AdBlock跟踪

I just want to find out, how many of my website users are using adblock (plus/pro/whatever). For this, I have implemented the following:

I track a visit of my HTML document (actually I track every request, but only the first one is a "visit". I use a session to check, if its the first visit) and save it to db
I have included a javscript file with the name /ad_server/banner/ad.js which is a php script which also tracks the visits and save it to db

In theory this should mean for example: 100 html_visits and 90 js_visits would mean, that there were 10 Users which requested the html file, but not the js file (which was blocked by AdBlock)

My problem now is, that I get 75.2% of users with an adblock, which is compared to different studies (25-30%) for my country a too large number. My website isn't tech specific, so the percentage shouldn't be that high.

Where is the error in my concept?

I append a timestamp to the js file url (/ad_server/banner/ad.js?1435143401) and disabled cache for that file (Cache-Control: no-store, no-cache, must-revalidate, max-age=0"; Pragma: 'no-cache') to prevent cache issues.

Seems like using a file named ad.js would itself be blocked by Adblockers. Since I can't see your code, I can't tell you if your logic is flawed or anything.

I wrote something like this the other day.

<html>
  <head>
    <script src="adcity.js"></script>
  </head>
  <body>
    <script>
      if( window.adblockerCheck === undefined ){
          // adblocker blocked our fake adcity.js file
          // send event to GA or other analytics provider
      }
    </script>
  </body>
</html>

And then all that adcity.js file has in it is:

window.adblockerCheck = true;

This is the simple version. At work I ended up doing a jquery $.ajax('adcity.js') and checking the status and stuff in the .complete() callback. That way we can check for different reasons that the request may ahve failed like 'timeout', etc.