Sensors for real-time process monitoring

The use of sensors for temperature and pressure under processing conditions is routine, and such sensors are standard features of commercial systems, but the measurement of 

Hardware sensors for the on-line monitoring of polymerization such as in batch reactors have been reviewed (Kammona et al. 1999). In Section 3.4 the use of ruggedized conversion sensors for fibre-optic near-infrared (NIR) spectroscopy during extrusion was described. In many ways, the requirements are simpler than for control of a batch reaction (as in a polymer-synthesis autoclave) since 

However, there is a particular requirement that governs the sensor  

For example, NIR spectra are temperature-sensitive, so control to 2°C is required. Ultrasonics using two probes at 5 MHz offers the possibility of a robust and less expensive method, but it is still under development and there are few commercially available systems. Chemometrics (Section 3.5) offers the best method of data analysis, and, by using PLS with three different data pre-treatments, it was possible to obtain real-time values of the composition of a fire-retardant to better than 0.1% accuracy (Fischer et al, 2006). 

As is noted in the following sections, dielectric sensors have usually been regarded as providing local viscosity data (Senturia and Sheppard, 1986), but detailed studies over a wide frequency range have shown that the absolute value of the static dielectric constant (so) may also provide data on conversion comparable to those obtained by DSC (Casalini et al, 1997). However, to achieve this it is noted that So must be measured to high accuracy (a relative error of 10 ) for this to be a viable technique. 

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