How a Scuba Diving Regulators First Stage Works

Like the diver’s beating heart that moves air from the lungs to the rest of the body, the scuba regulator’s first stage provides the critical connection between divers and the air in our cylinder, allowing us to work and play underwater.

The regulator first stage’s main function is to reduce the high tank pressure to an intermediate pressure that can be utilised by the second stage and provide air on demand to the diver. Modern regulator first stages are precision-made and designed to work under demanding conditions wherever divers care to explore.

We will be examining both piston and diaphragm first stages, and their differences and similarities to help you understand how this vital piece of equipment allows us to breath while enjoying the ocean’s depths.

1. The Parts

Both piston and diaphragm regulators have either a DIN or yoke style fitting to connect them to the scuba cylinder; an inlet filter to prevent contaminants from entering the regulator; a regulator body incorporating intermediate- and high-pressure chambers; a bias spring; medium-pressure fittings for second stages, inflator assemblies, and accessories; and high-pressure fittings for gauges and transmitters. Piston regulators have a piston-style valve assembly with a high-pressure seat separating the first stage’s high- and intermediate-pressure chambers, while diaphragm regulators have a diaphragm, lifter-poppet valve assembly and high-pressure seat performing the same function.

2. How It Works

As you inhale on the regulator’s second stage, pressure in the first stage’s intermediate chamber is reduced. The force of the bias spring and the ambient water (hydrostatic) pressure push inward on either the diaphragm or the base of the piston head, raising the valve and creating an opening between the intermediate- and high-pressure chambers. Air flows from the high-pressure chamber into the intermediate-pressure chamber and down to the regulator second stage via the connecting hose. When the diver stops inhaling, pressure inside the intermediate chamber increases until it is greater than that of the bias spring and hydrostatic pressure and the valve closes.

The first stage is designed to provide air at ambient pressure, so it must adjust for the changes in pressure as depth changes. To do this, a method is needed for the valve assembly to “sense” the ambient pressure changes and adjust accordingly. Piston regulators have the bias spring, the underside of the piston and a portion of the piston shaft exposed to the water to provide the hydrostatic pressure necessary for operation. Diaphragm regulators have one side of the diaphragm and the bias spring in contact with water to provide hydrostatic pressure but the rest of the components are sealed off from the environment on the other side of the diaphragm. In both cases, the amount of pressure required to open and close the valve assemblies varies with the ambient water pressure on the exposed surfaces.

3. Balanced vs. Unbalanced

A balanced first stage, whether piston or diaphragm, is designed so that tank pressure does not impact the operation of the valve. This ensures consistent breathing effort independent of depth or tank pressure. Currently, all diaphragm regulators in production are balanced. In diaphragm systems, balancing is accomplished by routing intermediate-pressure air to both sides of the complete valve assembly and passing the valve stem through both the high-pressure and intermediate-pressure chambers. In balanced piston first stages, the incoming high-pressure air does not directly act on the piston-valve, which also passes through the intermediate and high-pressure chambers. Unbalanced regulators have tank pressure acting directly on the high-pressure seat, and incoming high-pressure air will act to close the valve as intermediate pressure rises. This works fine while tank pressures are high, but can result in heavier breathing resistance when at higher ambient air pressure (deeper water) or when tank pressure is low. The unbalanced piston will still provide adequate air supply in these cases, but it will require more effort by the diver to breath.

4. Piston vs. Diaphragm

Now to the eternal debate over which is best: a balanced piston or diaphragm first stage. In reality, it boils down to a matter of personal preference and both designs work well at providing breathable air to their users. There are some distinguishing characteristics, however, that may aid in deciding which may be best for you.

