In the development of sensor applications, common mistakes include failing to select the most appropriate sensing principle/technology for the task. Below are some case studies where we were able to help teams find the best sensor for their needs.NeedSensorsMeasure blood oxygen level as part of a device to detect sleep apnea and to correct itWrong Sensor: A reflective pulse plethysmograph. Attractive because it is small, low cost, and because it can be placed almost anywhere on the body, but is not accurate enough to provide medically acceptable results without expensive and inconvenient calibrating. This sensor would not have provided accurate data for medical diagnosis.Right Sensor: A transmissive pulse plethysmograph, while accurate, is restricted to use on parts of the body that are thin enough to allow light to pass through. A design change allowed them to place it around a finger.Desire to measure core body temperature non-invasivelyWrong Sensor: A simple thermistor based measurement of the temperature of exposed skinRight Sensor: Depending on the requirements, there are many ways to measure core temperature non-invasively. Infrared sensors now can read the temperature of arteries through thin layers of skin with good accuracy. Heat flow sensors have also shown some promise. The less convenient “under the armpit approach” has been commercialized for many years. A simple temperature sensor on the skin, even at the wrist, may be able to infer core body temperature at times, with sensor fusion and a clever algorithm. The skin temperature may be close to the core temperature, taking into account the level of activity of the person, the air temperature, and if there is perspiration on the skin, all of which can be measured with sensors.Heart rate measurement that is convenient and always available.Wrong Sensor: While ECG electrodes provide a very good way to measure heart rate, they can be inconvenient. It is critical to have good contact, which often requires moist ECG pads, which is not acceptable for most consumer products and many medical devices. Large flexible electrodes can make good contact, but they usually require some adhesive that needs to be replaced periodically.Right Sensor: A pulse plethysmograph picks up pulse as well as measuring blood oxygen. Even the reflective type can be used to measure pulse rate and can be placed on the wrist where a transmissive pulse plethysmograph would not work. While it will not provide accurate Oxygen measurement, especially during movement, it will provide an adequate measurement for many applications, and it can provide an accurate heart rate even during movement with better comfort and convenience that will encourage usage.