Theory of operation & System overview

The RS series of  Spectrophotometer provides a non-contact, non-destructive method for constituent and/or property analysis utilizing NIR absorption technology.  In the NIR spectral region (900-2500nm) the second and third overtones of basic chemical bonds exist permitting compositional analysis of product much as the IR region may provide, but with a depth of penetration far greater.  This spectral region remains void of electronic transitions which dominate the UV and visible light wavelengths.  As a consequence, excellent product determination with limited sample preparation and control remains the hallmark of the NIR method.

Since the wavelength bands overlap and interfere, NIR analysis methods always rely on a reference method for calibration.  The quality of the calibration set will reflect the quality of the NIR method.  A calibration set should reflect the variance of all components and variables present in the process.  Such a statement may frighten newcomers to NIR, as no one can guarantee that ALL variables are accommodated or even known especially if the product is a complex organic or natural product.  Fortunately, the nature of the absorption bands, their width, and intensity are such as to render excellent measurement capabilities void of constant interference.  For any given band, only a very few other chemical structures respond at the same wavelength.  In many instances those structures may not even be present in the analyzed product.  In almost all instances a single compensating wavelength is sufficient to correct for any interferences.

As NIR requires a reference technique, there is no direct measurement method.  All NIR methods utilize a statistical correlation method of establishing the calibration.  Once a product calibration is developed, the method or equation can easily be transferred from one instrument to another usually requiring no additional adjustment other than the offset, and in a few instances requiring a slope adjustment as well.

Two instrument families provide excellent analysis methods utilizing NIR technology.  The first and more complex system involves scanning monochrometers.  These systems are capable of scanning a portion of the NIR spectrum and utilize all of the wavelength information to determine the desired constituents.  Intuitively this system provides the greatest ability to detect the constituents of interest; however, in reality this method also becomes strongly influenced by the regions of no-information or noise.  As a result, the calibration sample sets for scanning spectrometers are generally larger in numbers and must exhibit even greater variances than its simpler cousin the Filter Photometer.  In addition, as the more wavelength are used in calibration, they represent additional large numbers of degrees of variance in the sample, and require large sample sets to establish the proper correlative relationship.

The Filter Photometer uses fixed wavelength filters to select only the wavelengths which measure the constituent of interest, and may add an additional wavelength or two to correct for interacting components.  The bandwidth of the filter is controlled to assist in maximizing the measurement success.  Such a technique becomes simpler to calibrate as the number of variables is significantly less.  Nonetheless, the teaching set must represent as much of the variance of the process as possible independent of the constituent of interest.

The RS SERIES Filter Photometer utilizes a dual beam construction: a Sample Beam and a Reference Beam.  The Reference and Sample beams traverse the same temperature and time critical components: lamp, filters, and detector.  By comparing the Sample Beam response continuously to the Reference Beam results, any variation in lamp, filter, or detector response is immediately corrected providing an exceptionally stable measurement method.  Additionally, by maintaining a constant detector temperature, the response time of the detector also remains constant over very long periods of time helping to further stabilize the measurement results.

The RS SERIES Filter Photometer utilizes an internal microprocessor for continuous diagnostic monitoring to assure the integrity of each reported sample result.  Motor speed, detector temperature, power voltages, and light energy levels are just a few of the monitored components.

The rugged NEMA-4 construction of the RS SERIES Filter Photometer provides unparalleled process compatibility.  Air Purge of the optical surface as well as customized viewing ports assure maximum process reliability.  Vortex or Water cooler panels extend the operating environment of the instrument for hostile environments.  Fiberoptic probes provide remote process access when instrument size or temperature requirements prohibit proximity mounting.

The RS-232 interface permits connection to a single computer port, or the switchable RS-485 networking interface permits multiple units to co-exist on a single party line, while the dual channel 4-20 mA outputs permit simple connection to existing PLC process hardware.