  • The nature of a diaphragm first stage’s design means that it is environmentally sealed
  • There is less chance of components being affected by ice particles in cold water, or by silt, sediment or other contaminants in turbid waters
  • Diaphragm regulators tend to be complex with more parts than a piston regulator and have a smaller diameter valve, meaning lower overall volume of air to the second stage
  • Piston regulators are of simpler design and the size of the piston stem allows for a larger volume of air to be supplied to the second stage, making them popular for their ease of breathing effort
  • Piston regulators have more components exposed to the environment, making them more susceptible to contaminants affecting performance, icing in cold water and resulting free-flow from a stuck piston
  • Piston regulators may require more maintenance of exposed parts due to contamination

5. Additional Features

Regulator first stages are closed systems and are not designed with user-adjustable features, but there may be enhancements incorporated by the manufacturer. Both types of regulators may incorporate environmental protection, to include a second diaphragm to protect the bias spring of a diaphragm first stage, or by filling the exposed section of a piston regulator with a viscous fluid and sealing it off with a flexible ring. Some regulators incorporate automatic devices to close the high-pressure inlet and prevent contaminants or water from entering the first stage when the regulator is not connected to a tank. Some regulators may incorporate a special medium-pressure port with greater airflow specifically designed for high performance second stages.

How a Scuba Diving Regulators Second Stage Works

Have you ever wondered how regulators make it possible for scuba divers to breathe pressurised air underwater? Well, here is an illustrated explanation on half the equation: how a scuba diving regulator’s second stage works.

We have come a long way since the early 1940s when Jacques Cousteau and Emile Gagnan co-invented the Aqua-Lung, an underwater regulator modified from Gagnan’s demand regulator that fed cooking gas to a car’s carburetor in the exact amount needed.

Remarkably, today’s regulators rely on the same design principles used by Cousteau and Gagnan. You cannot breathe directly out of your tank because the high pressure would damage your lungs. Just like the Aqua-Lung, today’s second stages take pressurised air from the first stage and provide it to a diver at ambient pressure, allowing him or her to breathe normally underwater. Through the use of precision manufacturing, high tech materials and intensive testing, modern regulator design is efficient and reliable, and with proper maintenance and care, modern second stages will provide years of dependable operation.

1. The Parts: Mouthpiece, housing, cover incorporating a purge valve, flexible silicone diaphragm to separate the external water from the housing, demand valve assembly, demand lever and exhaust valve.

2. How It Works: Air from the first stage enters the second stage housing through an inlet fitting. At the inlet, an orifice with a circular “knife edge” provides a sealing surface for the main valve assembly. This assembly consists of a poppet with a hard-rubber seat at one end, a bias spring and the valve body. The bias spring applies pressure on the poppet, pressing the seat against the edge of the orifice and creating an airtight seal. The demand lever is attached to the valve body and compresses the bias spring to pull the poppet away from the orifice, allowing air to flow into the valve.

As you inhale, pressure in the second stage is reduced and the diaphragm is pushed inward by the surrounding water pressure. The diaphragm then pushes on the demand lever, opening the valve. With the valve open, air then enters the second stage through an opening in the valve housing until it equalizes with the surrounding water pressure, making it possible for a diver to breathe air at ambient pressure. When the diver stops inhaling, pressure inside the second stage increases, causing the diaphragm to relax and release the demand lever, closing the valve.

As the diver exhales, air is expelled through the exhaust valves. The design of the second stage keeps ambient pressure in the housing at the same pressure as the surrounding water, ensuring that the reg enables consistent breathing effort regardless of conditions and depth.

3. Additional Features: Many regs have cracking pressure adjustment knobs, Venturi adjustment controls, and a balancing chamber in the demand valve. In a balanced second stage, a balancing chamber is added to the end of a modified poppet with a drilled-out center chamber and a hollowed-out seat. When a balanced second stage valve is closed, air travels through the opening in the seat and poppet into the balance chamber, pushing the poppet back against the orifice. This means a lighter bias spring can be used and less effort is required to open the valve. A cracking pressure adjustment manually changes the tension on the bias spring to increase or decrease the amount of effort required to open the valve, and the Venturi adjustment moves a rotating vane to direct air flow toward or away from the diver’s mouth. Both features potentially reduce breathing effort and aid in managing freeflows.

Eight Sunk Boats, Two Divers – One Map

The Dive Master mapping project, is one of the more daunting tasks you will complete on your journey to becoming a PADI Dive Master.

The objective was set, map the wreck grave yard at Wraysbury Dive Centre, the team and plan agreed, the divers kitted up, BWRAF check completed – submergence into the deep. A short swim to the target area, in variable visibility – good buddy communication and navigation enabled the team to get to the wreck grave yard.

