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files for building a low cost diffusion dryer

The dessicant tray detailed in our nebulizer instructables is a much simpler way to control the humidity inside the test chamber. However, the diffusion dryer could reduce the decay rate of PSL bead concentration, since the PSL laden air makes minimal direct contact with the dessicant. Another way to resolve this could be just to pump in more PSL beads.

Making this diffusion dryer can be time consuming and the current design requires some manual machining on a lathe and mill. The design (concept and solid models) is very rough around the edges. Let us know if you plan on making one, and we can provide some more detailed information.

Solid Models

  • Diffusion_dryer.iam : assembly model for the diffusion dryer
  • Diffusion_dryer.ipt : master model for the following components
  • cap.ipt : end cap of the diffusion dryer. Milled from HDPE with a groove post-machined
  • inner.ipt : Holds the permeable membrane that the air will flow through. Milled from HDPE
  • mount.ipt : 3D printed mount to hold the dryer
  • tube.ipt : outside tubing for the dryer. Made from PETG tube stock.

Bill of Materials

Item McMaster # Price Notes
spring stock 9663K79 $4.45 for holding the flexible sleeving from collapsing under the weight of the dessicant.
expandable sleeving 9284K3 $2.08 separate the dessicant from the air. Other materials can be used depending on the type of particles that need to be dried (e.g. PTFE, Stainless Steel).
HDPE rod stock 8624K48 $26.37 stock material for milling out the various pieces.
tubing connectors 5372K123 $4.73 half inch tubing, 1/4 NPT. Pack of 10.
self tapping screws 99397A448 $11.30 M3.12 for tapping into plastic
o-ring: large 9452K54 $13.95 pack of 50. For piston seal.
o-ring: small 9452K188 $4.47 pack of 100. For face seal.
pipe nipple 4830K131 $1.50 the springs secure to this.

Build "instructions"

Spring attached via pipe nipple.

spring attached to the inner piece via a pipe nipple

Getting ready for turning between centers.

between centers

End caps post machined with o-rings attached.

end caps with o-rings attached

Lubricating the end of the tube with vaseline, to get the caps in easier.

o-rings can be hard to get in

Filling up with dessicant. filling up with dessicant

Known Issues

  • self tapping screws in shear. Should be okay for low pressures. Alternatively, a threaded rod running the length of the dryer can be used to fix the two inner parts together to take the axial loads.
  • the holes on inner.ipt should be blind holes. In the current configuration, it is a potential source of leaks. Heat set inserts could be used.
  • The cross sectional area of the outer O-ring is a bit large, and makes inserting/removing the end caps a bit difficult. Smaller o-rings could be used, a handle could also be added