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Technology Reviews

CPAP Hose Dryer: Filter Bottle

With the general idea being to dry a CPAP hose by pulling clean air through it, putting laser-cut MDF upwind of the inlet was a known-bad idea:

CPAP Dryer – overview

It did, however, dry the tubing and the construction was Pretty Close™ to being the proper size.

Making the stand from acrylic sheet eliminates the MDF stench:

CPAP Dryer filter – acrylic stand – fitting end

Incoming air passes through a dome-style N95 mask:

CPAP Dryer filter – acrylic stand – filter end

The mask sets the overall size of the stand:

CPAP Dryer – Filter holder – LB layout

Given that we’re not talking Level 4 Biohazard, any filter would work equally well. A dome mask has a nicely defined and self-supporting shape with a flange around the edge.

The flange provides a convenient way to build the clamp ring, starting with a scan from the face side:

Demetech Dome Mask – interior scan

Tracing the flange outline using GIMP’s Scissor Select tool and doing a little cleanup in Quick Mask mode produced a selection suitable for becoming a binary mask of the N95 mask:

Demetech Dome Mask – perimeter mask

Ex post facto, I realized the mask has a sufficiently regular outline to fit a much simpler Beziér spline:

CPAP Dryer – Filter holder – LB splines

That began in LightBurn as a circle fitting the lower part of the mask, converted to a path, then tweaked with the Node Editor to fit the top of the nose and add two nodes to pull the path inward on either side. In the unlikely event I make another bottle stand, the cut will be irrelevantly smoother.

The hole in the clamp comes from insetting that path by the flange width of 4 mm, whereupon the N95 mask pretty much self-centers in the hole:

CPAP Dryer filter – acrylic stand – filter clamp

You could draw a face on that thing…

The four small holes fit M3 aluminum rivet nuts:

CPAP Dryer filter – trimmed Rivnuts

They’re shortened by 1 mm (from the original length shown in the upper right) to fit 1 mm of mask sandwiched inside a pair of 3 mm acrylic sheets:

CPAP Dryer filter – Rivnut installed

The glowy edge-lit acrylic sheet has 4.8 mm holes for a snug push fit and the white clamp ring has 5.1 mm holes for a loose alignment fit. I drilled out the laser-cut holes for nice smooth sides.

I picked a bottle large enough to also hold the mask’s elbow, so that it would dry in the same stream of clean air. So far, the elbows dry well enough on their own, but the bottle remains a convenient size for fitting the mask on its end.

On the other end of the bottle, the lid gets a hose fitting turned from PVC pipe:

CPAP Dryer – filter hose fitting glue rings

The Official ResMed fittings on the masks and the AirSense 11 machine are about 20 mm long and just over 22 mm OD with a slight taper. The unheated hose has silicone rubber ends fitting very snugly around those cylinders, so I made the pipe fittings 25 mm long and 21 mm OD to ensure a low-effort, but still secure, fit.

The grooves cut into the fitting anchor a generous hot-melt glue blob sealing it to the lid:

CPAP Dryer – filter hose fitting inside

Yes, the foam disk and the hole through the lid were both laser-cut. Making perfect circles in thin organic material with zero drama is wonderful.

The downstream / mask end of the heated ClimateLine hose (left) is physically identical to the unheated hose ends, but the machine / upstream end (right) sports an electrical connector for the spiral heating element and the thermistor (in the white stud protruding into the mask end lumen):

ResMed ClimateLine heated hose ends

Yes, that does look a lot like a naked USB connector, as does the main power connection on the machine, and you can actually slide a Type A USB connector around it. The ResMed manual pointedly notes:

•Do not insert any USB cable into the AirSense 11 device or attempt to plug the AC adaptor into a USB device. This may cause damage to the AirSense 11 device or USB device.
•The electrical connector end of the heated air tubing is only compatible with the air outlet at the device end and should not be fitted to the mask.

ResMed AirSense 11 Clinical Guide

Protip: When you must carefully explain why T. C. Mits should not mate two obviously compatible and mutually antagonistic devices, your design-fu has failed.

The four ribs inside the upstream end slide over a 23.5 mm cylinder, which is enough larger than the 22 mm cylinder on the machine to wiggle the not-USB connector into place. Without a connector to worry about, I turned a sleeve adapting the smaller fitting to those ribs:

CPAP Dryer filter – heated hose bushing

It’s 27 mm long to keep the lip of the silicone seal away from the setscrew, 23.5 mm OD to exactly fit between the ribs, and a 21.5 mm ID slip fit over the bottle snout.

The tiny M3 setscrew lives in a hole tapped into the inner tube, because the sleeve is only 1 mm thick:

CPAP Dryer filter – acrylic stand – bushing center drill

The setscrew turns outward into a clearance hole drilled in the sleeve to lock it in place.

The outer PVC pipe in the vise is a simple cylinder fixture bored to match the sleeve, so I could grab it in the lathe chuck / vise without distortion. Just the force from a normal grip squishes the fixture enough to keep the sleeve from turning / moving / getting annoyed.

Improving the MDF fan box awaits a few parts, but, being downstream, isn’t on the critical path for drying hoses. The only trick is keeping the bottle inlet upstream of the fan exhaust.