A new device has entered the market: Nasal Pressure Transducer Cannula. This is now the preferred device because it measures the airflow itself, which is a much more calibrated signal and can detect UARS events, hypopneas, flow limitations, etc.
This raises the question of whether or not thermistors or oral sensors are still needed. Mouth breathing issues are important, and not to be confused with apneas. When you mouth breath, you are already engaging in obstruction. This raises another question, "how much of the obstruction do you need to clarify?" So, in summary, mouth breathing represents obstruction, so it's not always clear why you have to measure mouth breathing with another signal placed over the oral cavity.