This is a procedure for measuring the clearance time of a spray booth or room using a party fog machine or professional smoke machine. The clearance time should be checked just before the filters in the spray booth or room are changed.
It is recommended that a clearance time test should be undertaken as part of the thorough examination and test of the booth.
All local exhaust ventilation (LEV) systems should be thoroughly examined and tested at least once every 14 months, although some systems may require more frequent testing, including clearance times.
Procedure for measuring clearance time of a designated spray booth or room
- The spray booth or room should be empty when measuring the clearance time. This is because the volume of the car will effectively reduce the volume of a booth or room and give a lower clearance time. In addition, the smoke generated is made of a glycol aerosol and may leave a greasy deposit on any vehicle or body part in the booth or room.
- The booth or room should be set up for normal spraying operations except with the ventilation deactivated (see bullet point 3) and the lights on maximum to enable the smoke aerosol to be seen.
- Ensure that the extraction system is turned off. There are 2 reasons for this:
- in a booth or room with the ventilation running it would be very difficult to fill the room completely.
- filling the booth or room with the extraction turned off will give a clearance time showing the worst case scenario ensuring the room is clear before anybody enters, or sprayers remove their respiratory protective equipment (RPE).
- Fill the booth or room with smoke, making sure to distribute smoke evenly throughout it (an extension lead may be useful in allowing all areas of the booth or room to be reached).
- The booth or room shall be regarded as full when the facing wall is no longer visible when viewed across the short axis of the booth or room. Depending on the fog machine used, it may cut out and require time to reheat one or more times before this is achieved.
- Switch on the ventilation system and start a timer.
- During the smoke test the opportunity should be taken to do a visual inspection of the exterior of the booth or room and any associated ductwork to check for any leaking air.
- The booth or room shall be regarded as clear when smoke is no longer visible in any part of it. A lamp (viewed from a narrow angle towards the beam of light) may be useful in judging this, though ensuring that the room is truly full at the start of the measurement is more important than precise judgement of when the smoke has cleared. The difference between clear by eye and clear using a lamp is typically 30 seconds longer.
- Note the time at which the room is judged to be clear of smoke. This time should be rounded up to the next quarter minute. This should be put on a notice and displayed on the door or entrance of the booth or room, and all personnel who need to know should be told.
Protection from exposure – sensors
A practical alternative to ensuring people are not exposed to the invisible paint mist is to provide sensors which automatically warn spray booth or room users of the potential presence of isocyanates.
Protection from exposure – respiratory protective equipment
Appropriate RPE should be worn during the clearance time measurement; a disposable dust mask with a combination A/P3 filter will suffice. The smoke used is of low toxicity but may cause a slight irritation of the throat because of the high concentration, and it is not good practice to expose people to high smoke concentrations.
Smoke is most likely to linger and form eddies close to all the walls of a downdraft spray booth; in a room, the smoke will mix with the air and dilute evenly.
By disabling the extraction before filling the room with smoke a maximum clearance time is measured. As the extraction is usually running during normal spraying operations, the clearance time may be less than this but as a safety margin, it is important that the maximum time is observed.