Techniques/ Instrumentation for Online Measurement

Real time continuous measurements can be made by two methods:
1. Online
2. Inline or In situ

1. Online Analysers
In most cases online measurements are based on automated laboratory based measurement techniques.
The sample is usually injected using rotation valves or peristaltic pumps to the instrument using flow injection analyses (FIA) or sequential flow injection techniques (SFI), via a separate analytical line which is connected to the main process stream. The sample is then extracted, pre-treated if required and presented to the analytical instruments for the analysis required. This system can be either calibrated online automatically, or offline by skilled operator. Online analysers usually require a
suitable temperature and pressure, fixed background measurement conditions and frequent calibration to obtain analytically valid results.

2. In line Analysers
Inline analysers are simpler in design and can measure directly in the process line, using a probe that is chemically insensitive. However they are more susceptible to physical and chemical interference from the sample matrix as measurement backgrounds are more changeable and they usually require ex-situ calibration.

Despite their ability to acquire continuous real time data, their widespread application has been limited due to the inability to reliably obtain accurate, cost effective water quality data.