Having located ‘Spiller’ the team set about recording each wreck in turn, measuring the length, depth, position and relative position to the previous target. Constant communication and checking between the buddy pair – did not stop the line entanglement, but this was easily fixed – a reminder that the incident pit is never that far away. Careful dive gas monitoring, indicated that at 85 minutes into the dive, it was time to turn for home, a cup of tea and a review of the data collected.

The heading to the exit point was established and after 96 minutes the team surfaced, they had achieved a lot – not completed the map, but excited for the next dive!

Will you be joining the team, increase your skills, become a PADI Dive Master

4 Benefits of Being an EFR and How It Can Help Save a Life

With Rec2Tec Diving running an EFR course this evening (06/08/19) and the ultimate Rescue course – 7-8 September 2019, this article written by PADI Regional Training Consultant, Jeremiah Foo is excellently timed!

Emergency First Response (EFR) training is highly recommended for everyone to help save lives and attend to illness or injury in our everyday lives. It equips us with adequate knowledge and skills to deal with emergency situations before further support arrives. To get a better understanding of how EFR training will benefit us and our love ones, check out these five benefits of first aid training:

1. It Saves Lives

EFR training gives us the confidence and ability to react immediately to an incident, injury or illness. Take CPR (Cardiopulmonary Resuscitation) for example. It has been reported time and again that many lives have been saved due to fast reaction and CPR given during critical and life-threatening accidents, injuries or health complications.

2. It Builds Confidence and Clarity During an Emergency

EFR training doesn’t just teach us how to treat patients in need of first aid, it also gives us confidence to effectively manage an emergency without fear, confusion or feeling overwhelmed.

3. It Can Reduce Recovery Time

Rapid reaction to illness or injury, before further aid such as an ambulance arrives, can not only save lives, it also reduces recovery time of the patient.

4. It’s a Great Team-Building Exercise and FUN

When we get the opportunity to learn or refresh skills together with family, friends or colleagues that will help us look after one another, it brings us closer together. Many teams have reported more awareness of their families’ and co-workers’ well-being following first aid training.

Are you still wondering if Emergency First Response (EFR) training is the course for you? Keep reading about an incident that happened in China where HongJie Guan applied his EFR skills and performed CPR on an individual which ultimately, helped save his life.

First Responder in Action

Friday 24th May 2019, Cafeteria of the Finance Bureau in WenZhou, China.

While having breakfast at the canteen in the Finance Bureau, HongJie Guan and his wife noticed a crowd had quite suddenly gathered nearby. HongJie went over to see what the commotion was about, only to realise that a man in his fifties was about to pass out while still in his seat. HongJie immediately realised that the man must have choked on the food he was having and his airway was blocked. Hearing that Emergency Medical Services were already on the way, HongJie instantly asked the crowd to back off and allow the victim and him some space. Next, HongJie lifted the victim’s head and performed abdominal thrusts; the victim eventually threw up and the obstruction was cleared. HongJie continued with his breakfast.

After a few minutes the patient collapsed. HongJie rushed over again to assist and upon checking the victim’s responsiveness realised that he was not breathing. HongJie immediately began CPR with rescue breaths. This continued for about 7 minutes before the paramedics arrived and took over.

Updates from the hospital indicated that the patient started breathing after receiving CPR continuously for 47 minutes. He then underwent surgery. It was later confirmed that he’d had a blood clot due to a medical condition that had led to a heart attack.

The patient is now recovering.

Please do not wait until something happens before taking up first aid training. The Emergency First Responder courses provide us with the most important skills we could ever learn while also providing us with the confidence to car

How Nitrox Diving works?

What is Nitrox?

Nitrox is a gas that is generally used for recreational diving. The gas is simply a mixture of oxygen and nitrogen, which in recreational diving terms, is sometimes referred to as ‘enriched air nitrox (EAN)’. The oxygen concentration in the air is normally around 21%, however the oxygen concentration in nitrox is higher. Nitrox refers to any combination with levels of oxygen between 21 and 40 percent- 32% is a common oxygen content used.

The well-used term ‘decompression sickness’ (DCS), also known as ‘the bends’ starts when nitrogen enters the divers bloodstream and as they dive deeper, the pressure will also increase.

When the nitrogen builds up and the diver comes to the surface too quickly (without performing decompression stops), the diver’s body will need time to remove the absorbed oxygen and may experience decompression sickness as a result. DCS is extremely unpleasant and in some cases it can be deadly, which is why so much research has been driven into how we can prevent it from happening.

