Here is an excellent CO2 meter if you seek one for home or office use: https://www.amazon.com/gp/product/B01FYWU2IS/ https://www.amazon.com/gp/product/B01FYWU2I

Here is an excellent CO2 meter if you seek one for home or office use: https://www.amazon.com/gp/product/B01FYWU2IS/ https://www.amazon.com/gp/product/B01FYWU2IS/ I can vouch for the monotonic rise or fall in its displayed values as affected by ventilation.