Hello PaulS,
thank you very much!! Now I have what I wanted but in a much easier way than I thought before...
In my webduino script I created a second page named run.html. In the html head of this page I wrote:<meta http-equiv='refresh' content='10'> Just plain old html.
On the main page I have put a link with<a href='run.html' target='_blank'>online data</a>
With the target thing it opens in a new window. When the server runs I can open the link on the main page and a second window opens. Then it refreshes every 10 seconds and this is exactly what I wanted 
Thanks a lot
Elektrix