Automatic log out

Hi I currently use an automatic logout out system that sets a time out in two ways. (If the ip address on computer is recognized then set timeout to 10 mins, if not then set to 2 mins.) 1) The time out setting is used to create a meta refresh tag that will re-direct the user to the logout page after X seconds. 2) At the beginning of each page i set a variable with the current time and check to see if the difference between the previously set variable and the now current time is greater than X seconds, if so then log the user out. I know meta refresh is not to be relied on and that is why i use the 2nd method as well, but i have had varied reports that this system does not work, i.e. it logs people out to quickly. Does anyone know a better way of doing this or improvements?? it would also be nice to stop the pages displaying after a time out when a user presses the back button! Thanks a lot in advance Adrian