After 6 months of use on a baby with reduced mobility, I have to say that the sensor (which is on the baby's foot 24 hours a day) does not work very well, resulting in many times showing less oxygen than it actually has and constantly triggering the alarm! (the sensor I have is the baby one that costs about 130€). For its price, it is unacceptable that it requires 4 AA batteries, which are not included in the package. I bought rechargeable batteries and a charger. It should have come with a rechargeable lithium-ion battery and a charger. The service is good and prompt, we had a problem with the first cable-sensor and they changed it immediately.