Transmission probe

Figure 5. Single-point IR sensor head layouts a transmission probe with fibre coupling b transflectance probe with variable pathlength and single fibre coupling c (diffuse) reflection probe with single illumination fibre and collection fibre bundle d two-reflection ATR probe with fibre-optic coupling e multi-reflection ATR probe (DiComp -type) f ATR fibre...

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

At Merck KGaA in Darmstadt (Germany), in-line UV spectroscopy has been used for monitoring a distillation setup for solvent purification." A transmission probe was implemented at the top of the column. Solarization-resistant UV fibers guided the light to a diode array spectrometer. From the spectra, a quality parameter was extracted and fed to the process control system (PCS). As an example, the quality parameter would exhibit a transient behavior upon column startup. Below a defined threshold value of the parameter, the PCS would assume sufficient product quality and switch the exit stream from waste to product collection. The operation of the spectrometer does not take place in a direct manner, but rather via the PCS. 

The use of a transflectance probe, rather than a transmission probe, allowed good quality spectra to be obtained despite the low light transmission due to the high solid levels. 

The extmders were monitored using a pair of custom-made transmission probes inserted into the extruder die just downstream from the screws. Each probe consisted of a sapphire window brazed into a metal body a quartz rod behind the window piped the light to the end of the probe low-OH silica fiber-optic bundles connected the probe to the NIR analyzer, an LT Quantum 12001. Optical path lengths were typically between 0.3 and 2.0 cm. 

Wand- or dip-type probes are single-sided probes for optical transmission, absorbance, reflectance, and fluorescence measurements. Spectra-caliper probes are used for transmission measurements through solid samples such as plastic films, fibers, biological tissues, and intact plant leaves. Transmission probes are always used in pairs for optical transmission and absorption measurements. 

Landauer R (1989) Conductance determined by transmission probes and quantised constriction resistance. J Phys Condens Matter 1 8099... 

Figure 9.17 The principle of a spectrophotometer fitted with an immersion probe. Monochromatic light issuing from a spectrophotometer is guided towards an immersion cell and then returned to the detector. The route is confined by a fibre optic. Left transmission probe. Right ATR probe the sapphire prism has a refraction index greater than that of the solution. The schematic shows three reflections of the beam and its penetration into the solution (see explanation in Chapter 10, Section 10.9.3).

A) Transmission probe with facing configuration, (B) Transmission probe with side-by-side configuration, (C) Attenuated total reflectance (ATR) probe, (D) Scattering probe. 

This increased spectral information can be maximised still further by applying the same chemometric principles used in NIR. If a derivative is apphed to the data, we can resolve overlapping absorptions to differentiate between components with essentially similar UV spectra. This does not however address fundamental issues with probe design. UV spectroscopy, like NIR uses transmission probes... 

Various probes are available for introducing the radiation to the sample. Transmission probes can be either of an insertion or flow-through type. Reflectance probes utilise a reflective surface to return the radiation to the detector. Internal reflectance probes employ attenuated total reflectance at a sample surface to obtain an absorption value. 

A transmission pair analyzer combines the advantages of laboratory NIR analysis in a process instrument. The NIR monochromator is interfaced to the process via fiber-optic bundles and stainless steel transmission probes. This instrument is used primarily for on-line investigations of pilot plant or production plant applications. Typical applications include monomer moisture content, residual solvents, and solids content in polymerization processes, and solvent and solutions compositions in continuous chemical processes. 

The NIR spectrometer was integrated to the extruder via a custom in-line temperature-controlled die adapter with a fixed optical path length, as shown in Fig. 7.7. The single-fiber transmission probes have standard 1/2-20 UNF- mounting threads and are screwed into the die adapter such that they are perpendicular to the plane of the material. The probes have been designed to be seated flush within the die adaptor to minimize optical path length fluctuations, to be resistant to the exterior operating temperatures, and to withstand pressures up to 1500 psi. 

Hall and De Thomas (24) described the development of spectroscopic models for the on-line analysis of fat in milk and cheese and moisture in butter and cheese. For fluid milk analysis, the process interface consisted of an interactance immersion probe inserted into a pipe. This probe was usefui for minimally scattering fluid streams as it reflected light that passes through the sample back to the fibers, providing a double pass through the sample. For butter analysis, the process interface consisted of two probes mounted at 180° to each other welded to the pipe at the end of the creamer. For process cheese analysis, transmission probes were positioned in a process pipe with sapphire windows. 

FIGURE 37.26 Process liquid transmission probe. (Courtesy of HeUma GmbH Co KG)... 

Grob et al. [172] have reported the analysis of polyols in a pilot plant operation using a fibre optic transmission probe. Free epoxide (unreacted) and hydroxyl number were monitored to enable cost and safety control during full-scale production. The feasibility of NIR-ATR spectroscopy for the in situ characterisation of epoxy/amine cure reactions has been demonstrated [173]. 

For example, Hong and Lee (1983, 1985) argned that Sprow s results were exaggerated. Using light transmission probes to measnre interfacial area, they found only a small spatial variation in drop size. We can only conclude that spatial dependence can occur and is system dependent. It is less apt to be a problem, even for coalescing systems, if the vessel has short circulation times. 

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