The Flow Head on the Spirometer is rated at flow rates up to 10L/sec. It has the best linear range for flow rates equal to or below 5L/sec. A calibration syringe is available from other vendors that can be used to assess the precision of the Spirometer.
The accuracy of the Spirometer will depend on a number of factors. Volume is calculated by taking the integral of the flow rate. This is done via software calculations. As a result, there is not a definitive answer on the accuracy of the Spirometer.
The Vernier Spirometer is designed for educational use only. It is intended for qualitative data rather than quantitative measurements of lung volume and function.
Medical-grade spirometers would be much more accurate and have exact specifications and as a result are much more expensive.