Have you ever wondered what’s the purpose of that small lever (and knob) with plus and minus signs on your scuba regulator?
What happens if you turn it all the way towards the plus or minus signs? Does it improve your air consumption?
The small lever on scuba diving regulators is called a Venturi lever, whereas the rotator knob is called a spring adjustment knob and they both fine tune the way in which air or more generally gasses flow through your regulator.
Both Venturi valves and spring adjustment knob can be found in entry level as well as in more expensive, top of the line regulators.
To understand how a Venturi Lever works in scuba regulators (second stages) and how to use it, let us quickly review how a second stage works.
1. Functioning of a Scuba Diving Second Stage
The purpose of a second stage is to transfer highly pressurized air or gas from the first stage and provide it to a diver at ambient pressure.
More broadly a scuba second stage reduces air or gas pressure while a diver inhales, so that it becomes breathable.
Airflow inside a second stage is regulated by a diaphragm that flexes inwards when you inhale, and flexes outward when you exhale.
Inward flexing allows a downstream valve to release gas, whereas outward flexing prevents gas from re-entering in the regulator and hose.
When you exhale through a scuba regulator, gas is in fact released in the outer environment through exhaust valves.
2. Functioning of the Venturi Valve in a Scuba Regulator
The purpose of a Venturi lever in a scuba diving second stage is to assist the flexing of the diaphragm inside the scuba regulator and ease breathing when you inhale.
In fact, whenever the Venturi lever is on (tilted towards the plus sign), the diaphragm will flex more easily when you are inhaling.
On the other hand, when the Venturi lever is off (or tilted towards the minus sign) the diaphragm will not be mechanically assisted while inhaling.
Note that I did not explicitly say that breathing is “less easy” when the Venturi lever is off.
This is because breathing under water should always be effortless.
3. Where should the Venturi Lever be positioned at each stage of a dive?
Although in most regulators the positioning of the Venturi Lever will not make much of a difference while breathing, it could actually make a difference on gas coming out of a regulator when you are not inhaling.
Given that the activated Venturi lever will actually exert some pressure on the second stage’s diaphragm irrespectively of whether you are inhaling or not, it could actually ease gas free flowing from the second stage.
This is why I usually have the Venturi lever switched to off in my octopus (before, during and after a dive), and only switch it on in my primary second stage as soon as I reach one meter below the surface.
This allows to reduce the risk of regulators free flowing at surface and reduce the risk of my octopus free flowing throughout the dive.
Sometimes I also forget to switch the Venturi lever on throughout a dive and have personally not noticed any difference in breathing until now.
4. Functioning of Spring Adjustment Knob in a Scuba Regulator
The spring adjustment knob, rotator knob, or cracking pressure adjustment knob, has a similar function to the Venturi lever.
The main difference is that the Venturi lever moves a rotating vane shifting air direction inside the second stage, whereas the spring adjustment knob mechanically changes the tension on the bias spring to increase or decrease ease of breathing.
As for the venturi lever, I always prefer to have the spring knob off or fully rotated in the minus direction on my octopus.
When it comes to the primary second stage I usually have it slightly rotated towards the plus sign (one full turn).
Contrary to the Venturi Lever, I do notice a slight difference in the ease of breathing when my adjustment knob is on.
This is particularly the case in more challenging dives that are either deeper (below 35 meters) or longer.
5. Does setting the Venturi valve and spring adjustment knob improve air consumption?
Breathing below surface level should be an effortless exercise, irrespectively of how you set up the Venturi valve and the spring adjustment knob.
As we have seen, minor adjustments of both mechanisms can lead to slight improvements in the way gas flows through the second stage.
This however will not allow you to improve gas consumption in general.
And that is why, particularly for newly certified diver that are looking to buy a new scuba diving regulator, I usually suggest not to care too much about a venturi lever and rotator knob.
Despite those mechanisms are quite simple, they add complexity to the overall design of a regulator, resulting in slightly higher prices.
Scuba diving regulators with Venturi Valves and Rotator knobs could also be somewhat more expensive to service.
6. Summing Up
A Venturi Lever and a rotator knob in a scuba diving regulator have a similar function. They both help finetuning the way in which gas flows through your scuba regulator while you inhale.
Although the most important element of every scuba diving regulator is that it should make breathing effortless, venturi valves and rotator knobs can bake breathing even easier.
This is particularly the case in more advanced dives.
Venturi valves and rotator knobs are not a “must have” element in scuba regulators.
In fact, many scuba diving regulators do not even have such elements and work perfectly fine.
If you are looking to buy a scuba diving regulator and are undecided on whether it should have a Venturi Lever and / or a rotator knob, you should also take into account servicing costs.
Most scuba regulators need to be serviced on an annual basis or after a given number of dives. You should take into account that the more complex your regulator, the higher the annual servicing costs.