How to build a PAPRa Fan Box

How-To guide for building the Tetra PAPRa fan box (March 2021 version).

Quick Synopsis

The Tetra Powered Air Purified Respirator requires that the air be purified; this purification is accomplished by powering a fan to pull air through a filter and then pushing that purified air through a hose to a mask worn on the face. This section contains the instructions for building the box that holds the fan and filter.

NOTE: this version of the fanbox has a known problem-- the power connector specified on the PCB is proprietary, and does not properly link into the provided DC cables. The next version of the fan box will fix this problem.

The main goal of this iteration of the fan box is to reach N99 filtration levels by ensuring that the DC jack is dustproof and using an IP68-rated jack.

WARNING: we provide no warranty, implicit or explicit, that this device will work up to an N95 specification, when built outside of a facility rated to do so. The only way to truly test one of these devices using something like a Portacount under controlled conditions.

Parts Required

This photo shows all the parts laid out for building this component:

Assembly Diagram

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3D Printed Parts

These parts should be printed at 0.2 spacing. They should be oriented such that the large plates are closest to the build plate:

Table 1. Table Printed Parts
Part	Quantity	Material	Supports required?	STL File Location
The Fan Box (holds the fan)	1	PLA	Yes	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/STLs/fan_box/P-TET-000112-3%20Fan%20Box.stl
The Bottom Plate	1	PLA	Yes	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/STLs/fan_box/P-TET-000110-2%20MOLLE%20Fan%20Base%20Plate.stl
UniFrame Cover	1	PLA	Yes	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/STLs/fan_box/P-TET-000116-2%20UniFrame%20Filter%20Cover.stl
The Outlet (attaches hose to fan box)	1	PLA	Yes	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/STLs/fan_box/P-TET-000113-3%20Fan%20Box%20Outlet.stl

Gaskets

Gaskets should be cut from 2mm craft foam. Ideally, they should be cut by something such as Cricut (https://www.cricut.com/), but can also be cut using scissors or an Xacto knife so long as the templates are followed closely.

Table 2. Table Gaskets
Part	Quantity	Material	Pattern
Box Gasket	1	Craft Foam	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/Gaskets/P-TET-000111-1%20Fan%20Box%20Gasket.dxf
Fan Gasket	1	Craft Foam	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/Gaskets/P-TET-000114-0%20Fan%20Gasket.dxf
Filter Gasket	1	Craft Foam	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/Gaskets/P-TET-000115-0%20Filter%20Gasket.dxf
Outlet Gasket	1	Craft Foam	https://github.com/tetrabiodistributed/papr-print-files/blob/v3.0/Gaskets/P-TET-000117-1%20Funnel%20Gasket.svg

PCB

Table 3. Table PCB’s
Part	Quantity	PCB files location
Fan Box PCB	1	https://github.com/tetrabiodistributed/PAPRA-PCB/blob/f1064eef011a469fd023b5d7fd1b579d3a39d23f/pcb/PapraFanBoxPCB

Vendor Bill Of Materials

These are components that need to be purchased. Prices fluctuate, and so are not included here. Alternative versions can be sourced, and we welcome edits for tested alternatives.

Table 4. Table Vendor Bill Of Materials (BOM)
Count	Description	URL	Number in Package
1	UTUO Brushless Radial Blower Dual Ball Bearing High Speed 12V DC Centrifugal Fan with XH-2.5 Plug 120mm by 120mm by 32mm 4.72x4.72x1.26 inch	https://www.amazon.com/gp/product/B01CSNEO2G/	1
1	Donner 60CM Guitar Pedal Power Cable Cord 10-Pack	https://www.amazon.com/gp/product/B078XRM3W4/	10
8	50 Pcs M4-0.70 x 20mm Button Head Socket Cap Bolts Screws, 18-8 Stainless Steel, Allen Hex Drive, ISO 7380, by Fullerkreg,Come in a Plastic Case	https://www.amazon.com/M4-0-7X-Available-Stainless-Machine-Fastener/dp/B081JYXLK7/	50
1	2mm Craft Foam	https://www.amazon.com/Foam-Sheet-X18-2mm-White-pack/dp/B004M5QGBQ	10
7	Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section,	https://www.amazon.com/Frost-King-Self-Stick-Weatherseal-D-Section/dp/B00FQ5A5RM	204

