| Keep the measurement head cover installed during calibration and linearity test. |
Apart from the offset calibration and linearity test, the instructions in LT31 User Guide (M210667EN) also apply to the installations with enhanced weather protection hoods.
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Loosen the 2 counter nuts, 1 on each side, with a 8-mm hex socket driver.
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Loosen the 2 hex socket screws in the hood bushings and pull the protection hood from
the 2 horizontal plate studs.
The protection hood is connected with a steel wire to the horizontal plate assembly to avoid mechanical stress to the heater connection cable when the hood is removed from the receiver unit.
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After removing the protection hood from the horizontal plate studs, you can keep it
freely suspended on the steel wire. Attach the optical blocker and the filter holder to
the horizontal plate assembly with 2 thread inserts shown in the following figure.
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After offset calibration and linearity check, mount the weather protection hood
carefully. Plug the protection hood entirely onto the 2 studs on the horizontal plate.
Make sure that the gap between the rear edge of the protection hood and the horizontal
plate surface is as small as possible.
Make sure that the heater connection cable and the steel wire are not pinched. 
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To secure the protection hood tighten the 2 hex socket screws that are inserted in the
hood bushings.
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To protect the hex socket screws from loosening, tighten the 2 counter nuts.