vb.net/ASP.net Update Web Page when not the active Tab

I have a vb.net application that displays sensor information on a webform.  The web form updates the information every 15 seconds and works great while it is the active tab on the browser.
But once a different Tab on the browser is selected, on the next update the webform tab returns

404 - File or directory not found.

The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

The url in the window then shows the original URL with  a /15,%2015 appended to the end of it.  Such as http://myserver/mywebsite/15,%2015.  Hitting the refresh button on the browser doesn't reload the original URL.

How can I continue to update the browser tab even when it is no longer the active tab?  In this particular case the users will be changing between tabs regularly so I need to at best maintain updates on the webform tab or at least, not have it change to the error message and pause until the tab is active again.