Saving String Of Text To File Every Few Seconds

I want to save a string of text to a file every 1.4 seconds or so, and each day at midnight I want to create a new file for the days log data so I keep the file size down. The file stays open the whole time so that it is ready to accept data from several different forms which call a function (SaveToVoltsLogFile(str)), so I open it at module level like this:

Dim fsVlogStream As New FileStream(EVENT_LOG_PATH & VOLTS_LOG_FILENAME & DateString & ".ip", FileMode.Append, FileAccess.Write, FileShare.Read)
Dim swVlogWriter As New StreamWriter(fsVlogStream)
Now each time I call a function SaveToVoltsLogFile(str), the str data gets saved to the file:
Friend Sub SaveToVoltsLogFile(ByVal str As String)
swVlogWriter.WriteLine(str)
swVlogWriter.Flush()
End Sub

But by opening it this way, I cannot call a sub to close it and open a new file for the next day because putting the Dims into a sub means that the file is out of scope to other subs/functions, so my Save... sub won't work! So, what I think I need is a separate sub to open the file, another sub to close it, and another to write the str data. But how do I do this and keep the file in scope to all subs?