Hi vdao
The codes I use do not generally specify the use of the backing gas. They merely state that if you need to use backing gas to get an acceptable result in your PQR test, then you need to also use the same for production welding. The need for backing gas may be specified in client specifications, but generally it is a matter of basic welding engineering. Given certain materials, with certain processes, you just need to protect the root from oxygen (or nitrogen) to get an acceptable result. Typically GTAW welding of stainless steels.
The easiest way to purge a plate coupon (there are a number of ways) would be to tack weld an angle iron onto the back of the plate, and seal the sides with tape. (e.g. masking tape) Apply the gas from one end of the angle iron, and make a small drain hole in the other end to prevent excessive pressure build up when closing the weld. (Or else it will cause root concavity.) Place a piece of masking tape along the top of the joint, and progressively remove it as the weld progresses. For most stainless steels, ensure you keep the root purged till you have about 6mm of weld deposited.
Regards
Niekie