Health & Safety

  Confined Spaces: Air Monitoring and Ventilation

Multi-Gas Meters

Multi-Gas Meters

Multi-gas meters for confined space/hazardous space air monitoring at UWM; from left to right, Industrial Scientific TMX412, Dynamation CGM 929, MSA Passport 5 Star. Though similar, each instrument has unique features and operating characteristics.

These devices measure "real-time" oxygen content, flammability/explosion potential, carbon monoxide (CO) and hydrogen sulfide (H2S) concentrations. Many deaths occur in the United States every year because of failure to conduct environmental monitoring when performing confined space entry work.

Air Quality Requirements

From Wisconsin DILHR 32.64 Air Quality Requirements for Confined Spaces:

ILHR 32.64 Air quality. (1) ATMOSPHERE LIMITS. Except as provided in sub. (3), a confined space may not be entered, unless the atmosphere of the confined space has:

(a) An oxygen content of at least 19.5% or more but not more than 23.5%;

(b) A hydrogen sulfide content of less than 10 parts per million or a carbon monoxide content of less than 35 parts per million;

(c) A combustible gas content less than 10% of the lower explosive limit; and

(d) An exposure level, for any hazardous substance determined to be present, which is at or below the threshold limit value - short term exposure limit for any substance specified by the American Conference of Governmental Industrial Hygienists (ACGIH) in the publication adopted under s. ILHR 32.50 (4), or the short term exposure limits found in 29 CFR 1910.1000.


Axial Ventilation Blower
  • Our axial ventilation blowers (Model 1385D Air-Pac) are rated at 714 cfm for one 90 degree bend in hose, or 627 cfm if hose has two 90 degree bends.
  • Calculate purge times to achieve at least 7 air changes in the space prior to entry. Use the chart attached to the blower, or use the following equation: Minimum time required to ventilate space = ((7)*(volume confined space in cf))/(blower cfm).
  • As a general rule, always push clean air in.
  • If using a portable generator to power blower, make sure exhaust from blower is positioned downwind from the space.
  • Use GFCI protection if electrical devices must be used, including the blower.
  • Do not use blowers for enclosed spaces where damaged asbestos may exist (i.e., use blowers only in asbestos-free areas).