Info Technical Discussion | HOTDIVE Diving Equipment Development

[HotDive(Hide In Ocean)]

4. Filter​

From the pictures, it appears that you use flat polymer filters. I have nothing against polymer filters in principle, as I understand they help reduce cost. However, I assume you are not using these polymer filters in your oxygen units, as they would be utterly out of place. Are you using sintered bronze filters, as is standard in the diving and medical oxygen industry, or stainless steel mesh filters? If stainless steel mesh is being used, how are you mitigating what I see as a very high risk of oxygen fire within the filter? Literature repeatedly shows that stainless steel mesh is incompatible with oxygen service.1

I am not a fan of disc filters; would it not be possible to use a conical design, even without incurring the cost of a sintered bronze filter? The small surface area of a disc filter is inferior to a conical shape for safety and flow distribution. Flat discs clog up much quicker than their conical siblings.

For S3 (Yoke) and S1, S2, S3 (DIN) regulators, the filters are conical sintered bronze filters, and all filters used in regulators compatible with pure oxygen are conical sintered bronze filters.
In some specific regulator models (S1 Yoke and S2 Yoke), we use stainless steel filters. This allows them to be reusable, and maintenance can be performed by ultrasonic cleaning, which helps reduce overall usage costs for our customers.
In fact, the filtration efficiency of disc filters is not significantly different from conical filters. Many well-known brands, such as Apeks and TUSA, also use disc filters in their designs.

 

[Tanks A Lot]

Thank you for your detailed response, I appreciate it. I see we are approaching the situation from slightly different angles.

[...]
However, 316L stainless steel, when properly cleaned and maintained, is also widely recognized as suitable for oxygen environments. The key point is that stainless steel requires a sufficiently strong ignition source to sustain combustion. We employ numerous processes to mitigate these risks. Through proper design and strict oxygen cleaning procedures, 316L stainless steel components can operate safely even in high-oxygen environments.[...]

Absolutely, 316L is used in many industries for its desirable properties, including some oxygen applications. My main gripe with the SCUBA industry is that it is very difficult to compare it to other gas industries. Those often manage fleets of thousands of rental cylinders with strict protocols in place. In SCUBA, on the other hand, anyone can buy an oxygen cylinder, transfill whip, compressor, and start filling in their garage.

I believe the SCUBA industry, especially at the design stage, should approach things from a worst-case scenario. What happens if someone does everything wrong? Does the equipment guarantee disaster, or does it at least give them a fighting chance?

1. Ignition & Combustion Risk
[...]
Our Solutions:
We implement a comprehensive cleaning process to ensure all components meet strict oxygen service standards:
[...]

This is one of those areas where we clearly come from different angles. As a manufacturer, your procedures are correct. The issue with oxygen cleanliness is that it isn’t a one-off state, it must be continuously maintained. That “garage diver” comes to mind again. What happens if the cleanliness starts to deteriorate? Brass certainly doesn't guarantee a beneficial outcome. But I feel like the usage of stainless steel is shifting the balance slightly to the "bad outcome" state of the scales. Both systems will fail catastrophically when things get bad enough. The physics of it make me come to the conclusion that 316L reaches that state slightly faster than brass.

For me, good design mitigates risks from a gas-flow perspective. Are critical areas exposed to high-velocity gas? Are soft parts and springs shielded from adiabatic heating, flow friction, and particle impacts? Does the surrounding area act as a heatsink? Sherwood’s KVAB valves are a classic example: the way the parts are arranged is nothing short of genius once you dig into it.

2. Material Degradation
Sources of Risk:

• Metal Oxidation: 316L stainless steel, with its excellent chromium oxide passivation layer, exhibits outstanding stability in ambient high-oxygen environments, without the dezincification or stress corrosion cracking seen in brass.