Instead of using a standard mix, divers started using nitrox as it helps increase your allowed diving time due to offering less nitrogen and more oxygen. This means that less nitrogen can be absorbed into the bloodstream which therefore extends the ‘no decompression limit’, or in other words, you get longer bottom times.

While this does not completely prevent DCS, it reduces the risk of it occurring.

However, divers must also remember not to exceed maximum depth or bottom time!

Another benefit of using nitrox is shorter decompression times, this is because there is less nitrogen to be absorbed in the body, meaning that there is less nitrogen to expel from the body when resurfacing.

While oxygen is a key element to human life, oxygen at higher levels can be harmful to us too, this is called oxygen toxicity. If the oxygen concentration in the nitrox mix is too high, the diver may experience seizures, unconsciousness, breathing difficulty, damage to the lungs and eyes and in worst cases death.

Oxygen toxicity gives no or little warning before you experience these severe symptoms, which is what makes it so dangerous. It is safer to check your gas mix before you dive in order to eliminate risk of oxygen toxicity.

Deep Stops

World-recognised decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

by Associate Professor David J. Doolette

Gradient factors are mechanisms which modify the decompression stops prescribed by the Buhlmann ZH-L16 decompression algorithm. ZH-L16 is a “gas content” algorithm, which tracks the uptake and elimination of inert gas in notional tissue compartments and schedules decompression stops to not exceed specified maximum permissible inert gas partial pressures in the compartments. When such maximum permissible inert gas partial pressures are specified for decompression stop depths, they are referred to as M-values.

Gradient factors (GF) modify M-values (and consequently allowed gas supersaturation) to a fraction of the difference between ambient pressure and the original M-value. Thus, GF 80 modifies the M-value to 80% of the difference between ambient pressure and the original M-value. Typical proprietary implementations of the GF method require the diver to select two gradient factors: GF low modifies the M-values for the deepest decompression stop, and GF high modifies the M-value for surfacing (often designated as GF low/high, e.g. GF 20/80). The algorithm then interpolates a series of modified M-values in between these two user-specified points. If the GF low is set less than 100%, this forces deeper stops to limit supersaturation in the fast tissues early in the ascent, and setting the GF high to less than 100% will produce longer, shallower stops to reduce supersaturation in the slower tissues in the latter phase of the ascent.

In contrast to gas content decompression algorithms, bubble decompression algorithms (VPM-B is one such algorithm familiar to GUE divers) characteristically prescribe deeper decompression stops. In simple terms, bubble decompression algorithms favour deeper stops to limit supersaturation and thereby bubble formation early in the decompression, whereas traditional gas content decompression algorithms favour a more rapid ascent to maximize the inspired–tissue gradient of inert gas partial pressures to maximize tissue inert gas washout.

New Findings on Deep Stops

Deep stops came to the attention of early technical divers in the form of empirical “Pyle stops,” a practice serendipitously developed by ichthyologist and technical diving pioneer Richard Pyle, arising from a requirement to vent the swim bladders of fish specimens collected at great depth before arriving at his first decompression stop. There followed a strong trend toward the adoption of bubble algorithms, and also for the use of gradient factors to force gas content algorithms to impose deep stops (for instance, using GF low values of 30% or less). Based largely on supportive anecdotes, a widespread belief emerged among technical divers that deep-stop decompression schedules are more efficient than shallow-stop schedules. Efficiency, in this context, means that a schedule of the same or even shorter duration has a lower risk of DCS than some alternative schedule.

However, since about 2005, evidence has been accumulating from comparative decompression trials that shows deep stops are not more efficient, and possibly less efficient, than shallow stops.

