Category Archives: Dive Training

NAUI Intro To Tech with H-valves

The NAUI Intro To Tech course (Introduction to Technical Diving) is a skills and equipment focused course.

The skills that divers learn are invaluable to all diving, not just technical diving, but are necessary for progression into overhead and decompression environments where precision finning and buoyancy control are essential. Divers learn propulsion techniques with efficient finning, how to kick backwards, and turn in place.

Divers also learn the standard streamlined equipment configuration used in technical diving, and the importance of consistency and redundancy. The NAUI Technical Equipment Configuration (NTEC) is built from the ground up so that large equipment and equipment-dependent skill changes are not necessary are the diver progresses through technical diving.

All of the skills learned in NAUI Intro To Tech will serve a diver well in all aspects of diving even if they do not progress to further technical diving.

An H-valve is a special valve connected to a cylinder to allow two independent first-stage regulators to be connected, mimicking the redundancy of a double-cylinder configuration.

Many divers would like to learn the skills from a NAUI Intro to Tech course, but do not want to progress to using a double-cylinder configuration with a necessary drysuit. When using a single-cylinder with a H-valve, the diver can continue to use a wetsuit and learn all of the diving and most of the equipment skills.

NTEC ConfigurationDouble cylinders - steelDouble cylinders - aluminiumSingle cylinder
ValvesManifold doublesManifold doublesH-valve
Exposure suitDrysuit onlyDrysuit or wetsuitDrysuit or wetsuit

Other required equipment in the NTEC configuration for NAUI Intro To Tech is described in this post. Pro Scuba Dive Center has technical gear available to rent.

An assembled NTEC rig with a single cylinder and H-valve.

If you are interested in taking a NAUI Intro To Tech course please contact or Pro Scuba Dive Center.

Gas Consumption Calculations

Have you ever wondered how to compare your breathing efficiency? You cannot directly compare any two dives you have done, or two divers on the same dive profile using different cylinders. The dives are likely to be at different depths so the volume of each breath is different (Boyles law), and different cylinders have different capacities and rated pressures.

As a common reference point we convert our measurements to the surface, and imagine all the air has been decompressed from the cylinder into a large volume. That is why we call it Surface Air Consumption rate (SAC_{rate}).

To find our physiological rate we need to account for varying depth, and measure independently of cylinder size. We use the Respiratory Minute Volume (RMV_{rate}), that is the amount of gas that we breathe per minute at the surface. Taking 10 breaths at \mathrm{66\ [ft]} (\mathrm{3 [ata]}) would be the same as taking 30 identical breaths at the surface (\mathrm{1 [ata]} at sea level). Another problem is that we measure pressure, but this needs to be converted to an actual volume of air.

SACrateSurface Air Consumption rate [psi/min]The equivalent rate that the cylinder is consumed at the surface without depth pressure.
RMVrateRespiratory Minute Volume rate [ft3/min]The equivalent volume of gas in cubic feet, consumed at the surface.
PratePressure rate [psi//min]The rate of gas consumption at depth, as measured by a pressure gauge.
DfactorDepth factor [ata]How much consumption is increased by due to depth pressure.
TfactorTank factor [ft3/psi]A tank-specific number to convert tank pressure [psi] to gas volume in cubic feet [ft3].
VcylinderTank volume [ft3]The volume of gas in the cylinder

The equations we need are:

(1)   \begin{equation*}  % use "\text" in an equation* environment and "\mathrm" outside SAC_{rate} = \dfrac{ P_{rate} }{ D_{factor} } \hspace\textup{[psi/min]} \end{equation*}

(SAC_{rate} is specific to each cylinder, probably between \mathrm{20\mbox{--}50\ [psi/min]} )

(2)   \begin{equation*}  RMV_{rate} = SAC_{rate} * T_{factor} \hspace\textup{[ft^3/min]} \end{equation*}

RMV_{rate}  is our actual physiological consumption rate, approximately \mathrm{0.5\mbox{--}1.0\ [ft^3/min]} )

We start with a pressure rate (P_{rate}), say consuming \mathrm{2000\ [psi]} at an average depth of \mathrm{50\ [ft]} for \mathrm{30\ [min]}. Using an average accounts for the varying depth of most dives, we don’t usually dive true square profiles. We can use max depth in lieu of the average depth, but the calculated SAC_{rate} or RMV_{rate} will be higher. Fancy computers can provide average depth, but the Subgear XP10 rental computer does not.

    \[ \begin{split} P_{rate} &= \dfrac{ \left(  P_{start}\hspace\mathrm{[psi]} - P_{end}\hspace\mathrm{[psi]} \right) }{ T_{dive}\hspace\mathrm{[min]} } \\ &= \mathrm{2000\ [psi]} / \mathrm{30\ [min]} \\ &= \mathrm{2000\ [psi]} /  \mathrm{30\ [min]} \\ &= \mathrm{66.66\ [psi/min]} \end{split} \]

