from the Industrial hygiene department...
Ventilation of confined space
Application Questions and Problems topic
Posted by Med on 2 February, 2004 - 10:14 am
The percentage of oxygen in confined space is 7%. Ventilation system blow outside air into this space at a rate of 300 cfm. The volume of the space is 900 CF. After how long it is safe to enter this confined space. Answer is needed soon

Posted by Steve Myres, PE on 2 February, 2004 - 11:49 pm
This sounds suspiciously like a test question, especially with such an obvious answer.

Posted by Meir Saggie on 3 February, 2004 - 9:37 pm
What is the safe Ox percentage?

Posted by Matthew Hyatt on 5 February, 2004 - 1:02 am
19.5 to 21%

Don't forget about other gases, which could be displacing or robbing O2 from your environment.


Posted by Steve Myres, PE on 5 February, 2004 - 12:03 pm
OSHA/NIOSH minimum safe percentage is 19.5%.

If the airflow could be kept laminar (only old air is exhausted as new air is brought in) the air in the room would be completely changed and at atmospheric O2 concentration in three minutes.

If the air is assumed to be completely mixed upon entering the room, the 02 concentration will exceed 19.5% in 6 minutes, 43 seconds, assuming makeup air 02 concentration at 21%. After 10 minutes you will be at 20.5%. A similar calculation can be performed for other agents in the air.

Posted by Matthew Hyatt on 7 February, 2004 - 11:38 am
I guess after 6 minutes and 43 seconds, you would just entire that space without testing the air???

Your calculations are nice, but air monitoring is the preferred method of knowing if the air is safe or not. As a confined space instructor, I would never recommend to angone working in confined spaces to sit down, ran a calculation of how fast the air will be exchanged and what the possible new concentration is and then based upon that calculation enter a space. There is no way to know for sure unless you monitor the air.

M Hyatt

Posted by Matthew Hyatt on 3 February, 2004 - 1:17 am
Your O2 content must be 20 to 21%% and be maintained within that limit, use your air test equipment to monitor the air conditions at all times. Because you have such a low O2 content, you probaby have high Hydrogen Sulfide content as well. What type of space is it? How often is it entered. Are there other vents that air can be excahnged through? You should be running your air ventilation system all the time during any work in a confined space.

Do you have retrieval equipment? Air monitors? Trained personnel?

Drop me a line off line, I am a vertical rescue and confined space instructor.

If you are not sure of the various concentrations of O2, H2S and other gases (methane, CO, ect.) DO NOT ENTER THIS SPACE without ABS equipment - NO MATTER WHAT!!!

Matthew Hyatt
Techncial Consultants

Posted by Curt Wuollet on 5 February, 2004 - 12:39 am
That would depend on what the other 93% is. Might take a long time for cyanide gas.



Posted by Bob Peterson on 6 February, 2004 - 2:38 pm
My answer would be - until the gas monitoring instruments indicate it is safe.

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