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Just to settle this once and for all, I did some searching on my work data 
base for the MIL and commercial specs for welding and breathing helium. Here 
are the results:

There are 3 Mil-spec and one industry specs that come up:

MIL-PRF-27407B Performance Specification Propellant Pressurizing Agent, 
Helium (Last rev date 8/25/97)

BB-H-1168 Helium Technical

MSFC-SPEC-364C Helium (Last rev date 8/15/69)

Compressed Gas Association Specification CGA G-9.1 Helium

I could not get a copy of G-9.1 so I will stay with the Mil-Specs.

MSFC-SPEC-364C lists the same requirements as BB-H-1168, and  BB-H-1168 
refers to MIL-PRF-27407B. So lets stick with MIL-PRF-27407B.

Section 6.1 Intended Use: This section states that:

Type I Grade A helium is for "pressurization of rocket propellant systems, 
space vehicles and associated ground support equipment, welding and other 
shielding gas operations.

Type I Grade B helium is for "intended for use as a respirable breathing gas 
for divers during deep excursions into the oceans."

Type II is Liquid Helium.

So we are concerned with the difference between Type I Grade A and Type I 
Grade B.

Here are the differences from Table I. 

                                Grade A         Grade B
Purity                          99.995              99.997
Impurities (ppm by vol, max)        50 ppm max      -
Water                           9                   9
Hydrocarbons                    5                   1
Oxygen                          3                   3
Nitrogen + argon                14                  5
Neon                            23                  23
Hydrogen                        1                   1
Carbon dioxide                  1                   -
Carbon monoxide             1                   -

I then called my two compresses gas suppliers who will cert to MIL-PRF-27407B 
and asked for the difference between Grade A and B. The reply was "We test 
every cylinder of Grade B and only every lot of Grade A." I then asked how 
easily they meet the requirements of this spec and the reply was "We almost 
never even see the listed contaminates and when we do they are only just 
detectable."

I then stated to them "So the only difference between Grade A and B is how 
you handle the bottles, testing of every bottle, and the cert?" The reply was 
Yes, and about a 300% markup for the paperwork.


So, there is no difference in the purity or quality you get between welding 
(Grade A) and breathing (Grade B) helium.

There, now can we talk about something else?

Peter Johnson
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