Recommended Tools

These tools are recommended. URLs are for tools purchased and used in the building of the prototypes:

Table 5. Table Tool List
Description	URL
4-3/4 In. Bent Long Nose Pliers	https://www.harborfreight.com/4-34-in-bent-long-nose-pliers-63819.html
A hex screwdriver for the m4 screws	https://www.amazon.com/gp/product/B007ICWAJC
Flush cutter	https://www.harborfreight.com/micro-flush-cutter-90708.html
X-acto knife	https://www.amazon.com/Xacto-X3201-N0-Precision-Knife/dp/B00004Z2TQ
3D Printer (note the size of the print bed for the fan box)	https://www.creality3d.shop/collections/ender-series-3d-printer/products/creality3d-ender-3-pro-high-precision-3d-printer
A deburring tool	https://www.amazon.com/gp/product/B01L2XR4P2
#0 Phillips head screwdriver	https://www.homedepot.com/p/Husky-8-in-1-Screwdriver-with-LED-Light-232360016/301959976
CPAP hose cleaner (for maintenance)	https://www.amazon.com/Cleaning-DreamStation-Diameter-Stainless-Cleaner/dp/B08HLQV2VK/

Build Steps

Remove printed supports

Removing print supports can be done with a chisel, an x-acto knife, the bent-nose pliers, or your fingernails (which can be painful if something slips).

Assembly

Place the PAPRa PCB into the Fan Box:

Place & washer and tighten nut onto PAPRa Fan Box power jack:

Place Outlet Gasket in the appropriate spot:

Insert the Fan Gasket into the Fan Box:

Prepare tape around the Funnel and Fan:

Connect the fan power to the PCB:

Place the Box Gasket into the Bottom Plate:

Place nuts into the slots in the Fan Box and Screw the Bottom Plate onto the Fan Box. The Bottom Plate may bow out a bit:

Place the Filter Gasket on the Fan Box (note this image has the gasket offset from the slot in the top of the box):

Place the filter into the Filter Box, ripple side up. The gasket on the side of the filter may move a bit, which is fine, as that snugness indicates a good fit:

Flip the Filter Box and screw it into the Fan Box, taking care that the Fan Box Gasket does not move too much:

The gaskets can stick out somewhat from the body and still be effective:

Congratulations! You’ve now built a Tetra PAPRa Fan Box!

Questions

How frequently should I change the filter?

Changing the filter depends on how much you use the device. If you’re using the device where there are a lot of particulates in the air (such as a construction site or a woodshop), you may want to change every month or so. If you’re using the device where there are less particulates, every two to three months should be fine. HEPA filters just get dirtier and eventually the fan will have a hard time pulling air through the filter, and the filter should be swapped before that happens.

What happens if I drop a filter

You will likely have to get another filter. Even a small chip like this one:

reduces the efficacy of the filter because there isn’t a complete seal with the fan box and the filter.

Can I use any random HEPA filter?

Unfortunately, no. We have found in our testing that only some filters work. The one we’ve tested to work is the Germ Guardian filter specified in the BOM.

Is the fan box waterproof?

No, the fan box is not water proof. If you want to use this fan box in the rain, the filter itself would likely clog and become less useful. We are examining louvred designs to allow use in a mist or light rainfall.

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Last modified September 5, 2021: This commit is a first pass to describe the new fan box and m12 housing builds, as well as the flow testing done to validate the new version. TBD: adding in the deleted gaskets and STLs from v3.0 back into the main project and then updating the links in here (ea25d54)