Warm gas can hold more water vapour than cool air. This can be likened to a sponge - the warmer the gas, the larger the sponge. Figure 1 illustrates the saturation curve of water vapour versus temperature. As temperature increases, more water can be held in the vapour phase.

Dew point (Td) is defined as the temperature to which the gas would need to be cooled for condensation to begin. If the gas is cooled further, more water will condense; the temperature and dew point are the same, and the gas remains saturated.

Therefore, it should be noted that water vapour has a partial pressure and makes up part of the breathing and the sensed gas. Rebreather control systems only measure the oxygen content of a gas for PO2 control; however, integrated controllers with decompression rarely make adjustments for vapour in the assumed inert gas fractions. Such inaccuracies can only be calculated if the gas content is known for that particular rebreather, as the oxygen sensors being positioned in different locations in a loop will vary vastly in supersaturation.

Such accuracy in gas content would only be beneficial where such data is critical; modern decompression devices have either built-in conservatism or user-adjustable conservatism that can be manipulated to take into account those unquantified variables.

The effect of pressure on the dew point

As a diver, it is important to know that the dew point also changes with pressure. With an increase in total pressure, each partial pressure is also increased: an increase in total pressure causes a shift in the dew point. This increase is like squeezing a sponge - gas is unable to hold as much water vapour under pressure. In Figure 2, the total pressure is doubled, and therefore, the vapour pressure due to water vapour is doubled. The result is that the dew point shifts. In rebreathers, where gas at atmospheric pressure is compressed to 7 bar or more, the vapour pressure rises a long way above the saturation curve, and condensation is observed.

To give you some perspective, if the loop gas you breathe is at 35°C on the surface, at 60m, the loop gas would have to be at 76°C to maintain the same dew point.

It is therefore important to accept that condensation will occur and there is little we can do about it. What we can try to do is prevent that condensation from causing problems with electronics and sensing systems.

Moisture and sub-systems

Condensation is not an immediate problem; it is a potential problem relating to the sub-systems. Pure distilled water conducts little electricity; it is the impurities in the water which give it the conductive properties. When we assume that there will be organic components as well as scrubber dust and other debris, we should assume some conductivity. If saline liquid were to make it into the loop, then we can be assured of it.

There are two main aspects of those systems we are concerned about:

• the electronics, including power supplies
• the oxygen cells

Moisture effects on rebreather electronics

From our earlier conclusions, it is pretty obvious that the water vapour is present anywhere there is gas from the loop. It is possible to mitigate the vapour by segregating the electronic components, coating them and using water-absorbent membranes or cartridges at the interface, but this has the disadvantage of being of limited life before it too is saturated and requiring post-dive maintenance.

The only other way is to separate the electronics from the loop entirely in its housing. Unless the electronics are in 1ATM housing, then there are equivalent levels of vapour in the electronics or battery housing, regardless of their physical construction.

Electronics on a rebreather system are varied and should be considered from the integrated controllers to the cables interconnecting subsystems. Most electronics circuits in such equipment are potted or encapsulated in a hydrophobic or displacing substance, such as a polyurethane two-pack. Alternatively, there is a substance that we call conformal coating that can work well.

Cables and their termination are a weak point in that they must, at some point, be exposed to the gas in question. This leads to some gas permeating inside the cable sheath and causing corrosion, or even worse, voltage leaks, which we describe as ‘Creepage’.

Protecting your wiring

Galvanic-assisted corrosion is of particular concern, given the fact that solder joints are comprised of different metals or alloys that are in contact with one another. Add to this a high oxygen content environment, and salts and any metals are at particular risk. To many, this phenomenon is countered by 'regular' maintenance, which usually means “replace when bad”. It is just as simple to design and construct using principles to avoid this corrosion in the first place.

Polyurethane two-pack compounds, which are not loaded with fillers, are relatively cheap and encapsulating joints is a simple and effective method. Let me explain, soldered joints in rebreathers can corrode by >0.15mm per year when inadequately protected. Oh, and heat shrink is not protection.

Inevitably, when cables and conductors are inadequately protected, the pressures exerted on the system promote migration of oxidising agents up the cable through the gaps between conductor strands. When considering the low voltages involved in closed-loop control systems in rebreathers, we can see how design criteria can be seen as a critical function of its design, not a secondary consideration of assembly. Marine cables are now commonly available that minimise or prevent this occurrence.

The importance of oxygen cells

Oxygen cells rely on a very small output voltage that operates between an effective 8-150mV output. Creepage acting on a system with such a sensitive instrument voltage range is significant.

If water vapour were to condense in the loop and form moisture, which it will, then that moisture would first collect on surfaces with a temperature lower than the gas. Eventually, almost all the surfaces will obtain some moisture and ignoring the effects of ‘wicking’, the liquid will attach to most surfaces. The level of conductivity of the liquid determines the level of detriment to the system.

An SMB or coaxial cell has a dielectric separation of about 5mm, which is fairly good. This is due to the labyrinth pattern of the engagement of the male and female parts of a plug/socket.

A Molex plug does not perform as well, and the separation is only the distance between the pins, which is a couple of millimetres.

An SMB cable socket still has a cavity where the conductor is soldered to the centre pin, and this should be potted with polyurethane to prevent any problems.

