How do I stop Safari from refreshing pages on my iPad?

This is really annoying. I am not sure if it is related to time or to memory. (it's not auto-refresh set by the website). I often load up several web pages in tabs that I would like to read on the train to work. Unfortunately when I open the tabs, they refresh eating into my data plan. If I switch off data, then the pages still try to refresh and I can't see them at all.

surely there must be an iPad Safari option to stop pages from automatically re-loading?