However, since about 2005, evidence has been accumulating from comparative decompression trials that shows deep stops are not more efficient, and possibly less efficient, than shallow stops. Most studies have used venous gas emboli (bubbles) as an indicator of comparative risk of decompression sickness (DCS). Blatteau and colleagues compared dives using French Navy air and trimix decompression tables (relatively shallow stop schedules) to experimental schedules with added deep stops and longer total decompression time (similar to Pyle stops). Despite longer total decompression time, the deep stops schedules resulted in either the same or more VGE than the shallow stops schedules, and some cases of DCS. (1)

Spisni and colleagues compared trimix dives conducted using a deep stops schedule (ZH-L16 with GF 30/85) to an even deeper stops schedule with longer total decompression time (a UDT version of ratio deco) and found no difference in VGE.(2)

An as-yet-unpublished study compared trimix dives using a DCAP shallow stops schedule to a ZH-L16 GF 20/80 deep stops schedule with similar total decompression time, and the deep stops schedule resulted in significantly more VGE.(3)

A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives. (4)

What To Do About Gradient Factors

The emerging body of evidence against deep stops suggest common gradient factor setting should be modified to de-emphasize deep stops. Fraedrich validated dive computer algorithms by comparing them to well-tested U.S. Navy decompression schedules, including the schedules from the deep stop study outlined above. For that dive, ZH-L16 with a GF low >55% (e.g. GF 55/70) produced a first decompression stop between 70 and 40 fsw.(5)

Tyler Coen at Shearwater Research Inc. noted that GF settings recommended by Fraedrich modify ZH-L16 M-values so that approximately the same level supersaturation is allowed at all stop depths. To understand this requires delving a little further into M-values.

The emerging body of evidence against deep stops suggest common gradient factor setting should be modified to de-emphasize deep stops.

M-values are typically a linear function of stop depth. In older algorithms such as ZH-L16, the M-value generating functions have a slope greater than one (in ZH-L16, the slopes are the reciprocals of the “b” parameters), resulting in increasing supersaturation allowed with increasing stop depth. In more modern algorithms developed by the U.S. Navy since the 1980s, including the one used to produce the shallow stops schedule in the study outlined above, the slope of the M-value generating functions are generally equal to one, so that the same level of supersaturation is allowed at all stop depths. This results in modestly deeper stops than older algorithms, but still relatively shallow stops compared to bubble models.
With this information in mind, I set my GF low to roughly counteract the ZH-L16 “b” parameters (I have been using Shearwater dive computers with ZH-L16 GF in conjunction with my tried and true decompression tables for about three years). In ZH-L16, the average of “b” parameters is 0.83. I choose my GF low to be about 83% of the GF high, for instance GF 70/85. Although the algebra is not exact, this roughly counteracts the slope of the “b” values. This approach allows me to believe I have chosen my GF rationally, is not so large a GF low as I am unable to convince my buddies to use it, and satisfies my preference to follow a relatively shallow stops schedule.

This article was prepared by Assoc. Professor Doolette in his personal capacity. The opinions expressed in this article are the author’s own and do not reflect the view of the Department of the Navy or the United States government.

1. Blatteau JE, Hugon M, Gardette B. Deeps stops during decompression from 50 to 100 msw didn’t reduce bubble formation in man. In: Bennett PB, Wienke BR, Mitchell SJ, editors. Decompression and the deep stop. Undersea and Hyperbaric Medical Society Workshop; 2008 Jun 24-25; Salt Lake City (UT). Durham (NC): Undersea and Hyperbaric Medical Society; 2009. p. 195-206.

2. Spisni E, Marabotti C, De FL, Valerii MC, Cavazza E, Brambilla S et al. A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. Diving Hyperb Med 2017;47:9-16.

3. Gennser M. Use of bubble detection to develop trimix tables for Swedish mine-clearance divers and evaluating trimix decompressions. Presented at: Ultrasound 2015 – International meeting on ultrasound for diving research; 2015 Aug 25-26; Karlskrona (Sweden).

4. Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. 53 p. Report No.: NEDU TR 11-06.

5. Fraedrich D. Validation of algorithms used in commercial off-the-shelf dive computers. Diving Hyperb Med 2018;48:252-8.

Additional Resources:

PADI recently published an excellent post, “Evolving Thought on Deep Decompression Stops,” by John Adsit, on the subject of Deep Stops.
Alert Diver magazine published a profile and interview with Doolette in the Fall of 2016.

