Inside your while loop, unbundle the voltage waveform and wire the voltage reading value to a 'less than' vi function (top terminal). To the bottom terminal of the 'less-than' vi, wire in a constant of 12. Wire this result to the while loop control. In Labview 6, you can right-click on the loop control and select 'continue if true' (voltage<12). This setup well allow your while loop to repeat as long as your voltage readings are less than 12.
Good luck with it, Doug