AERI Desiccant Replacement Procedure

Tools and Equipment Required

  • Large flat head screwdriver
  • Desiccant:  approx. 5oz/150 mL silica gel beads, molecular sieve, or calcium sulfate (Drierite).  Molecular sieve is recommended.  Alternatively the entire desiccator module (SPL6300G) can be replaced.
  • Recommended: Dry nitrogen gas, with 0-15 psi regulator, interferometer purge fitting, 0-10 scfh flow meter, and several feet of ¼” I.D. Tygon tubing.


  • The interferometer in the extended range (ER) AERIs at the NSA and AMF-3 sites contains hygroscopic optics that will degrade upon exposure to humid air.  Use a dry nitrogen purge, if possible, to purge the interferometer case when performing a desiccant change.  Continue purging the case for several minutes after the desiccant assembly has been installed.
  • If purge gas is not available, minimize the time that the interferometer case is open to humid outside air by performing certain steps of the desiccant replacement procedure quickly.
  • Minimize the time that the replacement desiccant supply is open to outside air.


  1. Leave the spectrometer powered on during the desiccant replacement procedure.  This will keep the internal optics warm and gives added protection against humidity.
  2. (Optional) Purge the interferometer case by connecting the purge gas source to the quick-release fitting on the back of the interferometer case.  A slow flow of ~5 scfh is appropriate.
  3. Remove the desiccator module from the side of the interferometer case by unscrewing it counterclockwise (see Figs. 1, 2).  Use a flat head screwdriver to loosen it, if necessary.
  4. Temporarily seal the circular opening on the interferometer case with tape or plastic.
  5. Unscrew the metal cover of the porous polyethylene desiccator tube and remove the old desiccant (see Fig. 3).  The old desiccant may be regenerated or discarded.
  6. Refill the tube with fresh desiccant and replace the cover.
  7. Re-install the desiccator assembly.  Ensure that the o-ring is in its groove when mating the flange.
  8. If the purge is being used, continue purging for 15 minutes at ~5 scfh, then disconnect the purge line.


Figure 1.  Desiccator module location


Figure 2.  Desiccator module removal / insertion


Figure 3.  Desiccant inside desiccator module