The Baby monitor has an infrared sensor that continuously measures baby’s chest skin temperature. The value reported on the app is the average over last 5 minutes.
It can take 10-20 minutes for the temperature readings to become reliable after the Baby monitor is put on the baby. If the Baby monitor itself is significantly colder or hotter than your baby's skin temperature, this period might take longer.
Chest temperature generally follows their core temperature, but varies a lot more. So there can be times when the skin temperature doesn't accurately reflect the exact core temperature. Some things will affect their chest skin temperature more than their core, such as low or high temperature of the ambient environment (i.e. room temperature), layers of clothing, wetness of the skin or clothes and physical exertion.
If Bluebell alerts you about your baby being hot or cold, it’s recommended that you also measure their temperature with an in-ear thermometer. This can give you a more reliable core temperature reading.