Design Update

Although our “cocktail straw” Fleisch spirometer was a large design advancement, the internal diameter of the straws was not small enough to achieve laminar flow through the capillaries. After some fluid mechanics calculations, we discovered that the inner diameter of the capillaries will have to be <1.2 mm to produce laminar flow through our spirometer.

We began searching for materials that met this requirement, and realized that standard hematocrit tubes have appropriate dimensions. We ordered a pack of the polycarbonate style (hematocrit tubes are also generally available in glass) and tried updating our design to incorporate these tubes.

Hematocrit capillaries in place

Unfortunately, polycarbonate is a very poor heat conductor, and our attempts to cut the tubes with a fine-blade band-saw resulted in fusing our capillaries shut. As is, we are still looking for suggestions of capillary materials or any premade capillary systems that we could simply cut and plug in to our spirometer.

Ends of capillaries fused by cutter

On the software end, a team of interns has been actively refining computer algorithms that take an array of flow data points and calculate the various spirometry test values. The team is using the raw curve data from the NHANES III and comparing their calculated values with those calculated by NHANES. This process has been challenging as well, as NHANES has used data processing techniques that are not clearly defined to calculate the lung function indices. Our team is currently working on developing a program that will run numerous types of processing techniques, ranging from very low-level to advanced, on the data and compare which methods yield the results closest to those printed by NHANES.

The final front of development is focused on the internal circuitry components of the spirometer. Currently, we are working with the PIC18F13K50 microcontroller which will be responsible for taking the data from our iLite signal conditioner and passing it to the computer via USB. Because USB is an advanced and highly-standardized communication protocol, we’ve been devoting considerable time into learning how to program the microcontroller to successfully identify itself to the computer and then proceed to stream data to it. Also, although our circuitry will have relatively few components, all of them have specific electrical specifications that need to be met by incorporating appropriate accessory components (resistors, capacitors, etc.) into our design. Progress in this area will hopefully allow us to create a Printed Circuit Board (PCB) of our circuitry design by the end of summer.

The multi-faceted nature of the spirometer project has allowed the team to learn a great deal in a wide variety of fields, but we are still far from experts in any one area. Feedback on our design, including critiques of existing work and suggestions for future progress, are always appreciated!