Our depth factor (D_{factor}) accounts for depth, (using average if available) by scaling our measurement by the total pressure at depth plus the air pressure, in atmospheres-absolute \mathrm{[ata]}. The depth factor indicates how much additional ambient pressure there is from depth. \mathrm{33\ [ft]} of sea water is equivalent to \mathrm{1\ [atm]} of gauge pressure. Assuming that our computer tells us the average depth was \mathrm{50\ [ft]},

    \[ \begin{split} D_{factor} &= \left(\dfrac{depth_{average}}{\mathrm{33\ [ft/atm]}}\right) + \mathrm{1\ [atm]} \\ D_{factor} &= \left(\dfrac{\mathrm{50\ [ft]}}{\mathrm{33\ [ft/atm]}}\right) + \mathrm{1\ [atm]} \\ &= \mathrm{2.515\ [ata]} \end{split} \]

We can now calculate the equivalent consumption rate of the cylinder at the surface in \mathrm{[psi/min]}:

    \[ \begin{split} SAC_{rate} &= \dfrac{ P_{rate} }{ D_{factor} }\\ &=  \mathrm{66.66\ [psi/min]} / \mathrm{2.515\ [ata]} \\ &= \mathrm{26.5\ [psi/min]} \end{split} \]

SAC_{rate} is only valid for comparing identical tanks, so we want to remove the influence of the cylinder and work out the volume of gas consumed. We want to know volume, so we use a tank factor (T_{factor}) to convert from pressure to volume. We know the rated pressure of the cylinder when it is full, at Pro Scuba Dive Center the Worthington High Pressure steel cylinders are pressurized to \mathrm{3442\ [psi]} when full with \mathrm{80\ [ft^3]}.

    \[ \begin{split} T_{factor} &= \dfrac{ Volume_{cylinder}  \mathrm{[ft^3]} }{ Pressure_{cylinder} \mathrm{[psi]}  } \\ &= \mathrm{80\ [ft^3]} / \mathrm{3442\ [psi]} \\ &= \mathrm{0.02324\ [ft^3/psi]} \end{split} \]

We can now convert the rate we are consuming cylinder pressure \mathrm{[psi/min]} to the rate we are consuming gas volume \mathrm{[ft^3/min]} using the Tank Factor T_{factor}:

    \[ \begin{split} RMV_{rate} &= SAC_{rate} * T_{factor} \\ &= \mathrm{26.5\ [psi/min]} * \mathrm{0.02324\ [ft^3/psi]} \\ &= \mathrm{0.62\ [ft^3/min]} \end{split} \]

You can use RMV_{rate} to calculate how long you expect any sized cylinder to last at any planned depth. For example, a low pressure \mathrm{3000\ [psi]} aluminium \mathrm{63\ [ft^3]} cylinder at a planned depth of \mathrm{50\ [ft]} with a RMV_{rate} of \mathrm{0.5\ [ft^3/min]}:

    \[ \begin{split} Duration &= \dfrac{ V_{cylinder} / RMV_{rate} }{ D_{factor} }  \\ &= \dfrac{ \mathrm{63\ [ft^3]} / \mathrm{0.5\ [ft^3/min]} }{ \left( \dfrac{\mathrm{50\ [ft]} }{ \mathrm{33\ [ft/ata]} } \right) +\mathrm{1\ [ata]} \right) }  \\ &= \mathrm{50.1\ [min]} \end{split} \]

(The cylinder is fully consumed with no reserve gas.)

We will practice calculating RMV_{rate} after every scuba dive when we log our dives. You can use the above equation, or an online calculator, or an app (iPhone or Android), or a slide wheel, or an abacus …

SAC_{rate}  is often confused with Respiratory Minute Volume (RMV_{rate}), they are not the same thing. RMV_{rate} is more useful for gas planning because it applies to any sized tank

Some things to be aware of are:

  • You can only use RMV_{rate} to compare two dives of similar workload. If you use RMV_{rate} to plan how long you expect a cylinder to last you at a given depth, you need to be conservative to account for possibly swimming harder, more currents, being cold (consumes more oxygen), anxiety of an unfamiliar dive driving faster breathing, etc.
  • You cannot use the above equation for metric tanks, they are measured differently and the tank volume does not mean the same thing. It is not sufficient to convert \mathrm{[psi]} to \mathrm{[bar]} and \mathrm{[ft^3]} to \mathrm{[liter]}s.
    For the sake of comparison \mathrm{0.5\ [ft^3/min]} is an example of a low RMV_{rate} and \mathrm{1.0\ [ft3/min]} is a high RMV_{rate}.
  • Smaller people often require less air (oxygen) to sustain their smaller bodies under the same conditions.
  • Performing drills and exercises in a class is not a good indicator of true RMV_{rate} because a lot of gas is vented by regularly inflating and deflating the BC, not to mention free-flows 🙂 You will get a more accurate measurement from the fun dives on the final day of class.
  • Remember that SAC_{rate} and RMV_{rate} are equivalent surface pressure values, at \mathrm{99\ [ft]} you will consume gas 4 times faster!


