My question about gas chromatography sulfur chemiluminsecence detector. I test unknown sample gas by GC-SCD (calibrated ) and the result of *H2S is 279 PPM , *but when I test the same sample with the GC-TCD (calibrated ) the value of *H2S is 2500 PPM . I'd like to inform you that both GCs are calibrated and have very good operation conditions with stable parameters . the question is if the sample gas with higher H2S over detection limits of SCD detector (1000 ppm). why the result it 279 ppm Best regards

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4 down vote favorite share [g+] share [fb] share [tw] I am developing my site using server side sessions using redis as backend for saving the session. Now the issue which is bothering me is of user leaving the website without logging out. I mean user simply closes the browser which causes the cookie to be deleted. Now session of that user still exists on the server and will not be used again as new login requires creating a new session due to security reasons. To avoid the case where hacker steals the old cookie and use it after user login again with same old session id. In essence user leaves the website without explicitly logging out and his session will be deleted after certain time limit of inaccessibility. I am thinking time limit of 30-60 minutes. Also with every new request from user his cookie will also be updated to keep track of when the user last time accessed the site. But nowadays, people let site remain open for long time without accessing it. For example users open facebook and gmail in new tabs and forget about them for 2-3 hours and still they are not asked to login again. Is letting a 2-3 hours old cooke access the session secure? My concern is someone steals user cookie and use it 2-3 hours later. Thinking on this topic has also forced me to question how facebook manages security if user can use a session where they are not accessing it for long periods of time and still they remain logged in. Or is it not secure for me to keep logged in when am not accessing the site session for longer period of time? It can be the case also there is some pinging mechanism using which sites keep track of user having their site open in a browser and when browser closes they are notified and can work accordingly. My website is a social network and needs all those security and usage features which a social network may need. I am new to web security and web development in general and may be the case where my above questions may seem a little basic. If you feel that is the case kindly point to some good reference where I can read and find answers to my question.

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