Tim Cook may have described the Apple Watch as ‘the most personal device we've ever created’ but reports claim the gadget doesn't tolerate some individual’s tattoos. Owners of the coveted watch have found the timepiece's sensors malfunction when worn on tattooed wrists. Dark, vibrant ink seems to cause the watch’s heart rate monitor to lose connection and give inaccurate readings. Scroll down for video . Some owners of the Apple Watch have found that it malfunctions if worn on tattooed wrists (pictured). Some dark, vibrant ink seems to cause the watch’s heart sensor to lose connection and give inaccurate readings . The issue first came to light on Reddit and Twitter where a number of users reported similar issues, such as having to repeatedly enter a password to unlock the phone. This is required every time the device detects  it has left a wearer’s wrist. Tests suggest that tattoos in dark and solid colours interfere with the device the most. This is because the heart sensor reads a pulse by measuring light absorption though the skin - a process called photoplethysmography. 'This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light, Apple wrote on its support page. 'Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. 'When your heart beats, the blood flow in your wrist - and the green light absorption - is greater. Between beats, it’s less. 'By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute - your heart rate.' It is thought that  red and green inks reflect light in a similar way and the sensor can't distinguish between the blood and tattoo. It appears  the pigmented ink of some tattoos affects the watch's sensor’s ability to read an individual’s heart rate, which in turn leads the watch to think it has stopped making contact with the skin. Apple has yet to comment on the problem, despite there being speculation that the apparent fault could also stop some people using Apple Pay. Tests conducted by iMore suggest that tattoos in dark and solid colours interfere with the device the most. This is because the heart sensor reads a pulse by measuring light absorption though the skin - a process called photoplethysmography. 'This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light, Apple writes on its support page. 'Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. 'When your heart beats, the blood flow in your wrist - and the green light absorption - is greater. Between beats, it’s less. 'By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute - your heart rate. So the problem seems to occur when a wearer has something on their skin that reduces the lights reflectiveness, such as ink under the skin’s surface. It appears that the pigmented ink of some tattoos affects the watch's sensor’s (illustrated) ability to read an individual’s heart rate, which in turn leads the watch to think it’s stopped making contact with skin . Reddit user guine55fan thought his 42mm Sport edition watch had a bad wrist detector sensor beacuse it locked every time the screen went dark and wouldn;r receive notifications. He tried holsing it against his hand, which unlike his wrists and arm, aren't tattooed. 'My hand isn't tattooed and the Watch stayed unlocked,' he wrote. 'Once I put it back on the area that is tattooed with black ink the watch would automatically lock again.' The user has told Apple about the problem who tried to solve the problem, but are also reporting it to their engineers. This doesn’t include natural skin pigmentation. iMore tested the watch’s sensors on tattooed and non-tattooed parts of wrists, as well as elsewhere on the body and found that on non-inked parts, the sensors gave uniformed readings. However, ‘on tattooed sections, sensor readings varied wildly depending on colours and shading,’ Serenity Caldwell reported. She said  solid colours in darker inks such as black and red ‘confused’ the sensor the most, causing misreadings of up to 196 beats per minute (BPM), for example. The average heart rate is 72 BPM. Tests on lighter colours such as purple and yellow caused slightly elevated misreadings of 80BPM, compared to 69BPM on the wearer’s non-tattooed wrist, but didn’t interfere with skin contact detection, unlike with darker shades of ink. The team was unable to reproduce problems reported on Twitter of light, but patterned tattoos creating errors, leading them to conclude that the design and type of tattoo makes a difference in how well the watch works. Apple has yet to comment on 'tattoo-gate' but admits on its support page that 'many factors' can affect the performance of the Apple Watch heart rate monitor. 'Skin perfusion is one,' it says. 'A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. 'If you’re exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading.' Apple said that motion is another factor. 'Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.' The sapphire glass screen in the Apple Watch has remained scratch-free after being subjected to being rubbed with sandpaper, keyed and even hit with a hammer. The test was carried out by Cardiff-based iPhone repairs specialist iPhoneFixed.co.uk, which used a sapphire screen from a 38mm Apple Watch for its experiment. Two out of the three models of Apple Watches have sapphire glass screens - the Apple Watch and Apple Watch Edition. The glass is made from synthetic sapphire, which is created by applying heat and pressure to aluminium oxide powder, before the crystals are processed to remove impurities. The tiny round disks created are ground into shape, and polished, to become glass. Sapphire glass is said to be twice as tough as normal glass and almost as hard as a diamond. To test the durability of the screen, it was rubbed against a stone wall and then with a coin, before being ‘scratched’ by a key. The video capturing the attempts at destruction, shows no scratches were achieved to the screen. It also withstood being rubbed with abrasive sandpaper at some force and hit by a hammer – albeit not very hard. But most impressive, was the screen’s ability to withstand an electric drill being dragged across its surface, without leaving a mark. The test led iPhoneFixed to claim: ‘Sapphire glass is pretty strong and virtually impossible to scratch’.
Tattooed Apple Watch users  reporting problems with its heart rate sensor .
Sensor takes readings by measuring light absorption though the skin .
Dark, solid tattoos seem to confuse the new device the most .
Apple has yet to comment on the problem, despite people taking to Twitter .