Real-time continuous measurements can be made by two methods:
- Inline or In situ
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 required analysis. This system can be either calibrated online automatically, or offline by skilled operator.
Online analyzers usually require a suitable temperature, fixed background measurement conditions and frequent calibration to obtain analytically valid results.
In spite of the inherent advantages of online sensors/monitors, their wide application is still limited due to the following reasons:
Online monitoring suffers from more problems than laboratory-based methods as many on-line monitoring technologies developed are direct adaptation of traditional, laboratory-based analytical methods not originally designed for field applications, but required to operate in extreme and variable environments. Consequently, these methods require frequent calibration and maintenance.
The analyzers are often influenced from cross responses due to matrix variations between the standards and samples analyzed, as the measurement conditions are not controlled. • Changes in sample matrix affect online analyzers making it difficult to obtain continuous, reliable measurement in the field.
Significant economic and logistic costs are associated with maintaining remote equipment, as it is difficult for operators to detect problems, such as sensor fouling.
The problems associated with conventional online analyzers stems from the fact that univariate linear calibration models derived from Gauss’s theory of least squares are employed to determine unknown concentrations.
To obtain reproducible and accurate results, the samples and standards therefore must be analyzed, under the same measurement conditions. As consistent measurement conditions required for reliable performance are rarely available in the field, thereby affecting the performance of CEQMS/CEMS and can cause deviation in the data from online instrumentation.
The users/regulatory authorities need to frequently validate their online results with laboratory based methods.
Inline analyzers 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.