Measuring paint spray booth clearance time

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 to give a worst-case time. It is recommended that a clearance time test should also be included in the 14-month thorough maintenance examination of the booth or room; however, more frequent testing may be required.

  1. 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/room.
  2. The booth or room should be set up for normal spraying operations except with the ventilation deactivated (see No. 3) and the lights on maximum to enable the smoke aerosol to be seen.
  3. Ensure that the extraction system is turned off. There are two reasons for this:
    1. in a booth with the ventilation running it would be very difficult to fill the room completely;
    2. Filling the booth 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 RPE.
  4. Fill the room with smoke, making sure to distribute smoke evenly throughout the room (an extension lead may be useful in allowing all areas of the room to be reached).
  5. The room shall be regarded as full when the facing wall is no longer visible when viewed across the short axis of the room. Depending on the fog machine used, it may cut out and require time to reheat one or more times before this is achieved.
  6. Switch on the ventilation system and start a timer.
  7. During the smoke test the opportunity should be taken to do a visual inspection of the exterior of the booth and any associated ductwork to check for any leaking air.
  8. The room shall be regarded as clear when smoke is no longer visible in any part of the room. 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.
  9. 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.

Note 1:

A practical alternative to ensuring people are not exposed to the invisible paint mist is to provide sensors which automatically warn spray booth/room users of the potential presence of isocyanates. The research report 'An automated system for indicating spray clearance times of MVR spray booths and rooms - RR742' describes the design and assembly of a device consisting of a timer switch, programmed with the clearance time, which is triggered by a sensor that detects when the spray gun is turned on and off. Two types of sensor are evaluated and recommendations made for different operational setups. The system is relatively inexpensive and can be retrofitted to existing spray booths/rooms.

Note 2:

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.

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Updated 2021-02-01