A Spirometer is a device which measures a persons lung capacity. These devices are very commonly used in hospitals to make sure patients do not have any breathing complications during recovery. If you walk into a recovery room, you will often see a low-tech spirometer that looks a lot like a large measuring cup with a big tube coming out of the side. For our project, we undertook to build a more high-tech version of this device using a capacitor emerged in water and a low pass filter. This device allows patients to blow into a tube and read their lung capacity right off a computer screen.

Essentially our device involves a vessel containing a tube to blow into and a large capacitor. This vessel is emerged in water, but as you blow into the tube, it raises out of the water and the capacitor value changes accordingly. The changing capacitance value if then calibrated to calculate lung volume. The combination of these components allows a patient or doctor to digitally calculate a patients lung capacity.

Step 1: Supplies

What is needed:

1 x Resistor (1 MΩ)

1 x Large Parallel Plate Capacitor (40 mm x 300 mm with ~0.8 mm spacing)

1 x Variable Frequency Power Supply

1 x Vessel (must have a volume of at least 5.5 L)

1 x Tank (must be able to fit the vessel and contain enough water to fill it)

Lab View Software

Connecting Wire

*There is quite a bit of flexibility with the above supplies. As long as the set up is essentially the same, the software can be calibrated to work with different resistors or different capacitors.

Step 2: How It Works

This spirometer works by relating a changing capacitance value to lung volume. The capacitor is initially completely submerged in water, but as you blow into the tube, air from your lungs fills the top of the vessel and causes it to rise. As this occurs, the capacitor gradually becomes less and less submerged, therefore changing its capacitance value. Using the transfer function of the low pass filter with know input frequency and resistor values, we can determine the capacitance of the circuit. The circuit can then be calibrated using known air volumes to relate capacitance to lung volume. Take several data points of air volume and capacitance, plot them, and fit a line. When the equation for this fit line is incorporated into the lab view program, the circuit becomes a real time lung volume transducer!

Step 3: Circuit Diagram

The diagram features the variable frequency power supply, the 1 MΩ resistor, the capacitor emerged in the tank, and where the voltage measurement is taken by the computer

Step 4: Building the Vessel

There are a few important constraints for the vessel:

1. It must have a large enough volume to measure full lung capacity

2. It must be completely water tight except for the a single water entry hole

3. It must remain pretty much upright as it lifts out of the water

For our vessel, we modified two large juice bottles so that we had enough volume. Essentially, we took 2 of the same bottles, cut the very bottom off one and the middle off the other, and then resealed them together (pictured above). This gave us plenty of volume and was fairly straightforward to build. Additionally, the design of the bottle made it easy to keep the vessel water tight. To make sure it remained upright, we designed a narrow tank with a diameter just larger than that of the vessel.

Additionally, the vessel must contain a capacitor which remains in a fixed position and is parallel to the length of the bottle. This is essentially to getting an accurate measurement. It is also important to make sure the wires and connections of the capacitor are completely sealed from the water so that they don't short.

Finally, a tube needs to be secured in the bottle to direct airflow. The end of the tube must be at the very top of the bottle and the tube should be flexible enough as to not hinder the movement of the bottle as it is rising out of the water.

Step 5: Building the Tank

The design of the tank does not have nearly as many restraints as the vessel. Pretty much, it just needs to be water tight and have a large enough volume to fit the water and the vessel. We designed a tank that allows the vessel to rise out of the water upright by making the diameter of the tank just larger than the diameter of the vessel. A smaller tank, such as a paint bucket, can also be used, but there must be something added to keep the vessel upright as it rises from the water.

Step 6: Calibration

Programing the computer to measure lung volume from this device is pretty straightforward. The device works such that the value of the capacitor depends directly on how much of it is submerged in water. The above equation relates capacitance to the input voltage frequency of the circuit. Once this equation is programed into the computer, you can use it to determine different capacitance levels with different air volumes.

At this point, the calibration can begin. Since capacitance is directly dependent on the height of the water in which it is submerged, and this height depends on how much air is blown into the vessel, we can assume that capacitance is directly proportional to lung volume. Therefore, if the capacitance is measured at several different know lung volumes, a trend line can be fitted to the data, and the equation for this line can be used to determine lung volume from capacitance value.

A copy of our Labview code is attached.