IMHO, i thinkk you would stand a greater chance of trapping slag if you run two passes on your root. May I ask what the allowable root gap opening is? I like to use the widest gap allowable, (usually 1/8") and cheat that to the full limit. I would just use enough weave to make sure you are breaking down the top and bottom edge of the plates. Do the coupons have a knife edge, or a land? Make sure you are running hot enough so you can deposit a light layer on your first pass, but not undercutting the top and leaving cold roll on the bottom. Watch the top and bottom edges of your puddle, not the back as you will see the slag and confuse that with your puddle. Make sure your lead angle and work angle are correct. If you have a chance to practice, do it. Get your machine set to where when you tap your slag when cleaning, it all falls off without having to make it sound like a woodpecker going after a hollow tree. If your slag wants to hang tight, you probably are traveling too fast, or too hot. This is where watching the top and bottom edge of your puddle fall into place. If you can see just when the puddle starts to bubble or lift out, that is when it is time to move to either the top or bottom depending on which edge your filling. When I mentioned slight weave at the begining of this post, what I meant, was just enough to fill both the top and the bottom. Usually not more that the diameter of the rod. If you are running a 1" coupon, make sure you preheat to at least 150 degrees, or what ever the WPS spells out.
Practice, Practice, Practice.
Hope this helps. Good luck
Mike
If they give you a standard B-U4a joint to weld in the horizontal position, it should have a 1/4" root opening. The smallest the root opening could be is 3/16" and if you do the fit-up as part of the test you are allowed to go to a 1/2" maximum root opening. I used to give this test plate to the welders in the shop and I required a 1/8" electrode on the first two stringer passes. With a 1/4" root opening there is plenty of room to tie into the backing bar and insure no slag inclusions at the root of the weld joint. After the first two stringer passes I would let them use either a 5/32 or 3/16" electrode, whichever the welder was more comfortable with welding. I am sure you will have no trouble taking the weld test. Keep you amps down and lay in two nice stringer passes in the root. Best of luck.
Randy