Immersion probe

For immersion probes we also get similar improvements using piezocomposite transducers as demonstrated by the third example. In Fig. 8 we compare pulse form and frequency spectrum for a 2 MHz probe Z2K with 10 mm transducer diameter. The echo of the composite probe has 11 dB more amplitude and is clearly shorter than for the old design, also indicated by the increase in bandwidth from 45 to 76 %. 

Fig. 8 Pulse shape (top) and spectrum (bottom) for a 2 MHz immersion probe with PZT (left) and composite transducer (right)...

Examples of nir analysis are polymer identification (126,127), pharmaceutical manufacturing (128), gasoline analysis (129,130), and on-line refinery process chemistry (131). Nir fiber optics have been used as immersion probes for monitoring pollutants in drainage waters by attenuated total internal reflectance (132). The usefulness of nir for aqueous systems has led to important biological and medical appHcations (133). 

We have observed a dependence of the yield, polymerization degree, and polydispersity of polysilanes on temperature and also on the power of ultrasonication. In the ultrasonication bath the simplest test of the efficiency of cavitation is the stability of the formed dispersion. It must be remembered that the ultrasonic energy received in the reaction flask placed in the bath depends on the position of the flask in the bath (it is not the same in each bath), on the level of liquid in the bath, on temperature, on the amount of solvent, etc. When an immersion probe is used the cavitation depends on the level of the meniscus in the flask as well. The power is usually adjusted close to 50% of the output level but it varies with the reaction volume, flask shape, and other rection conditions. The immersion-type probe is especially convenient at lower temperatures. 

Fig.io.n i immersion probe with standard ground joint and reaction vessel for UV/visible spectroscopic analyses under normal pressure. 

Like the filtered flow cell, the filtered immersion probe (either transflection or transmission) may be used in sample environments in which bubbles or particles make the use of unfiltered samples impossible. 

Figure 4.2 Schematic of basic probe designs transmission cell, immersion probe (retroreflecting and transmission), attenuated total internal reflection.

There have been substantial changes in Raman sample interfaces recently. The approaches now can be divided broadly into two categories based on the sampling volume. The probes intended to sample small volnmes include the more traditional noncontact or standoff probes, immersion probes, and optical microscopes. Large volume sampling approaches are newer and include WAl and SORS probes and transmission confignrations. 

Figure 7.4 Collection of commercial Raman probes designed for different installations (a) laboratory scale probe with interchangeable immersion or noncontact optics, shown with immersion option (b) probe shown in (a) installed in laboratory fermentation reactor (c) production scale immersion probe (d) probe shown in (c) installed in a glass reactor (e) gas phase probe with flow through cell (f) probe shown in (e) installed in process piping (g) wide area illumination (WAI) noncontact probe after completion of a pharmaceutical tablet coating operation. Adapted, with permission. Copyright 2004 Kaiser Optical Systems, Inc.

We recognize that immersion probes for direct introduction into a vessel are available for some sensors and a very limited number of spectroscopies. Therefore immersion probes represent a more specialized and limited case. 

To allow routine remote measurements on an entire batch without the necessity to sample, or to monitor the concentration of a product on a fabrication line, immersion probes are used (Fig. 11.15). These devices incorporate two fibre optics to conduct the light to the sample and retrieve the light after absorption in the medium under investigation. 

Figure 11.15—Principle of a spectrophotometer fitted with an immersion probe. Monochromatic light from the spectrophotometer is guided toward an immersion cell and then brought back to the detector. The reference beam is also guided by a fibre optic.

One of the basic unit operations is dry mixing that is used in most manufacturing schemes. In Fig. 10.6, an example of the use of Raman monitoring for blend control is shown. In this case the blending procedure is very fast and the mixture is well blended within the 5 min used. De Beer et al. used an in situ Raman immersion probe setup to study an ibuprofen-xanthan gum... 

The ability to separate the fiber-optic tip and sample by as much space as required makes Raman sampling extremely flexible. For example, 15-foot immersion probes to insert in the top of chemical reactors have been made. The laser beam travels down the 15-foot probe shaft, finally coming to a focus just outside of the optical window at the probe s tip. A photo of such a probe is shown in Figure 5.4(a). Non-contact probes could also be made to come to a focus at similar distances, but then the beam must be enclosed to address safety concerns. 

Probes can be engineered for almost any environment, but these requirements must be clearly defined before probes are built. No probe can survive all environments so there may be some design trade-offs. Many immersion probes have a second window as a safety backup to ensure the process remains contained in the event of a window seal failure. Probes have been designed with motorized focus adjustments, automatic solvent or steam washes or air jets to keep the window clear, and sealed in water- and pressure-safe containers. Three commercial probes, two immersion and one non-contact, are pictured in Figure 5.4. Different optics can be attached to the face of the non-contact probe, much like conventional cameras using different lenses. 

Finally, the impact of possible calibration instability on the prediction quality must be understood. In this case, spectrograph and laser wavelength calibrations were very stable and easy to update, but of minor concern because of the broad bands being used. However, since the intensity calibration function is non-linear, any changes in it could unequally affect bands used in the calibration ratio and introduce error in the prediction. Newer equipment offers easy intensity calibration routines but this can be difficult to use automatically with immersion probes since it requires that they be removed from the process. 

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