One of the clever features of the new Apple Watch is that it can monitor your heart rate. The back of the watch housing contains green and infrared LEDs, paired with light sensors. The infrared lights are used to measure your heart rate every 10 minutes, and if that doesn’t produce an accurate reading, the Watch switches to the green LEDs. The amount of blood in your skin varies with each heart beat, and the sensors are designed to detect the minute differences that are used to infer your heart rate.

Recent reports have indicated that some wearers are having difficulty getting accurate readings, especially if they have tattoos on their wrists where they wear their watch. Apple has now acknowledged the problem, adding the following paragraph to their Support page about the heart rate feature:

Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.

Apple’s solution to inconsistent heart rate readings from the Watch is also worth noting: “You can connect your Apple Watch wirelessly to external heart rate monitors such as Bluetooth chest straps.” It seems like an expensive and inconvenient work-around, but it may be necessary if you want an accurate record of your heart rate.