316L certainly has the edge mechanically; it is a wonderful material. However, I doubt dezincification in a gaseous high-oxygen environment is any real concern, I’ve never seen evidence of that. I would like to be shown wrong on this. Stress corrosion cracking in brass is also rare as far as I know. When SCC is mentioned, my first thoughts go to aluminium and then to stainless steel. Stainless is known to be susceptible to SCC in the presence of HCl, though obviously that should never happen with a regulator (Yet I know many "technicians" using muriatic acid...).

• Oxygen-Compatible Seals: FKM/Viton® O-rings are used. The high bond energy of the C-F bond (485 kJ/mol) provides excellent thermal stability and oxidation resistance, making it the industry-standard choice for high-pressure oxygen service.
  [...]

I usually dislike relying on branded products, but I’ve seen too many substandard O-rings. Viton is certainly not the only good FKM compound. Are there any verifiable ways to guarantee quality FKM if Viton isn’t specified?

• Currently, we only use FKM/Viton® seals and MCG111 silicone grease on certain specific models.

I’ve long thought SCUBA gear would be better off using only PFPE grease. Personally, I’m not the biggest fan of Christolube 111; there are better, if sometimes more expensive, alternatives. Wouldn’t it make sense to drop silicone grease completely from the line-up, even if cost is a factor? I think your gear would benefit from it.

[...]

• System Design Philosophy: We adhere to the principle that “a clean gas supply is the first prerequisite for safety.” Oxygen-compatible regulator design can only be effective under clean gas supply conditions.
• Gas Handling & Monitoring:
  We recommend using high-quality filters (combining coalescing filters, activated carbon, molecular sieves, etc.) between the gas source and regulator to remove oil mist, moisture, and particulates. The gas used in our regulator tests undergoes compression (with built-in compressor filtration) + condensation + separate filtration.
• Importance of Clean Gas Supply: Only with pure gas can the high-oxygen compatibility design of the regulator truly function.

I think this is where our perspectives diverge most clearly. You’re correct that a clean gas supply is the first prerequisite for safety; no debate there. Perhaps it’s unfair of me to introduce worst-case scenarios, but I do think about design more in terms of flow patterns and “hot spots”. Almost anything can be made oxygen-safe if flow velocities and adiabatic heating are tightly controlled.
A good design shifts the outcome in the "idiots" favor, so to speak.

[...]

• Medical Oxygen Systems — 316L components are widely used in hospital central oxygen pipelines, ventilators, and anesthesia machines in direct contact with medical oxygen.

On the subject of medical oxygen systems, I don’t entirely agree. As far as I know, most use copper tubing, per BS EN 13348 or ASTM B819. Granted, these often operate at much lower pressures.

• Aerospace & NASA Applications — 316L stainless steel is explicitly approved for oxygen systems when proper cleaning standards are met (e.g., ASTM G93, NASA-STD-6001).

Regarding standards: ASTM G93 is a cleaning guide rather than an approval of materials. Most documents I’ve read stop short of prescribing specific materials, instead advising engineers to assess suitability for the intended task. ASTM G93 does provide detailed recommendations on cleaning stainless steel, that is correct. If that is an "endorsement", I'm not qualified to say.
NASA-STD-6001, at least in the version I know, focused on test setups and materials evaluation. It mostly referenced 304L. I see a new version was released in June 2025, so I may need to revisit that. If it makes more explicit material recommendations, I’ll be pleasantly surprised.

 

[Tanks A Lot]

• Hyperbaric Oxygen Chambers — Valves and fittings in medical and diving oxygen chambers are often made of 316L stainless steel.

On hyperbaric oxygen chambers, I can’t fully agree either. From what I know and have worked with, most valves are brass, though you are right that most fittings are stainless steel, the pipes included.

• Food & Pharmaceutical Production — In oxygen-enriched cleaning and sterilization processes, 316L stainless steel pipes and valves are widely adopted for their corrosion resistance and cleanliness.

There’s no question that 316L can be used in oxygen systems. My question is more whether it’s the best choice for SCUBA. I would argue brass is still the clear winner in terms of oxygen suitability.

