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Up Topic Welding Industry / Technical Discussions / Shielding Gas - Dew Point
- - By Tyrone (***) Date 09-26-2013 11:15
Hi folks,
All our WPS states "Dew Point:  -57 deg. C Max".  A question was asked, "Can the units be changed to Parts Per Million (ppm)? Our measuring instruments record ppm."
All the old timers are gone, so I can't lean on anyone here.  I see a lot of gas suppliers have conversion charts (Temperature to PPM).

I'm leaning toward changing to ppm since the data can then be used to determine if the system is in control (run charts, Cp, Cpk, etc).

Anyone have any thoughts?

Tyrone
Parent - - By fschweighardt (***) Date 09-26-2013 13:25
Thats gonna be about 16PPM, and -71deg F.  No reason the units cannot be changed.  Its like feet or inches.  What is the gas in question.  Does this come from the code of construction, AWS A5.32, or is it part of client requirements.

Reason I ask is that if you are working to A5.32, the 16 ppm is higher than allowed for most of the pure gases in the 1997 R2007 edition, and is way tighter that specified in the new (2011) edition.
Parent - - By Tyrone (***) Date 09-27-2013 10:57
The gas I'm look at is C25 and the code is Mil-Std-1941 Metal-Arc Welding of Homogeneous Armor.
I didn't realize gas mixtures have different dew points, (duh?!).  I'll have to do more digging into the others.
Thanks fschweighardt
Tyrone
Parent - - By fschweighardt (***) Date 09-27-2013 11:07
It looks like the MIL-1941 does not reference any other gas codes at all.  I would specify the C-25 mix from your supplier with a +/- 2.5% tolerance on the CO2, and require dewpoint testing down to the level called out in the code.  I might further request that the THC (Total HydroCarbon) levels be tested, with a max of 16 PPM. Finally, I would specify that H20 + THC must be less than 16 PPM as well.

It appears that the MIL code is trying to limit the presence of H2, and if you have your supplier run the above tests, you will meet both the code, (and I believe the engineering intent) of the gas purity specification.
Parent - - By welderbrent (*****) Date 09-27-2013 12:27
Another thing to remember is that just because the tank that you have on hand says one thing does not mean it is not qualified for the other.  Many times they fill all their bottles the same but don't want to have to do the stricter tests to prove it so just leave the lower rating that it is classified to for most jobs specs.  (hope my rambling made sense there)

Just like buying a wide flange beam for A992 and then noticing the specs call out A572.  Most companies give you a dual cert at the beginning because the beam normally fits both sets of criteria.  Some do not, you have to call and ask for it.  On RARE occasion it does not meet both sets of standards. 

But, back to the gases, if the customer has specified one thing and your gas specs don't show it then you need to get it fixed.  They may bring you another tank from the same batch that filled the first one but as long as they tested it to show it meets the customer's specs then all is good.

Have a Great Day,  Brent
Parent - - By fschweighardt (***) Date 09-27-2013 12:40
Sadly, gases are not included in 5.01 (yet) so there is not a uniform way to define batches/lots.  Generally you can get a batch analysis on a certain quantity of cylinders and the sampling is done on one cylinder from that batch.  If everything is done properly, that single sample cylinder is an excellent representation of the purity levels of the entire quantity.  However, if there are some "dirty" cylinders as part of the batch, you might get some bottles that exceed the specific impurity requirement.

We have a product that uses dedicated cylinders for gas types (Ar/O2, Ar/CO2, etc) and we keep those bottles in that service.  We also use residual pressure valves on this type of gas so we have additional confidence that no impurities get backed up into the bottle.  Careful evaluation of of your gas vendors product filling procedures and properly specified tests for specific impurities will get you the gas you want/need.  Beware "Welding Grade" nomenclature, it doesn't reallly mean anything. 

For best results carefully specify every individual impurity you are worried about, tests of the cylinder(s) before/during/after the fill procedure, batch size, raw product specifications, and solid documentation.

We have a little saying "Good gas ain't cheap, and cheap gas ain't good"
Parent - - By welderbrent (*****) Date 09-27-2013 13:15
As to some of your comments there, I have witnessed dock employees, who knew better, not draw down a tank before refilling it.  After all, it won't take as much gas if the previous customer left some in it and we are filling it with the same product.  Why take the time and go to the added expense? 

All these things add to the possibility of contaminants that will cause problems in some applications, Ar for GTAW not clean enough can be a major frustration.

Have a Great Day,  Brent
Parent - - By fschweighardt (***) Date 09-27-2013 13:27
Pretty common with "standard" industrial cylinders.  This is why we have a line of gas for more critical work that incorporates vacuuming the cylinder, analyzing the residual gas, filling with analyzed bulk Ar and CO2, testing the filled composition and different impurities, and documenting all of the above.
Parent - - By Tyrone (***) Date 09-30-2013 10:46
I've asked QA to analyze the data they've collected every day for the last 365 days.  I would like to know if we have a problem to begin with.
Thorough testing/documenting of the gas could become really expensive on an ongoing basis.  I hope the results are way below 16ppm.
Thanks for the input guys.
Tyrone
Parent - - By fschweighardt (***) Date 09-30-2013 23:11
are you on cylinders or bulk that is piped into a shop
Parent - By Tyrone (***) Date 10-01-2013 10:22
It's bulk, piped and mixed in the shop.
- - By 803056 (*****) Date 09-30-2013 19:44
Check AWS A5.32 for shielding gases.

Al
Parent - - By Tyrone (***) Date 10-01-2013 10:40
Thanks Al,
A5.32 provides a lot more information than the Mil specs.  I'll adopt those requirements into our internal spec (if we're out of control).
Tyrone
Parent - - By fschweighardt (***) Date 10-02-2013 02:38
Consider reviewing both versions of 5.32, as they differ significantly in how they deal with impurities.  The old one has quite stringent limits on moisture, etc. on pure gases, but I dont remember anything specific on a mixed gas impurity level.  The new one has limits on the mixed gas, (as well as pures) but the overall moisture requirement is a good bit higher.  You may not get to where you client want you to be if you use the new standard.

As far as analyzing your bulk gas, consider starting with load analysis of the delivered argon and CO2.  Your supplier should be able to assist with that part.  If that checks out, you may want to do a portable analysis at the end of the piping system, and perhaps at the inlet to the feeder.
Parent - By Tyrone (***) Date 10-02-2013 10:21
I wonder why they relaxed the requirements on the new version?  I'm hesitant to specify any revision/version levels on internal documents.  Managing them would be a nightmare throughout the organization.
Thanks for the suggestions.
Tyrone
Up Topic Welding Industry / Technical Discussions / Shielding Gas - Dew Point

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