-- card: 12734 from stack: in.STK
-- bmap block id: 13343
-- flags: 0000
-- background id: 2575
-- name: 
----- HyperTalk script -----
on openCard
  set scroll of card field 1 to zero
end openCard



-- part 1 (field)
-- low flags: 00
-- high flags: 0007
-- rect: left=240 top=47 right=308 bottom=497
-- title width / last selected line: 0
-- icon id / first selected line: 0 / 0
-- text alignment: 0
-- font id: 22
-- text size: 10
-- style flags: 0
-- line height: 13
-- part name: 


-- part 2 (button)
-- low flags: 00
-- high flags: 8004
-- rect: left=25 top=277 right=331 bottom=78
-- title width / last selected line: 0
-- icon id / first selected line: 14953 / 14953
-- text alignment: 1
-- font id: 0
-- text size: 12
-- style flags: 0
-- line height: 16
-- part name: Return
----- HyperTalk script -----
on mouseUp
  visual effect dissolve
  go to card id 5605
end mouseUp




-- part contents for card part 1
----- text -----
Measuring Air Contaminants

Before entering a confined space it is required to measure  the oxygen
level, flammable vapor level-%LEL if applicable, and toxic contaminant level if applicable.

What is %LEL?

By definition LEL is the Lower explosive limit of a component mixture
in air.  For example Ethanol has a lower explosive limit of 3% Ethanol in air. Any mixture of Ethanol and air above 3% and less than the Upper Explosive limit is considered a flammable mixture.  %LEL  can be defined as a percentage of this lower explosive limit.   A reading of 10% of the LEL for Ethanol is really .3% Ethanol in air.

Meters that measure %LEL usually are calibrated for only a single compound such as Methanol or Ethanol.  Therefore its important to set very conservative limits on %LEL for safe confined space entry.  Of course a reading of zero %LEL is  desirable.  However, OSHA's guidelines suggest that readings as high as 10% LEL are permissible.

Often the same instrument is used to measure %Oxygen as is used for %LEL.  If this is the type of meter to be used always measure %Oxygen first and then %LEL. 

19.5% is considered the minimum safe oxygen level for entry without respiratory protection.  Below 19.5%
a condition called anoxia (resulting
from lack of oxygen) can result in
headaches, fatigue, blackout and death.
This is the leading cause of death in
confined space indcidents.

It should also be noted,  that oxygen enriched atmospheres >22% can also be dangerous as oxygen is toxic at high 
concentrations. 

Measuring for Trace Contaminants-

Often its necessary to measure for toxic components in the part per million level to determine if respiratory protection is required for safe entry without risk from the toxics.  An oxygen analyzer and %LEL
meter cannot provide enough sensitivity to measure specific compounds.  In this case an diffusion tube system called Draeger can be
used.  A sample is pulled with a pump
through glass diffusion tubes which 
contain a chemical specific reactive
compound which turns color on exposure
to contaminants.  Hundreds of tubes are sold which are specific to chemicals such as Styrene, Toluene,
Methanol, etc...

Always consult the operation and owners manuals of the systems you are using.  Make sure  oxygen analyzers are calibrated routinely and checked on a frequent basis to ensure that they will function when needed.