Note: "PE 983" will refer to the original Perkin-Elmer instrument. "Olis 983" will refer to the spectrophotometer after it has been modernized by Olis, Inc.

Creating an Olis 983 IR Spectrophotometer
from the Perkin-Elmer 983

An elegant digital data acquisition method has been developed to collect more precise data from the IR spectrophotometer

Spectral Range: 5000 to 180 cm^-1

The new Olis data acquisition scheme collects the correct value for %T without any adjustment or calibration using a deft rearrangement of the six values for sample and reference and an ingenious exploitation of the exponential nature of the detector's output. A secondary benefit of the change from the Perkin-Elmer 983 to the Olis 983 is the removal of all obsolete electromechanical components and all problems associated with use and maintenance of them, including calibration. No changes to the optics of the instrument are made.

Six changes are made to the classic PE 983 IR hardware
to support this new computerized data acquisition:

• All electronic circuit boards within the card cage, and the card cage, are removed and discarded, removing many pounds of obsolete circuitry.
• The chopping motors are replaced with high precision stepping motors.
• The slit drive motor and cam are removed and discarded; a precision linear stepping motor is fitted to the monochromator, so that a total of 7,200 steps are now used to drive the slits from 0.0 to 4.0 mm.
• The grating table and the filter wheel stepping motors are retained, but they are now powered by all new Olis electronics.
• The source's AC transformer is removed and a new DC power supply is added.
• The phase relationship between the beam splitting chopper (BSC) and the pre-sample chopper (PSC) is shifted 120°, changing the order of signals seen by the thermocouple (TC).

Three important improvements are made in the detection system.

1. The two choppers are operated at slower speeds, allowing the detector’s output to more nearly reach its correct level.
2. The phase relationships between PSC and BSC is changed to produce much closer temporal correspondence between sample and reference signals.
3. Advantage is taken of the fact that the detector's signal exponentially approaches the correct level (see below).

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