What weight per volume of barium sulphate do you need for x-ray contrast?

It depends entirely on the barium procedure you are attempting. A single contrast barium swallow for example needs around 60-155% w/v. If you need visualization of the stomach, the numbers will be different. The smallest amount of Barium will show up on X-ray, however, due to its high atomic number. This is evidenced by the dramatic chest x-rays of people who have had barium aspiration.