I am no longer surprised when I teach a PRCS or an Emergency Response course and the students who have been doing entries into PRCSs and responding to emergencies but are unaware of primary atmospheric conditions. This is how I explain it…
We have 100% volume of air in the space.
That 100% volume is broken down into:
21% Oxygen (NOTE: 20.8% at sea level is more accurate; I am just rounding up for simplicity in this article)
78% Nitrogen
1% Trace elements
Of course, it is the Oxygen that we need to survive. And OSHA/NIOSH has set the minimum % of O2 at 19.5%.
FACT #1: We will ALWAYS have 100% volume of “air” in the space – the make-up of that 100% air is what matters.
FACT #2: 1% = 10,000 ppm
FACT #3: 10% = 100,000 ppm
FACT #4: 100% = 1,000,000 ppm
So, let’s say we have an atmosphere of 19.8% Oxygen. Most entry supervisors (poorly trained ones) will look at the Entry Permit and say, “19.8% is greater than 19.5%,” so the atmosphere is OK. But is it?
The atmosphere has lost 1% of Oxygen. That 1% air volume is still inside the space, but it is NOT oxygen – SO WHAT IS THAT 1% (or 10,000 ppm)?
There are a lot of chemicals that have IDLH levels far less than 10,000 ppm. Add that fact to the fact that most direct-reading instruments are capable of reading only 4-5 gases/vapors (O2, LEL, H2S, CO, etc.). So the gas/vapor inside the space is at 10,000 ppm, AND it is a gas/vapor that the meter is NOT capable of detecting – we could be sending an entry into an IDLH atmosphere!
For example:
The meter reads 19.8% Oxygen (e.g.,>19.5%, which is the lower acceptable range for 99.99% of entry permits I have seen).
That is 1% of Oxygen that is NOT inside the space, meaning 1% is something else.
Suppose 1% or 10,000 ppm is Hydrogen Cyanide (HCN) with an IDLH of 50 ppm. This means that the atmosphere contains 200 times the IDLH of HCN.
HCN is also odorless and colorless so that it won’t be smelled or seen, and the meter will NOT identify it unless it has an HCN detector.
This is why I always encourage clients to use a more NARROW RANGE for Oxygen. If O2 is not 20.8% (at lower elevations), most people would recognize something is “not right.” Still, too often, they will associate the variability of the reading with a “meter issue” and approve the entry when the O2 is above 19.5% (the OSHA minimum and most Entry Programs minimum).
I set my Oxygen range at 20% – 21% for an “entry supervisor” to issue an entry permit.
I do the same for my LEL%, but setting its max at 2%, not OSHA’s 10%. (NOTE: I have discussed this LEL% and the “correction factors” in several articles.).
Suppose the atmosphere is outside my narrower range as DEFINED and QUANTIFIED in the program and on the entry permit. In that case, the program requires a safety engineer/professional to evaluate the atmosphere and approve/deny the permit based on this SME’s evaluation.