I will take some videos or photos in the production workshop during my spare time and keep sharing them with you. I hope you will find them interesting.

I certainly enjoy the photos and videos you share from production, please keep them coming.

Thank you for raising this question about cold-water performance. The Hotdive S1 is not designed as a cold-water regulator. It is an unbalanced piston model, primarily developed for warm and moderate water environments. [...]

On the cold-water discussion: there is no shame in stating that the S1 is not a cold-water regulator. While a few unbalanced pistons have been adapted for cold conditions, most are not.

For diving in cold waters or even under ice, we recommend our S3. The S3 features a balanced diaphragm design, constructed from SUS316 stainless steel, and incorporates environmental sealing and thermal conductivity optimisation to enhance cold-water performance. It was specifically developed for low-temperature diving scenarios. I will post a detailed introduction about S3 later. Regarding the concern of whether stainless steel can pass cold-water auxiliary testing, I have a strong answer: we have customer feedback reporting successful dives with the S3 in the Arctic Circle, at water temperatures as low as -2°C.

For the S3, I don’t doubt your customers have had success even in the Arctic Circle. First stages with environmental sealing are very difficult to ice up externally; the second diaphragm is too far from where ice forms. Getting the second stage to cooperate in those conditions is much more challenging, kudos for that.
May I ask what exactly you did for thermal conductivity optimisation? I don’t see fins or other obvious surface enlargements that would improve heat exchange.

For S3 (Yoke) and S1, S2, S3 (DIN) regulators, the filters are conical sintered bronze filters, and all filters used in regulators compatible with pure oxygen are conical sintered bronze filters.

That is excellent news.

In some specific regulator models (S1 Yoke and S2 Yoke), we use stainless steel filters. This allows them to be reusable, and maintenance can be performed by ultrasonic cleaning, which helps reduce overall usage costs for our customers.

Then you’re clearly more adept than I am! I’ve never had much success cleaning filters: disc, conical, sintered, or mesh. That is probably a "me" problem.

In fact, the filtration efficiency of disc filters is not significantly different from conical filters. Many well-known brands, such as Apeks and TUSA, also use disc filters in their designs.

I don't think I can entirely agree. While the efficiency doesn't differ with brand new filters, disc shaped filters undeniably clog up faster than conical ones. That is the entire point of the conical shape, enlarged surface area. That's one of the reasons that most companies moved on from disc filters.


While we may not agree on everything, I appreciate the civil discussion and your openness in sharing details. Thank you!

 

[lairdb]

I’d just like to say that I am fascinated by and appreciate the discussion between @Tanks A Lot and @HotDive(Hide In Ocean) — clear expertise from both parties, civil and constructive tone, etc.

 

[elmo]

Thanks for the detailed discussion @HotDive(Hide In Ocean) and @Tanks A Lot

You are correct on this point. From an electrochemical perspective, 316L stainless steel indeed ranks higher than brass in the galvanic series, which theoretically could pose a “large cathode/small anode” galvanic corrosion risk when used in conjunction with brass. My primary recommendation is for our customers to pair stainless steel regulators with hoses that have stainless steel fittings. Using brass fittings generally does not cause significant issues either.

This is because the external surfaces of brass components are usually chrome-plated, and under normal conditions, brass does not come into direct contact with stainless steel regulators. If “brass—stainless steel regulator” direct contact does occur, it indicates that the chrome plating has been corroded or worn off, exposing the base material. In this case, the root cause of corrosion is not the presence of stainless steel, but rather plating failure. Moreover, once brass is directly exposed to seawater, it is already subject to corrosion.
In contrast, 316L stainless steel has a natural passive film (chromium oxide layer) that can regenerate even if the surface is scratched, without relying on plating. This makes it more durable in diving environments.
From a galvanic corrosion perspective, the electrochemical potentials of 316L stainless steel and chrome are close, so there is almost no significant potential difference between them. The real difference lies between brass and stainless steel, but if the chrome plating on the brass remains intact, such contact is almost nonexistent under normal use. Therefore, the key is to regularly inspect and maintain the plating on brass components to prevent potential galvanic corrosion.

