In Greater Detail

The PE 983 is equipped with a thermocouple detector and a high-gain detector signal amplifier. By photodiode standards, the response time of the detector system is very long; the system responds to the (instantaneous) change in light intensity by exponentially slewing to the appropriate output level. The rate of change is about 85 s-1 (time constant 12 milliseconds). Thus, the time to settle to the level appropriate to the light intensity is about 40 milliseconds if one wants the output to be at least 95% of the amount that is expected. However, the PE 983 operates with a 30 Hz chopper, which means that light switching occurs every 16.7 milliseconds; and the slow response of the detector system means that the signal from the detector has reached about 75% of the expected level, at which point the choppers switch the light and the detector exponentially slews to the new level. Since the detector's output never reaches the true level, the correct value for the transmission of a sample cannot be computed directly. The %T scale of the instrument must be "calibrated," since calculation cannot produce the correct answer from incorrect raw values.

The original Perkin-Elmer 983 IR spectrophotometer uses an AC synchronous motor for its beam splitting chopper (BSC) and another for the pre-sample chopper (PSC). The BSC operates at 30 HZ and the PSC at 10 Hz. Considerable circuitry and firmware are used to ensure that the choppers are running in synchrony so that the switching among sample beam, reference beam and dark periods is known and stable.

The new OLIS 983 uses dedicated microcomputers to control the new stepping motors. Now, the BSC operates at 12.5 Hz and the PSC at (12.5/3) Hz. If the two motors start out of sync, the PSC motor is slowed gradually until the two motors come into the correct phase, at which point the PSC is restored to its normal speed [(12.5/3) Hz]. This computerized operation and synchronization of the two choppers is direct, exact, reliable, and reproducible. The BSC speed of 12.5 Hz allows collection of 2 data points per second.

The Perkin-Elmer 983 produces a certain sequence of signals (figure 1) at the detector as the two choppers operate (Ref1, DarkS, Ref2, Sam1, DarkR, Sam2; see page 3.2 of the PE 983 service manual, figure 3-9). With this sequence used, collection of R and S signals is widely separated in time, so that errant signals are collected for R and S if the detector signal is changing, as it does during filter or grating changes.

The OLIS 983 uses a shifted sequence (Ref2, Sam1, DarkR, Sam2, Ref1, DarkS). In the new sequence, collection of S and R signals are as close together in time as possible, greatly minimizing errors if the detector signal is changing.

In the OLIS 983, the original coarse and fine position sensors at the grating table are retained and used. Now, however, a simple gate presents a signal to the controlling microcomputer when coarse AND fine sensors are both active. The grating table position is thereby determined very precisely. All subsequent movement of the gratings are based on this "calibration" point. The table rotates 0.002 degrees per motor step, maintaining the same step resolution as the original.

PREVIOUS  |  1  2  3  4  |  NEXT