Winging it

After the last tinkering with CO ₂  sensors on the VO₂ max device, Sensirion released an updated chip the STC31-C with finer resolution (0.2% vs 0.5%) and a more "raw" reading mode. The idea had been to use the CO₂ sensor to calculate the respiratory quotient, the VO₂ max device uses the O₂ sensor primarily to determine consumed oxygen. The O₂ sensor appears to be more accurate, reliable and consistent than the CO₂ sensors, however being electro-chemical, the O₂ sensors have a fixed lifespan. So the other idea is to see if it's possible to calculate the VO₂ max based on the expired CO₂ volume instead. The initial attempts had used the SCD30 sensor, however, despite being ridiculously accurate these sensors can only read up to 4% CO₂ levels, well below expired breath CO₂ levels and no use for exercise levels. So I ordered a few of the new STC31-C  to play with and built up a new module to test. PCB with an STC CO₂ sensor and SHT temp sen...