Cal. Code Regs. Tit. 17, div. 3, ch. 1, subch. 7.5, app 2 - Method for Measuring Inward Face Velocity

Inward face velocity must be measured at least once every calendar year and whenever the air pollution control system is changed in any way that may impact air flow to ensure that the ventilation system is working properly. Measurements must be conducted in accordance with the procedures specified in this Appendix 2 or an alternative method approved by the permitting agency.

1. Hood Measurement:

Divide the face of the hood, the slot area, or the normal plane, at the capture velocity measurement point into equal area rectangles (see Figure 1). The side of each rectangular area should be no longer than 12 inches. Measure the air velocity (fpm) at the center of each rectangle using a calibrated anemometer or other measuring device approved by the permitting agency. The velocity measuring device must have an accuracy of at least +10% of full scale. The measuring device must be in good condition, of proper velocity range, and operated according to the manufacturer's instructions. The measuring device must be calibrated in accordance with the manufacturer's recommendations. Do not block or disturb the airflow while taking the readings.

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Figure 1: Airflow distribution measurement for an exterior hood and an enclosing hood

Measure the volumetric airflow rate through the hood by measuring the velocity at the center of each equal-sized rectangular area (i.e., by performing pitot traverses.) If no suitable location exists for performing complete pitot traverses, measure the slot velocity and use this data to estimate the volumetric airflow rate through a hood.

2. Walk-in Booth Measurement:

For a cross-draft walk-in booth (i.e., air enters through filters in the front of the booth and leaves through filters in the back of the booth):

Divide the length of the booth into at least three cross-sectional areas to obtain the velocity profile in the booth. One cross-sectional area must be located near the exhaust plenum, one close to the supply plenum, and the other in the middle of the booth. Figure 2 illustrates the location of cross-sectional areas. Record the distance between each cross-sectional area and the exhaust or supply plenums. The distance between each cross-sectional area must not exceed ten feet.

Lay out imaginary grid lines through each cross sectional area. Use the intersections of the grid lines as locations to measure velocities inside the booth. The intersection points must be no more than six feet apart. Record the location of each point on the grid. Measure the air velocity (fpm) at each intersection point on the grid using a calibrated anemometer or other measuring device approved by the permitting agency. The velocity measuring device must have an accuracy of at least +10% of full scale. The measuring device must be in good condition, of proper velocity range, and operated according to the manufacturer's instructions. The measuring device must be calibrated in accordance with the manufacturer's recommendations.

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Figure 2: Airflow distribution measurement inside a cross-draft walk-in booth

For a down-draft walk-in booth (i.e., air enters through filters in the ceiling of the booth and leaves through filters that cover trenches under a metal grate floor):

Divide the height of the booth into at least three cross-sectional areas to obtain the velocity profile in the booth. One cross-sectional area must be located near the exhaust plenum, one close to the supply plenum, and the other in the middle of the booth. Record the distance between each cross-sectional area and the exhaust or supply plenums. The distance between each cross-sectional area must not exceed ten feet.

Lay out imaginary grid lines through each cross sectional area. Use the intersections of the grid lines as locations to measure velocities inside the booth. The intersection points must be no more than six feet apart. Record the location of each point on the grid. Measure the air velocity (fpm) at each intersection point on the grid using a calibrated anemometer or other measuring device approved by the permitting agency. The velocity measuring device must have an accuracy of at least +10% of full scale. The measuring device must be in good condition, of proper velocity range, and operated according to the manufacturer's instructions. The measuring device must be calibrated in accordance with the manufacturer's recommendations.

3. Average Value of Readings

Calculate the average value for all velocity readings, if all individual readings are within +20% of the average value. Do not include turbulent readings when calculating the average (turbulent airflow may be indicated by negative or zero velocity readings.) Record and make available for inspection by the permitting agency the entire velocity profile to show the airflow distribution. Examples:

Hood A - Velocity Readings (fpm)

100	90	110
85	115	100
105	95	100
Average Velocity =
900 fpm / 9 =
100 fpm

Hood B - Velocity Readings (fpm)

200	200	0
200	50	0
100	-5 *	-45 *
Average velocity =
750 fpm / 7 =
107 fpm **

* Negative values indicate airflow in reverse direction and are not included in the average.

** This is not a valid average, because individual readings are not within +20% of the average. The booth airflow needs to be adjusted and balanced before the velocity is measured again.

Notes

Cal. Code Regs. Tit. 17, div. 3, ch. 1, subch. 7.5, app 2