Aperture check for 900 Series Spectral Instruments
In order for the instrument to function properly the aperture setting in the instrument (RAM configuration setting) has to match the currently installed components. If this setting or one of the components is incorrect this can cause various error messages. It is possible that the instrument will (A) display a cell error message, (B) fail to perform a zero reflectance calibration or even (C) read differently than it should.
The following components are installed in matched sets:  the aperture itself,  the snap-in target window, and  the nose piece, which covers the optics. A 968 (& most other 900 series spectrophotometers) supports three different aperture sizes, 4mm, 8mm, and 20mm. The 4mm aperture is simply screwed over the 8mm aperture. The 20mm aperture has its own larger target window and nose piece.
Determining the physical aperture size: Will a finger fit through the opening in the snap-in target window? If so, aperture is 20mm. If not, will a pencil fit through the opening? If so, aperture is 8mm, else aperture is 4mm.
Make sure that the same is true for both the target window and the nose piece. The 4mm and 8mm share the same nose piece, but have different target windows
Verify that the aperture setting in the instrument (RAM configuration) is correct:
Press the two left-hand keys simultaneously; then press "Y" to enter the cal mode.
Press the right-hand key under the word "CHANGE".
Press the key beneath "NO" when prompted for new cal values.
Verify the displayed aperture setting.
If required, press the right-hand key to toggle between 4, 8, and 20mm.
Press left-hand key twice to return to the white calibration.
If the aperture size was changed the instrument will require both a white and a zero reflectance calibration (see Related Support).
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