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 |
| 1 | Breathing machine (or cough assist which can produce breaths of 2 liters in 2 directions at least 25 times per minute) | 1 |
| 2 | Washers 6 mm | 42 |
| 3 | Threaded rod 135 mm | 6 |
| 4 | M6 Hex nut | 42 |
| 5 | M3 bolt (1.5 cm long) | 2 |
| 6 | M3 nut | 2 |
| 7 | M3 bolts (5 mm long) | 8 |
| 8 | CO2 sensor mounts | 2 |
| 9 | Arduino Mega mount | 1 |
| 10 | CO2 Tanks | Enough for number of tests |
| 11 | Flow meter | 1 |
| 12 | 6 mm pneumatic tubes | 8 |
| 13 | Venturi inlet | 1 |
| 14 | Venturi outlet | 1 |
| 15 | Tube OD=16 mm ID=13.6 mm L=50 mm | 1 |
| 16 | Tube OD=40 mm ID=32 mm L=1000 mm (too be cut) | 1 |
| 17 | Pipe branch (aftakking) | 2 |
| 18 | One-way valve with hole | 2 |
| 19 | One-way valve without hole | 2 |
| 20 | One-way valve with restriction and no hole | 1 |
| 21 | One-way valve with restriction and with hole | 1 |
| 22 | Brackets 40 mm tube | 6 |
| 23 | Reservoir left side end caps | 2 |
| 24 | Reservoir right side end caps | 2 |
| 25 | 16 mm venturi tube connector | 1 |
| 26 | 32 mm tube connector | 2 |
| 27 | Dummy head | 1 |
| 28 | Base plate left | 1 |
| 29 | Base plate right | 1 |
| 30 | Build plate feet | 4 |
| 31 | Appropriate glue | Plenty |
| 32 | Mouth piece (to center the tube) | 1 |
| 33 | Mini-umbrella valves | 6 |
| Electrical components | ||
| 34 | Arduino Mega | 1 |
| 35 | SprintIR-WF-20 CO2 sensors | 2 |
| 36 | Wires | Plenty |
| 37 | USB A to B | 1 |
| 38 | Analog Differential pressure sensor Range: 500 Pa | 2 |
| 39 | Relay | 1 |
| 40 | Fan power supply | 1 |
| 41 | Fan | 1 |
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:
CO2 Sensors
General comment:
0-20% CO2 concentration by volume measurable. Digital read out via UART. SprintIR-20-WF from GSS.
Specification sheet:
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.
Step 3
Connect the one-way valves as shown in figure 4 and figure 5.
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.
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.
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.
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.
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.
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.
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.
