within 0.36–0.54 m/s: Pass
ACCEPTANCE CRITERIA
Pass if
- Average velocity typically 0.45 m/s ±20% (0.36–0.54 m/s) at the working position, as per your SOP and Annex 1 guidance.
- Individual points within the same range, or within ±20% of the average, as defined in your SOP.
- Smoke study shows unidirectional flow without dead zones or reflux into the work area.
IF IT FAILS
What to do
- Label the unit 'Not for use'.
- Clean or replace the pre-filter, check fan speed settings and remove any obstruction, then re-measure.
- If velocity stays low, replace the HEPA filter and re-qualify (velocity, integrity and particle count).
MORE
Other LAF calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.