Gas / Arc Welding, which to choose

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Hi all,I'm new to welding stuff, just wanna ask when to use Gas or Arc Welding?The engineer in site can immediately tell which to use, but didn't explain very well on the decision basic. Want to know the rationale behind.E.g. pressure line piping - TIG for first passes,  with arc welding on top.Why?Can anyone tell?Thank!
Reply:Engineering decisions are made from past experiences and testing.  If we were to look at your example I can remember when gas tungsten arc welding (GTAW) on the root pass was very rare for pressure piping.  This would be in the early 1970's  Before 1980 I can remember welders having to qualify doing GTAW roots in order to be able to get work in pulp mills.  Today roots with GTAW are very common.  What changed was the demands for higher quality welds.  Remember that when someone says that X-ray quality welds are perfect does not know what he is talking about.  I remember hearing this from a civil engineer.  The amount and size of flaws is all laid out in the particular welding codes.  A GTAW root is likley to give you a better weld than the root done with a cellulose electrode like E 6010.  To understand what is required in each situation you need to read/study the welding code that is being used.  Welding codes are constantly being updated as new processes and new alloys are developed.   From my view I noticed that the requirements by the engineers and the code requirements lag as much as ten years behind what is currently on the market in terms of welding processes and alloys.  If these new things are used a complete battery of tests are needed for the engineer to feel that the weld will perform as good or better than welding code procedures.  After ten years or so engineers on  committees study the reports and decide if they will change the current welding code to match the new ways of doing things.  That is why you rarely hear of boiler explosions or other pressure failure.  If you read any books about steam boats in the 1800's you will get an idea of the number of fatalities resulting for "seat of the pants" design.
Reply:Thanks! That is to say, using GTAW will give better quality of weld root than using arc welding in general.I think it is always better to refer to the welding code being use. But i am really new to welding, how should i start with the welding code? I can simply find out the base material being welded, not even able to identify which type of electrode to be use.Let say, we are having a valve body with material SA-105, then what decision i can make & reference i can look up?Thanks again.