The cells themselves contain electronic components. This includes a resistor network and a thermistor or similar device to affect the temperature compensation. These components inside the cell should be encapsulated or coated in a conformal coating.

The important part of the cell concerning oxygen sensing is the face. Water condensing on the face of a sensor will have the effect of slowing down the response time of a sensor, and although humidity 0-99% relative humidity (non-condensing) has a minimal effect on the sensor, the water vapour in the humidified gas will appear as a lower than expected oxygen output.

So, conclude that in a rebreather, calibration of a cell with any occlusion on the face can lead to an incorrect correction factor being applied to the control system.

Remember this: the response time of a cell is a function of two parameters:

1. The membrane. This is fixed at manufacture and stays constant throughout the sensor's life
2. Henry's Law. The difference in pressure between outside and inside the sensor.

They are the facts governed by the physics of a sensor. However, external effects can alter what happens, such as water on the membrane and sensor age.

Near the end of the sensor's life, the anode surface is much smaller, so response time must increase (probably a very small effect until the last hours of the sensor). Measuring response time has no purpose as it is less enlightening than measuring a sensor's output at high PO2.

This graph is one of many from Erik C Baker’s ‘Clearing Up The Confusion About Deep Stops’ paper.

A smart way of mitigating decompression risk

Bühlmann gradient factors are an integral part of decompression planning. By understanding how they work and the factors that influence them, you can make informed decisions about your dive profiles and minimise the risk of decompression sickness. You’ll typically find the default settings on your dive computer tend to be aligned with safer diving. Using these conservative values is likely to be a sensible option if you’ve struggled to understand some of the complexities detailed in this blog. Consult a qualified diving instructor for specific guidance based on your dive conditions.

There is a specific paragraph in the 2024 order which may cover diving activities:

“…possession is for the purposes only of making the weapon available for, or participating in, a permitted activity – i.e., a historical reenactment or sporting activity;”

While scuba diving isn’t specifically mentioned as a permitted purpose, there could be an argument that it may fall under ‘sporting’ activity. Of course, common sense should always be the priority here. We suspect that if it’s very evident that you’re a diver and have a bladed article in a collection of diving equipment, then you have a reasonable excuse not to be considered a thug!

Finally, some words of caution to take in. This is a blog from a seller and producer of diving equipment and it is not a substitute for professional legal advice.

Is this a symptom of a decline in diving?

Earlier this year, Darcy Kieran of the Business of Diving Institute (BODI) reported on his Scubanomics blog that 2023 had been healthy for scuba diving markets across Europe and the USA. However, there was a note of caution in Kieren’s research, as he noted “repeat customers are increasingly important for dive centres and resorts in an industry that, historically, has been reliant on new scuba divers”.

This latter point has sadly proven fatal for several physical dive shops over the past few years. Sheffield's SDS Watersports was another UK casualty over the recent summer. This is why we believe it’s important to help grow this activity and entice new generations to explore underwater realms. It’s pleasing to read about Lancashire’s Phoenix North West Sub-Aqua Club continuing to attract younger students.

Statistics show that scuba diving and snorkelling in England took a hit during the pandemic years, which was unsurprising, but these activities have bounced back since then. Web searches for diving terms over the past four years have gradually increased, albeit in small increments and occasional bursts of interest.

GoDive's first website, from 2001.

Looking to the future

Attracting the next generation of divers should certainly be a priority for the diving scene. Unfortunately, "gatekeepers" can often pose a barrier to entry for new divers. These individuals may have outdated attitudes, stringent requirements, or a lack of understanding of the modern diving experience.

Encouraging new divers and fostering stronger relationships between dive centres and their customers are crucial steps in ensuring the continued growth and vitality of this beloved pastime.

Preventing nosebleeds on a dive

We’ve already conveyed the importance of equalisation techniques on your descent. We also pointed out how nasal congestion puts a diver at higher risk of a nosebleed..

If you are suffering from a cold or an allergy which affects your sinuses, we wouldn’t recommend proceeding with your dive.

Nasal moisture is paramount in preventing nosebleeds. Another pre-dive exercise you can do is to gently equalise your ears every few minutes.

Another pre-dive exercise you can do is to gently equalise your ears every few minutes.

How to handle an underwater nosebleed

Understandably, the sight of blood in your mask is a thing of nightmares. Yet we cannot state enough that the diver’s maxim – don’t panic! According to research, anxiety/stress is believed to be one of the top three risk factors in scuba diving incidents.

If you’re having a nosebleed during a dive, we recommend this procedure:

1. Remain calm.
2. Alert your buddy.
3. Lightly pinch your nostrils together for a while.
4. Aim for slow and controlled breathing.
5. Clear your mask if possible
6. Make a slow ascent.

When you reach the surface, remove your mask and gently clear your nasal passages.

After your dive

If nosebleeds persist regularly, you should consult a doctor before commencing any further dives.

For those who feel determined to continue with diving, here are a couple of tips:

• Try out other equalisation techniques.
• Play it safe with shallower dives.

In conclusion

Nosebleeds are quite common among divers. In most cases, while they’re unpleasant to deal with, they’re not usually a cause for major concern. We’ve stressed the importance of frequent equalisation during your descent.

If you experience a nosebleed, remember to remain calm, alert your dive buddy and try our recommended actions.

Of course, this blog is no substitute for professional medical advice. Always reach out to a specialist if nosebleed becomes a recurring problem.

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