A WOMAN died while using sub-standard breathing equipment at the National Diving Centre in Chepstow, an inquest heard.

Experienced diver Mary Restell 54, from Devon, her husband Roy and a friend were using trimix – a mixture of three different gases - for the first time during the dive.

The inquest at Gloucester Coroner's Court, held yesterday, heard that she was using her own equipment and some of it was not up to standard.

The combination of trimix, which is more difficult to breathe than compressed air, and a regulator that was not functioning properly, led Mrs Restell to have breathing difficulties and become unresponsive deep under water at the former quarry.

Diving enthusiasts Mrs Restell and her husband, from Peter Tavy near Tavistock in Devon, had made more than 1,350 dives over many years but always using compressed air.

Mr Restell told Gloucestershire Coroner Katy Skerrett they had always enjoyed diving together and had become experienced divers, training in decompression techniques nine years ago.

On November 8 2014, he said they met with good friend and diver Claire Boxall to plan the deep dive at the diving centre near Chepstow on November 15, where she would dive with them. They had used the centre previously and knew the layout.

"It was the first time we had used tri-mix – a mixture of oxygen, helium and nitrogen, and the dive was scheduled to take 44 minutes," he said.

"We were going to descend to 60 metres (200 feet) spend 15 minutes at that depth and then start our decompression steps to return to the surface."

Mrs Restell had seemed fine at the surface as they checked their equipment and if she had not been they would not have dived, he said.

They descended to 60 metres together without incident, but when they started to swim back in the direction of the pontoon prior to starting decompression, she seemed unsure and unfocussed and was making ineffective movements.

"She began to sink and I tried to manage her buoyancy as well as my own. There was no understanding in her eyes and although there was some water in her mask, she made no attempt to clear it," he said.

"Suddenly, she dropped below 60 metres and I saw a rush of air from her mouthpiece. She signalled that she could not breathe.

"The only thing I could do was to risk decompression sickness and the bends by taking her to the surface as quickly as possible."

The staff did their very best to to help Mrs Restell but she died at the scene, and although he suffered from the bends, he made a good recovery.

Expert witness Thomas Anthony, who provides technical support to Navy divers, was asked by the coroner to check all the equipment Mrs Restall had used.

He said it seemed as though she still had plenty of gas at the time of the incident, but two regulators in her equipment were not performing correctly and did not appear to have been serviced recently.

"This meant she would have to make a lot more respiratory effort to get the amount of gas she needed," he said.

"Also, the gas itself is dense and more difficult to breathe at that depth, which would have added to her problems.

"A third factor that could have affected her is the high nitrogen content in the gas mix, which can be narcotic.

"And because she had to work hard to breathe there would also have been a build-up of carbon dioxide (in her blood)."

He said all this seemed to fit with Mrs Restall becoming unresponsive at depth.

"It was her first dive using tri-mix but I would normally expect formal training in its use, and a progressive approach to that kind of dive," he added.

Summing up, the coroner said the cause of death was a gas embolism and a lung-related pulmonary barotrauma and pulmonary tumour.

Mrs Restell had been in good health before the dive but had had died from a build up of gas in the blood, said the coroner.

"Joining all the facts together, there is only one conclusion I can come to, and that is the accidental death of a woman who had been doing something she loved," she said.