Regulator Series No. 1 – Poseidon Xstream
Let's take a look at the Poseidon Xstream regulator — a first stage regulator that stands apart from the crowd the moment you lay eyes on it. The first thing you'll notice is the birdcage-like top cap. Inside sits a spring assembly; this design allows more water to flow through the spring, preventing it from freezing. However, it also makes the top structure somewhat fragile — I have personally witnessed an Xstream regulator fall to the ground and completely come apart.
Next is the interior, where you'll find a remarkable red ball. In natural geometry, a sphere is the easiest shape to self-center. Imagine placing a ping-pong ball in a funnel — it naturally rolls to the very center without any deviation. This replaces the conventional sharp-tipped pin-on-flat high-pressure seat design. The smooth spherical surface never scores the contact area, so unlike conventional regulators, it won't fail from wear-induced intermediate pressure (IP) creep. On top of that, this design delivers an impressively high airflow rate (2,100 L/min). This is critically important for extreme-depth diving; paired with the Poseidon second stage regulator, the system has earned a certification for dives beyond 200 m (Norsok U-101). In plain terms: it's extremely durable and practically indestructible.
Inside the first stage regulator there is also a heavy-duty spring. Its purpose is to stabilize the intermediate pressure. As scuba tank / cylinder pressure drops, the thick spring pushes the red ball seat downward; the smaller spring at the bottom supporting the red ball is then compressed, generating greater force to push against the ball and compensate for the reduced differential between high pressure and intermediate pressure. In my own tests, the intermediate pressure remained unchanged regardless of the tank pressure reading.
At the base of the first stage regulator there is also a burst disc / pressure relief valve. This is related to the unique design of the Poseidon second stage: rather than venting when intermediate pressure rises too high, the second stage actually seals more tightly — meaning IP above 500 psi could potentially rupture the hose. Hence the burst disc is fitted to relieve excess pressure. Most people overlook its existence entirely; poor rinsing leads to it seizing up completely with rust, rendering it non-functional. The intermediate pressure on the Xstream is slightly lower than that of typical regulators, with a rated output of 8.5 Bar.
Feel free to share this with any friends who dive on Poseidon gear. The design principles behind the second stage will be covered in the next installment.
Poseidon Second Stage Regulator
Poseidon Second Stage Regulator
Poseidon Second Stage Regulator
Regulator Series No. 2 – The Poseidon Second Stage
The Poseidon second stage looks nothing like a conventional second stage's rounded, bulbous body — Poseidon's is shaped more like a harmonica.
The internal mechanism is completely different as well: there is no second-stage intermediate-pressure seat/diaphragm of the conventional kind. A standard second stage uses a spring to press a silicone seat against a valve orifice to stop airflow. Over time, sustained compression causes the seat to lose its seal and leak, or spring fatigue reduces clamping force — also leading to leaks. Neither problem exists in the Poseidon design. Instead, Poseidon drills numerous small holes around a cylinder and inserts a balloon-like bladder inside. The many holes provide a very high airflow volume. When the bladder is inflated, its expanding surface plugs the holes and stops airflow. The tail of the bladder is connected to a tiny exhaust valve; when the valve seat is depressed, the bladder deflates, the holes are no longer blocked, and air flows through. This design is similar in principle to a car's power steering system — a small input force is amplified into a much greater output. Consequently, only a very small diaphragm area is needed to actuate the exhaust valve and trigger airflow. This makes the regulator extremely sensitive; to prevent the surrounding water pressure from affecting the diaphragm, an elegant cover plate is fitted over it.
This is yet another design that is robust and practically failure-proof.
When swapping scuba tanks / cylinders and you need to purge the second stage, never press the purge button too deep — it's very easy to deform the delicate exhaust valve stem.
Also, the second stage may free-flow when you first open the tank valve. This happens because the pressure inside the bladder hasn't built up enough yet to seal the holes. As pressure gradually rises, the leaking stops. The higher the pressure, the better the seal — the exact opposite of a conventional regulator. So also be mindful not to dive the tank down below 10 bar, as the pressure may drop too low to maintain the seal and all the air will leak out.
Adjustment is also quite unique: because there is no valve seat orifice in the traditional sense, all conventional tools are useless. Sensitivity is set by adjusting the contact distance between the exhaust valve stem and the diaphragm.
