Wha happens to the weight of astronauts when they are in orbit

That depends on the size of the orbit.  

In "low Earth orbit", like the Mercury, Gemini, early Apollo flights, and aboard the ISS, the astronauts' weight is about 85% of their weight when they're standing on the Earth's surface.  

BUT ... since everything in orbit is in "free fall", they don't feel the weight of their own body or anything else.  The feeling is like being completely without any weight.    

measuring things 1 litre of water