Lifting a person with helium balloons

This has been a Frequently Asked Question  ever since I started working with balloons in 1987 !

On the recent BBC TV show Hyperlinks I saw a girl floating with the aid of eighteen helium  balloons, each with a four foot diameter (1.25meter). Each balloon lifted 3.5kg.    Http://www.bbc.co.uk/hyperlinks 

Of course if the balloons had been filled with hydrogen my school chemistry tells me that she would have only needed nine balloons - correct me if I am wrong.  

I have read of the Texan who floated aloft in his lawn chair, he must have used almost twice the number to lift the greater weight.  I would appreciate a link to a site giving details of his flight.

One point to remember is that the balloons themselves have a significant weight. Latex rubber balloons are a compromise between strength and light weight - they could be made thicker but the smaller sizes would not float.  The five-inch decorative balloons can just achieve a neutral buoyancy when helium-filled, the eleven or twelve inch ones float well.  Larger balloons such as the four foot diameter balloons are made of thicker rubber but of course the area of the balloon increases as the square of the diameter whereas the volume increases as the cube so there is plenty of lift to spare  from one of these.

The metallic foil or Mylar balloons have little lifting power because the material is comparatively heavy, some of the worse balloon designs in the past  have not floated at all  !  (from my personal experience)

This is a man from Ross-on-Wye in England who tied himself to 600 large balloons. He hitched a lift on a hot-air balloon for the first 5,000 feet then carried on up to 11,000 feet before parachuting down.

As you can see from the photo, each balloon is at least three feet in diameter.  I do not know by how much the diameter of each balloon would increase at higher altitude. I would think they would start out with the balloons  very much under-inflated to allow for this expansion.