You can read parts 1 and 2 here:
Q: Are manometers or other pressure-drop devices required by law on exhaust filters?
A: A means is required to make sure that you are moving the required amount of air for paint spraying. Your choices depend upon the code or standard adopted by your local authority. According to the NFPA® 1 Fire Code, the requirements spelled out in NFPA® 33 Standard for Spray Application Using Flammable or Combustible Materials 2011 Edition would apply.
NFPA 33 states that “Spray areas equipped with overspray collection filters shall have an effective means to ensure that the performance requirements of Section 7.2 are met.” [33:7.2.1] NFPA 33 also offer explanatory material that says that “Acceptable means to comply with 7.2.1 include, but are not limited to, visible gauges, audible alarms, approved interlocks, or an effective inspection program.” [33:A7.2.1] says that “Acceptable means to comply with 7.2.1 include, but are not limited to, visible gauges, audible alarms, approved interlocks, or an effective inspection program.” [33:A7.2.1]
According to the International Fire Code, the requirements spelled out in Chapter 24 would apply. The IFC states that “Visible gauges, audible alarms or pressure-activated devices shall be installed to indicate or ensure that the required air velocity is maintained.” [IFC:2404.7.8.3]
Therefore, some means is required to ensure that the required air movement is maintained within the spray area. Both codes give you options. The intent of these requirements is to alert the operator of the point where the filters are loaded and can no longer pass the required amount of air. At that point, the filters need to be replaced.
Q: What is the best placement of the sensor tubes coming from the magnahelic gauge for a downdraft pit type booth?
A: The answer depends on what pressure differential you are trying to measure. If you are measuring cabin pressure compared to building pressure, the high pressure sensor tube should be mounted in the cabin away from any booth openings. The low pressure tube should be left open to the shop. If you want to measure static pressure loss across the pit filter, the high pressure sensor tube should be mounted in the cabin away from any booth openings, and the low pressure sensor a few inches below the pit filters.
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