Inline and online measurement
What does the term mean?
Inline and online answer different questions. Inline describes the measuring location: the sensor encounters the material directly in the process, for example in a pipe. Online describes an automated connection to operation and may also involve a connected sample line.
Relevance to process measurement
Process control depends on how quickly a representative value becomes available. Installation location, sample transport time and signal processing matter alongside the measuring principle. The word online alone does not specify the actual delay.
How can process and laboratory values be compared?
Choose a location that represents the process state being monitored. A sample line also requires attention to flow and transport time. Record the laboratory sampling time and match it to the appropriate process reading.
During product changes, the two locations may otherwise represent different states. Ongoing readings help reveal trends, while laboratory analyses remain references for calibration and checks. Agree how the two will work together before commissioning.
Understanding delays between the process and the display
An online display may provide new values at regular intervals whilst still describing a process state that has already passed. In the case of a sample line, there are delays involved in transporting the sample from the sampling point to the sensor. The evaluation may also average values over time. For a control task, it should therefore be clear to which point in time the displayed figure refers. A high update rate alone does not answer this question. The measurement location and the path taken by the sample must be considered together.
During commissioning, a documented product change can be used for comparison. Note when the condition at the sampling point changes and when this change becomes apparent in the measurement. Assign laboratory samples to the same condition. A comparison between a single laboratory value and any subsequent display value is of little significance, particularly in the case of fluctuating products.
For the project planning stage, please specify whether a measurement is to be taken directly in the main line or in a connected section. It is also helpful to provide the intended application, typical variations in the product and the existing sampling arrangements. From this, it can be determined which time-related parameters need to be checked and documented during operation.