Using a velocity meter for a more cost efficient method to determine the high (red) mark.
1. Measure the velocity inside the booth when the blower is turned on and the filters are new (i.e. at the green mark). According to OSHA regulations the velocity should be at least 100 ft/min (0.5m/s) for most cross draft spray booths. You should refer to the regulations to determine what the velocity would be for your booth design.
2. If the velocity is less than that required by OSHA, you should ask a qualified, certified electrician to increase the fan rotation until you attain at least 100 ft/min. If you don’t have an electrician who can do this, call your spray booth vendor.
3. After you ensure that the initial velocity is more than 100 ft/min, slowly plug up the filters by placing small pieces of masking paper over the filters. Read the velocity inside the booth.
4. If the velocity was more than 100 ft/min at the green mark, you can keep building up resistance on the filters until the velocity drops to 100 ft/min. That would be the high level mark (red) at which you would need to change filters.