In other words, if a brass fitting and a stainless steel regulator do come into direct contact, it is already a sign of plating failure, not a “new problem caused by the stainless steel material.”

What about the opposite? If your hoses have stainless fittings and are used on brass regulators do we risk the first and second stages becoming the anode?

 

[HotDive(Hide In Ocean)]

Thank you for your detailed response, I appreciate it. I see we are approaching the situation from slightly different angles.


Absolutely, 316L is used in many industries for its desirable properties, including some oxygen applications. My main gripe with the SCUBA industry is that it is very difficult to compare it to other gas industries. Those often manage fleets of thousands of rental cylinders with strict protocols in place. In SCUBA, on the other hand, anyone can buy an oxygen cylinder, transfill whip, compressor, and start filling in their garage.

I believe the SCUBA industry, especially at the design stage, should approach things from a worst-case scenario. What happens if someone does everything wrong? Does the equipment guarantee disaster, or does it at least give them a fighting chance?


This is one of those areas where we clearly come from different angles. As a manufacturer, your procedures are correct. The issue with oxygen cleanliness is that it isn’t a one-off state, it must be continuously maintained. That “garage diver” comes to mind again. What happens if the cleanliness starts to deteriorate? Brass certainly doesn't guarantee a beneficial outcome. But I feel like the usage of stainless steel is shifting the balance slightly to the "bad outcome" state of the scales. Both systems will fail catastrophically when things get bad enough. The physics of it make me come to the conclusion that 316L reaches that state slightly faster than brass.

For me, good design mitigates risks from a gas-flow perspective. Are critical areas exposed to high-velocity gas? Are soft parts and springs shielded from adiabatic heating, flow friction, and particle impacts? Does the surrounding area act as a heatsink? Sherwood’s KVAB valves are a classic example: the way the parts are arranged is nothing short of genius once you dig into it.


316L certainly has the edge mechanically; it is a wonderful material. However, I doubt dezincification in a gaseous high-oxygen environment is any real concern, I’ve never seen evidence of that. I would like to be shown wrong on this. Stress corrosion cracking in brass is also rare as far as I know. When SCC is mentioned, my first thoughts go to aluminium and then to stainless steel. Stainless is known to be susceptible to SCC in the presence of HCl, though obviously that should never happen with a regulator (Yet I know many "technicians" using muriatic acid...).


I usually dislike relying on branded products, but I’ve seen too many substandard O-rings. Viton is certainly not the only good FKM compound. Are there any verifiable ways to guarantee quality FKM if Viton isn’t specified?


I’ve long thought SCUBA gear would be better off using only PFPE grease. Personally, I’m not the biggest fan of Christolube 111; there are better, if sometimes more expensive, alternatives. Wouldn’t it make sense to drop silicone grease completely from the line-up, even if cost is a factor? I think your gear would benefit from it.


I think this is where our perspectives diverge most clearly. You’re correct that a clean gas supply is the first prerequisite for safety; no debate there. Perhaps it’s unfair of me to introduce worst-case scenarios, but I do think about design more in terms of flow patterns and “hot spots”. Almost anything can be made oxygen-safe if flow velocities and adiabatic heating are tightly controlled.
A good design shifts the outcome in the "idiots" favor, so to speak.


On the subject of medical oxygen systems, I don’t entirely agree. As far as I know, most use copper tubing, per BS EN 13348 or ASTM B819. Granted, these often operate at much lower pressures.


