Operations/Analysis Unit 4
4.4Operational Theory Cont'd
The energy source for the analyzer is most commonly provided by as a high intensity quartz iodine lamp located in the source module. Quartz iodine was chosen because it produces sufficient NIR to operate the system and maintains a nearly constant brightness over its lifetime. (See Figures
This energy is then fed through the sample, which is temperature controlled, and into the detector module where it passes through a rotating filter wheel before reaching the lead sulfide (PbS) detector (PbSe also used for longer wavelengths (cooled and
The filter wheel, driven at 30 RPS or 1800 RPM by a synchronous AC motor, contains two optical filters with bandpasses selected for each applica- tion, thus providing reference and measuring pulses from which the required information may be obtained.
The detector receives pulses at the rate of 60 PPS, or two pulses per revolution of the filter wheel. Every other pulse is from the measuring filter, while the alternate pulse is from the reference filter, so that pulses through the measuring filter alternate with pulses through the reference filter. A filter position sensor, which is an optical device having an integral light source and light detector, differentiates between the two.
The two entrained pulses received by the detector each revolution are amplified through a preamplifier which is physically located inside the sealed compartment with the filter wheel and detector. This signal is then sent to a clamping circuit where an exact zero reference is established.
This clamped video signal is then fed through a gain control network, which is controlled by the automatic gain control loop, through another amplifier, to the electronic switch. This switch is controlled by the switch driver network which derives its information from the filter position sensor in order to separate the entrained video signal into its component parts of a measuring peak and a reference peak. These peaks are then fed through a balancing network and channeled into separate peak height detectors which produce DC voltage levels which are exactly equal to the peak height or absolute magnitude of the voltage from the base to the peak of each of the pulses.
At this point the reference signal is fed back to the automatic gain control loop to maintain the desired system gain. In addition, both the mea- suring and reference levels are fed to selector switches in order to enable
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