Introduction

The use of masks under strenuous conditions can be quite uncomfortable and laborious. It is hence imperative that the air inside the mask is as fresh as possible and the act of breathing is easy. This places limits on the total amount of “dead” volume due to the re-breathing of CO2 from the previous breath(s). This effect can cause the overall oxygen levels to fall and the wearer to breath more heavily and quickly to make up the difference. This leads to exhaustion, uncomfort and severe drops in cognitive performance which leads to mistakes, low morale and motivation. Measuring the CO2 levels is hence imperative to ensure safe and comfortable use of the mask.

List of required components

NR:Type of component:Quantity
1Breathing machine (or cough assist which can produce breaths of 2 liters in 2 directions at least 25 times per minute)1
2Washers 6 mm42
3Threaded rod 135 mm6
4M6 Hex nut42
5M3 bolt (1.5 cm long)2
6M3 nut2
7M3 bolts (5 mm long)8
8CO2 sensor mounts2
9Arduino Mega mount1
10CO2 TanksEnough for number of tests
11Flow meter1
126 mm pneumatic tubes8
13Venturi inlet1
14Venturi outlet1
15Tube OD=16 mm ID=13.6 mm L=50 mm1
16Tube OD=40 mm ID=32 mm L=1000 mm (too be cut)1
17Pipe branch (aftakking)2
18One-way valve with hole2
19One-way valve without hole2
20One-way valve with restriction and no hole1
21One-way valve with restriction and with hole1
22Brackets 40 mm tube6
23Reservoir left side end caps2
24Reservoir right side end caps2
2516 mm venturi tube connector1
2632 mm tube connector2
27Dummy head1
28Base plate left1
29Base plate right1
30Build plate feet4
31Appropriate gluePlenty
32Mouth piece (to center the tube)1
33Mini-umbrella valves6
Electrical components
34Arduino Mega1
35SprintIR-WF-20 CO2 sensors2
36WiresPlenty
37USB A to B1
38Analog Differential pressure sensor Range: 500 Pa2
39 Relay1
40Fan power supply1
41Fan1

Fan

General comment:

Regular computer fan.

Analog pressure sensors

General comment:

500 Pa sensor, depending on the type of fan used in the setup.

Specification sheet:

https://www.sensirion.com/en/download-center/differential-pressure/sdp800-proven-andimproved/

CO2 Sensors

General comment:

0-20% CO2 concentration by volume measurable. Digital read out via UART. SprintIR-20-WF from GSS.

Specification sheet:

https://docs.rs-online.com/ece4/0900766b8168eabd.pdf

Before you start

Before you start you should print the following parts:

  • Venturi inlet
  • Venturi outlet
  • 40 mm brackets
  • Pipe branches
  • One-way valves
  • Dummy head
  • Mouth piece
  • Sensor mounts
  • Arduino Mega mount
  • Reservoir end caps

Building steps

The files are available for all the components.

In theory, all these components can be 3D printed and used, except for the nuts and threaded rods which are likely not printable in sufficient quality (with M6 sized threads). However, for the baseplate plexi-glass is recommended because of its stiffness. Furthermore, the rubber footing adds grip to the setup which makes the setup easier to work with.

The one-way valves are constructed by inserting silicon umbrella valves into the dedicated 3D printed one-way valve housing as shown in Figure 1. The umbrella is secured on the small hole as shown in Figure 2. For each direction there are 3 valves in series where the middle one is given additional pressure drop to ensure that there is enough bleed air. The using the double connector the valve assembly can be constructed as shown in Figure 6.

After this the venturi assembly can be made and connected as shown in Figure 7. The bleed air holes must be placed in the direction shown here. This is fasted to the valve assembly.

The current setup can be connected to the 3D printed head and bolted to the baseplates, together with the sensors and the Arduino Mega, as shown in Figure 8.

Once the setup is secured to the baseplate and the bolts have been tightened, the bleed tubes can be connected as shown in Figure 9, Figure 10 and Figure 11.

The electrical connections as follows (See Figure 12, Figure 13 and Figure 14):

  • Pin 14 to pin 3 sensor 1.
  • Pin 15 to pin 4 sensor 1.
  • Pin 16 to pin 3 sensor 2.
  • Pin 17 to pin 4 sensor 2.
  • Pin 50 to pin 2 pressure sensor.
  • Pin 52 to pin 2 sensor 1.
  • Pin 51 to pin 1 pressure sensor.
  • Pin 53 to pin 2 sensor 2.
  • Pin A0 to pin 4 pressure sensor.
  • The rest to ground.