Regarding standards: ASTM G93 is a cleaning guide rather than an approval of materials. Most documents I’ve read stop short of prescribing specific materials, instead advising engineers to assess suitability for the intended task. ASTM G93 does provide detailed recommendations on cleaning stainless steel, that is correct. If that is an "endorsement", I'm not qualified to say.
NASA-STD-6001, at least in the version I know, focused on test setups and materials evaluation. It mostly referenced 304L. I see a new version was released in June 2025, so I may need to revisit that. If it makes more explicit material recommendations, I’ll be pleasantly surprised.

As you mentioned, the entry barrier in the diving industry is relatively low. People can easily purchase a full set of equipment and become so-called “garage divers,” often without proper regulation or oversight. While we cannot completely prevent such practices, what we can do is to share knowledge and raise awareness, helping more divers recognise the seriousness of potential risks and improve their safety awareness. At present, this is the most effective way to minimise accidents.

Regarding the potential theoretical risks of stainless steel regulators, we are continuously studying and exploring them, always seeking safer and more effective ways to reduce the likelihood of issues. Whether it is adiabatic heating, flow friction, or particle impact, we carefully analyse these possibilities and take them into account in our designs. Our regulators are not static products; they are constantly being improved and refined. I am committed to ongoing optimisation to further enhance both safety and reliability.

 

[HotDive(Hide In Ocean)]

Thanks for the detailed discussion @HotDive(Hide In Ocean) and @Tanks A Lot

What about the opposite? If your hoses have stainless fittings and are used on brass regulators do we risk the first and second stages becoming the anode?

If the regulator body is made of brass and the hose connector is stainless steel, this combination is more stable in seawater than a stainless steel body with small brass components. Galvanic corrosion follows the principle that the less noble metal corrodes preferentially, so a large brass body acts as the anode while a small stainless steel part serves as the cathode, resulting in a slower corrosion rate. With the addition of nickel-chrome plating, the use of dezincification-resistant (DZR) brass, and regular freshwater rinsing after diving, this setup is very reliable in practice, and proper maintenance can effectively prevent corrosion issues.

 

[pisauron]

Great thread and I learned a lot!

I have been using successfuly the T2 & T4 2nd stages up to 60m depth of warm salt water.
They both preformed very well in my opinion.

I haven't tried the 1st stages yet.

Can you show examples of your backplate bcd?

 

[HotDive(Hide In Ocean)]

Great thread and I learned a lot!

I have been using successfuly the T2 & T4 2nd stages up to 60m depth of warm salt water.
They both preformed very well in my opinion.

I haven't tried the 1st stages yet.

Can you show examples of your backplate bcd?

Thank you very much for your support and feedback. We will continue to work hard to bring you more professional and reliable diving equipment.
I will gradually introduce our product series in detail, including regulators S1–S3 and T1–T4. This will be a fairly long sharing post, so I hope everyone stays tuned.
Next, I will give a dedicated introduction to our BCD buoyancy control devices—I believe this product will definitely catch your interest! Stay tuned.

 

[HotDive(Hide In Ocean)]

I usually dislike relying on branded products, but I’ve seen too many substandard O-rings. Viton is certainly not the only good FKM compound. Are there any verifiable ways to guarantee quality FKM if Viton isn’t specified?

There are always components on the market with superior performance, but when choosing, factors such as cost must be carefully considered. For example, FFKM seals perform exceptionally well, but their high cost means they can only be used in specific scenarios and cannot be widely applied. As a manufacturer, we are committed to finding a balance between quality and cost, providing users with products that offer excellent performance at an acceptable price. Within the same product category, we use higher-quality materials while offering more reasonable pricing, which is why we continue to gain recognition from our customers.

As you mentioned, you don’t blindly follow big brands. For most consumers, big brands are often the default choice, while many smaller brands—equally good or even superior in some aspects—are overlooked. I completely agree with your view. Although small or emerging brands may not be as well-known as the big names, they often focus more on product quality and technical development. Just like Hotdive, we aim to let more users experience top-quality products at a minimal cost through high-quality materials, thoughtful design, and reliable performance.