The horizontal mechanism design places the exhaust port on a single side, which happens to be well away from directly in front of the mask — exhaled bubbles won't obstruct your line of sight. Feel free to share this with fellow Poseidon divers so they understand how it works.
Poseidon Second Stage Regulator
Poseidon Second Stage Regulator
Regulator Series No. 3 – Why Most Rental Regulators Use Piston First Stages
Think back — have you ever had this experience? When you were learning to scuba dive, you felt like the regulator got harder and harder to breathe from on the way back, as if each breath was heavier than the last.
It's not your imagination. The reason is that most training equipment uses a piston-type first stage regulator. The advantages of a piston first stage are: (1) it's inexpensive, (2) it has a simple construction, and (3) it's inexpensive and has a simple construction. This is why it has become the dominant choice for rental gear. The most common models seen across Southeast Asia are the Aqua Lung Calypso and the Scubapro MK2.
However, this design has an inherent flaw: intermediate pressure drops as tank pressure drops. I took an MK2 unit through a complete overhaul — replacing all consumable parts (high-pressure seat) and lubricating with the manufacturer's specified grease — then connected it to tanks at varying pressures and measured the intermediate pressure output:
- 200 bar > 9.25 bar
- 170 bar > 8.8 bar
- 80 bar > 8.3 bar
- 50 bar > 8.2 bar
The change is clearly visible. And with typical rental equipment that has been used for a long time without proper servicing, the variance is even greater. Beyond the IP shift, there is also a limited airflow volume issue, as the internal orifice is roughly the size of a ballpoint pen tip. That is another critical factor — it means airflow at depth is not as smooth and effortless as it should be. For these reasons, a piston first stage will never make it onto my shopping list.
How do you identify a piston first stage? The simplest way is to look at the shape: if it has one long, protruding stem, that's it.
Regulator Series No. 4 – Where Can You Find a Regulator with Stable Intermediate Pressure?
In the previous installment, we covered how piston first stages suffer from intermediate pressure that drops as tank pressure falls. Many divers then ask: are there no regulators out there with stable IP? Of course there are. There are also some that claim to be "balanced" but whose IP still varies — but since I'm telling the truth here, the manufacturers are probably already annoyed with me, so don't bother asking me which ones.
Today I tested the Scubapro MK25 (balanced piston) and the Apeks XTX40 (diaphragm-type) first stages.
Results are as follows (digital pressure gauge resolution: 0.05 bar):
MK25 — one of the rare balanced piston designs with stable IP output:
- 200 bar → 9.55 bar
- 80 bar → 9.5 bar
- 50 bar → 9.5 bar
- 30 bar → 9.5 bar
XTX40:
- 200 bar → 9.1 bar
- 80 bar → 9.15 bar
- 50 bar → 9.15 bar
- 30 bar → 9.2 bar
Different manufacturers each have their own clever approaches to stabilizing IP output — even some piston designs achieve stable intermediate pressure (Sherwood being one example). But IP isn't the only factor affecting performance; airflow volume is equally important. At the very least, when buying a regulator you should know exactly what grade and price tier you're getting, rather than just grabbing whatever looks affordable.
Don't forget your annual regulator service — find a trustworthy shop and dive with greater peace of mind.
Bonus Section – Hearsay Is Often Wrong
First: always rinse your regulator in fresh water. Stop listening to the myth that you can't soak it. As long as the dust cap on the first stage inlet is properly secured, water will not get in. Think about it — the inside of a first stage is pressurized to 9 bar or more at all times, which is equivalent to a depth of 80 m. What's a few tens of centimetres of water in a rinse bucket going to do to that?
Second: don't neglect your octopus / alternate second stage. In most dives, the octopus never gets used — meaning its internal mechanism never gets exercised. That makes it even more prone to seizing up with salt deposits. Give it a press every now and then! If you have access to a scuba tank / cylinder, you can even submerge it in fresh water and purge it, then attach the tank and press the purge again to blow out any remaining water with air.
Third: whenever the first stage regulator is removed from the tank, always point the inlet port downward. This is the moment when seawater most easily enters the regulator — seawater on the body of the unit, seawater on your hands, all of it can drip straight into the inlet at that moment. Most regulator metals are chrome-plated brass; once the protective chrome layer is rusted through or peeled away, the damage is irreversible. Consistent, proper rinsing is always the best form of maintenance.
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