Step 1

Create a left and right base-plates with dimensions of 350 mm by 200 mm each. Advised is to use a puzzle pattern to connect them.

Step 2

Produce all the one-way valves using the umbrella valves as shown in figure 1, figure 2 and figure 3.

Figure 1: Silicon umbrella valves and the dedicated 3D printed one-way valve housing.
Figure 2: Securing the umbrella valve on the one-way valve housing using a small hole on the top.
Figure 3: Restricted valve for extra pressure drop.

Step 3

Connect the one-way valves as shown in figure 4 and figure 5.

Figure 4: One-way valves for inhale. The middle valve has extra pressure drop.
Figure 5: One-way valves exhale. The middle valve has extra pressure drop.

Step 5

Use the pipe branch to connect both the inhale and exhale sides of the system. Note that all connections must be air tight.

Figure 6: One-way valve assembly.

Step 6

Assemble the venturi as shown in figure 7. Make sure the positions of the bleed air connectors are in the same place. Under these connectors, a hole must be drilled. The connector-hole seal must be air tight.

Figure 7: Venturi assembly.

Step 7

Snap the reservoir caps onto a length of 40 mm OD pipe with length of 200 mm. Insert a pipe into the back of the dummy head and fasten all the components to the base plate as shown in figure 8. Figure 9 and figure 10 provides a side and top view of the setup.

Figure 8: The different components fastened to the baseplate through drill holes.

Step 8

Connect the 6 mm pneumatic pipes between the different components as shown in figure 9 and 10. The bleed pipe for the inhale ends in the dummy head mouth and enters through the side of the pipe before the dummy head. It is held in the center using the mouth piece. This is shown in figure 11. These pipes and their connections must be air tight.

Figure 9: Connection bleed tubes (side view).
Figure 10: Connection bleed tubes (top view).
Figure 11: The bleed tube for the inhale line extends into the mouth via the throat.

Step 9

The wiring of the Arduino Mega can be done. Note that the pin connections and the electrical connections as follows (See Figure 12, Figure 13 and Figure 14):

  • Pin 14 to pin 3 sensor 1.
  • Pin 15 to pin 4 sensor 1.
  • Pin 16 to pin 3 sensor 2.
  • Pin 17 to pin 4 sensor 2.
  • Pin 50 to pin 2 pressure sensor.
  • Pin 52 to pin 2 sensor 1.
  • Pin 51 to pin 1 pressure sensor.
  • Pin 53 to pin 2 sensor 2.
  • Pin A0 to pin 4 pressure sensor.
  • The rest to ground.
Figure 12: Connections to Arduino Mega.
Figure 13: Connections to CO2 sensors.
Figure 14: Connection to pressure sensor.

About the Setup

The setup consists mainly of a breathing machine and CO2 source which is connected to the pneumatics shown in Figure 3. The way these sections are connected are specific to the breathing machine and CO2 source of the specific setup.

Figure 15: Connection of the CO2 and breathing machine to the pneumatics.

The pneumatics are shown in Figure 4. The pneumatics separate the inhale and exhale air from each other and facilitates the measuring of the CO2. Most of the air is forced through the one-way valves during the inhale and exhale. However, the resistance of the valves is changed to generate a pressure drop across the valves. This pressure drop is used to bleed off air into the buffers, located on the side of the setup. The path of the air through the one-way valves is shown in Figure 5. The bleed air for the different sensors are shown in Figure 6 and Figure 7. The bleed air for the inhale is taken from the end of the mouth as shown in Figure 9. Lastly, the flow velocity and volume are measured through the pressure drop over the venturi, shown in Figure 8.

Figure 16: Pneumatics of the CO2 test setup. Air enters from the left and exits through the dummy head on the right during exhale and vice verse during inhale.
Figure 17: Direction of flow through one-way valves. From left to right is exhale and from right to left is inhale.
Figure 18: Bleed air path for CO2 sensor inhale.
Figure 19: Bleed air path for CO2 sensor exhale.
Figure 20: Bleed air for CO2 sensor inhale from mouth to buffer.
Figure 21: Venturi pressure sensor bleed air.

Downloads

https://1drv.ms/u/s!AqC74CqqYVbpgeAWwgrmCQX7ErizZw?e=YXJFsX