The Math behind the ZH-L16 Model: Bühlmann established, by means of many hyperbaric chamber experiments with volunteers, how much supersaturation the individual tissue compartments can tolerate without injury. He expressed the relationship through the following equation:
pamb. tol. = (pt. i.g. – a) ·b
or
pt. tol. i.g. = (pamb / b) + a
pamb. tol. – the ambient pressure tolerated by the tissue
pt. i.g. – the pressure of the inert gas in the tissue
pt. tol. i.g. – tolerated (excess)pressure of the inert gases in the tissues
pamb – current ambient pressure
a, b – parameters of the model ZH-L16 for each tissue. “a” depends on the measure unit of pressure used, while “b” represents the steepness of the relationship between the ambient pressure pamb. and the pressure of inert gas in the tissue pt. i.g. The first equation shows which lower ambient pressure pamb. tol. will still be tolerated at the actual pressure of inert gas in the tissues pt. i.g. The lower equation shows which level of supersaturation pt. tol. i.g. can be tolerated at a given ambient pressure pamb for a given tissue.

Dr. David Doolette began scuba diving in 1979 and was introduced to the sinkholes and caves of Australia in 1984. Around this time, he alternated between studying for his B.Sc. (Hons.) and working as a dive instructor, when he developed an interest in diving physiology. He planned and conducted some of the first technical dives in Australia in 1993. Since being awarded his Ph.D. in 1995, he has conducted full time research into decompression physiology, first at the University of Adelaide, and since 2005 at the U.S. Navy Experimental Diving Unit in Panama City, Florida.
He has been a member of the Undersea Hyperbaric Medical Society since 1987, received their 2003 Oceaneering International Award, and is a member of their Diving Committee. He has also been a member of the South Pacific Underwater Medicine Society since 1990 and served as the Education Officer for five years. He is a member of the Cave Diving Association of Australia, the Australian Speleological Federation Cave Diving Group, Global Underwater Explorers, and the Woodville Karst Plain Project. He remains an avid underwater cave explorer, both near his home in Florida and abroad

Swanage Dive Weekend 30 August – 1 September 2019

Swanage

Salt, salt, salt and more salt – if you want to expand your diving from the fresh water lakes and quarries we typically dive through out the year, this is the trip for you. The trip is for all qualified divers, with something for everyone.

With spaces booked on the excellent shuttle services run by Swanage Boat Charters, four current bookings are in place for two dives on Saturday and two dives on Sunday, the first dive on the Saturday is on the Valentine Tanks – max depth 15m and the second shuttle dive on the Carantan – max depth 30m. The Sunday sees us diving the Carantan first and then the Valentine Tanks second.

Swanage also has a pier, and it is possible to dive this – tides allowing, and other shuttle services can be booked, if the demand exists for drift or scenic dives.

We will be staying in shared rooms at the YHA on Friday and Saturday night, spaces and booking are limited, to secure your place or for further information email: training@rec2tecdiving.co.uk

Carantan Class

About the Carantan

Seized by the Royal Navy from occupied France in 1940, this 400-tonne submarine chaser was 120ft long with a narrow beam of just 20ft. By December of 1943 she was operating under the control of the Free French and was escorting a submarine towards Portsmouth when a fierce storm caused her to capsize. This tragic outcome may have been speeded along by a large Boer War-era gun bolted to her foredeck and top-heavy brass plating used for her superstructure.
Only 6 of the crew of 23 were saved, making the Carantan a war grave. Lying on her side, what is left of the Carantan juts up around ten feet off the seabed at the highest point, although much of it is smashed and broken. Where plating had come away from the skeletal frame of the superstructure in places.

Valentine Tanks

The Valentine Tanks

These tanks were in fact sea-going vessels in their own right. Before their demise, they were taking part in Exercise Smash around Poole Bay in preparation for the forthcoming D-Day landings of 6 June, 1944. The idea behind the top-secret design was that a canvas frame fitted around the tank would, when raised, displace enough water to enable them to float. They were known as “DD” or “Duplex Drive” Valentine tanks, and a propeller was also mounted at the rear to move the vehicle forward and to within beach range, where the tank’s tracks would take over the job. Today, divers usually visit only two examples of these tanks because, conveniently, one pair has been roped together to help divers to locate both on a single dive. They lie upright only 70m apart.

PADI Sidemount – Vobster – 30/31/03/19

Over the weekend of the 30-31/03/19, Rec2Tec Diving was at Vobster Quay in Somerset for a weekend of sidemount diving, with two students working towards their PADI Sidemount qualification. The weather was kind, the water cool and the viz fab - the students did really well too.

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