See it used here: http://en.wikipedia.org/wiki/Fed-batch. In the table giving the different strategies to control growth in a fed-batch culture, one finds, for example: "on-line measurement of glucose". What does it mean? Is there an off-line meassurement? What does it mean?
On-line means that's it's generally continuous, or constantly taking the measure, through a tube connecting the batch.
Offline is used for things that are taken out of the reactor, or removed from the process.
Thus it's "on" or "off" the system depending on whether it's contained within the normal piping/fludics of the system: "On" is in the system, "Off" is out of the system.
$\begingroup$ So in an on-line measurement, the sample used to make the measurement is returned to the culture? $\endgroup$– a06eOct 30, 2013 at 23:37
1$\begingroup$ @becko Normally what is not expended is returned, but often it's not really sequestered very much to begin with. Think of a temp probe suspended in a tube of flowing liquid. It can measure the temperature with very little "diversion" and would be an on-line measurement. $\endgroup$– Atl LEDOct 31, 2013 at 0:29