A young college student experience is tachycardia after a night of heavy drinking. She is advised to undergo a stress EKG. As she exercises, the recently calibrated pulse oximeter records a heart rate ranging from 25 bpm at rest to 50 bpm to while jogging. The test is stopped and restarted twice, and each time the pulse ox immature yields a resting heart rate of 25 and a jogging rate of 50. Which aspects of this instrument does the EKG technician question?