Calculate the \boldsymbol{SAC_{rate}} and \boldsymbol{RMV_{rate}} for a training dive, avg depth \mathrm{20\ [ft]} for \mathrm{30\ [min]}. The diver is using a Worthington HP steel cylinder \mathrm{80\ [ft^3]} at \mathrm{3442\ [psi]}. The diver begins with a full cylinder and ends with \mathrm{1800\ [psi]}.

    \[ \begin{split} P_{rate} &= \dfrac{ \left(  P_{start}\hspace\mathrm{[psi]} - P_{end}\hspace\mathrm{[psi]} \right) }{ T_{dive}\hspace\mathrm{[min]} } \\ P_{rate} &= \dfrac{ \left( \mathrm{3442\ [psi]} - \mathrm{1800\ [psi]} \right) }{\mathrm{20\ [min]} } \\ &= \mathrm{1642\ [psi]} / \mathrm{30\ [min]} \\ &= \mathrm{54.73\ [psi/min]} \end{split} \]

    \[ \begin{split} D_{factor} &= \left(\dfrac{depth_{average}}{\mathrm{33\ [ft/atm]}}\right) + \mathrm{1\ [atm]} \\ D_{factor} &= \left(\dfrac{\mathrm{20\ [ft]}}{\mathrm{33\ [ft/atm]}}\right) + \mathrm{1\ [atm]} \\ &= \mathrm{1.606\ [ata]} \end{split} \]

    \[ \begin{split} T_{factor} &= \dfrac{ Volume_{cylinder}  \mathrm{[ft^3]} }{ Pressure_{cylinder} \mathrm{[psi]}  } \\ &= \mathrm{80\ [ft^3]} / \mathrm{3442\ [psi]} \\ &= \mathrm{0.02324\ [ft^3/psi]} \end{split} \]

    \[ \begin{split} \boldsymbol{SAC_{rate}} &= \dfrac{ P_{rate} }{ D_{factor} }\\ &=  \mathrm{54.73\ [psi/min]} / \mathrm{1.606\ [ata]} \\ &= \boldsymbol{\mathrm{34.07\ [psi/min]}} \end{split} \]

    \[ \begin{split} \boldsymbol{RMV_{rate}} &= SAC_{rate} * T_{factor} \\ &= \mathrm{34.07\ [psi/min]} * \mathrm{0.02324\ [ft^3/psi]} \\ &= \boldsymbol{\mathrm{0.791\ [ft^3/min]}} \end{split} \]


NAUI Mobile App

The NAUI Mobile app has been released on the Apple App Store and Google Play.

NAUI Mobile presents digital certification cards for all online eLearning courses completed on the NAUI CORE platform since the launch of NAUI CORE in April 2018. Certifications registered on the legacy platforms are listed in the app, but do not include the digital certification card product.

Users can see all of their NAUI certifications and use the free nitrox calculator tools.

Users must have a free NAUI CORE account to use the app, available to everyone. Digital certification cards to replace existing plastic cards are available for a limited time for a special of $5, until April 30th, 2019.

Click here for more information.

Monterey Bay Dive Site Locations

Click on any of the addresses to open the location on Google Maps.

Quick Reference

More InformationGoogle Maps
Pro Scuba Dive Center4637 A Scotts Valley Dr, Scotts Valley, CA 95066
San Lorenzo Valley High School PoolHwy 9, Felton, CA 95018
Monterey State Beach (Del Monte Beach)Del Monte Ave and Camino El Estero, Monterey, CA 93940
San Carlos BeachCannery Row and Reeside Ave, Monterey, CA 93940
K-Dock, MontereyFigueroa St and Del Monte Ave, Monterey, CA 93940
Lovers Point17th St and Ocean View Blvd, Pacific Grove, CA 93950
Cowell BeachBeach St and Pacific Ave at Municipal Wharf, Santa Cruz, CA 95060

Pro Scuba Dive Center

4637 A Scotts Valley Dr, Scotts Valley, CA 95066

Nearest landmarks are Santa Cruz County Bank, and Mint Bar.

  1. Store is hard to see from the road, entrance is next to Palo Alto Medical Foundation, and across from Santa Cruz County Bank.
  2. Classroom is located at the rear of the shop next to the cleaning station and rinse tubs.

San Lorenzo Valley High School Pool

Hwy 9, Felton, CA 95018

Nearest landmarks are Castelli’s Deli-Cafe.

Pool is located at the rear, travel clockwise in the parking lot on the perimeter.

  1. Enter from Highway 9 at the traffic lights
  2. Turn left and drive around the school clockwise through the parking lots past the baseball fields.
  3. Continue until near the end of the parking lot, park near the shipping container.
  4. Assemble your gear in the south-east corner of the pool, there is a back gate to enter the pool.
  5. Assemble and test your gear.
  6. Please keep gear on the concrete because chlorine kills the grass.

Monterey State Beach (Del Monte Beach)

Del Monte Ave and Camino El Estero, Monterey, CA 93940

Nearest landmarks are McDonalds and Monterey Bay Kayaks

  1. Opposite McDonalds on Del Monte Ave, and near Monterey Bay Kayaks.
  2. Entrance to the parking lot is very close to the traffic lights.
  3. Parking is metered and accepts credit cards or cash. Maximum cost is $10 per day.
  4. Assemble and test your gear.

San Carlos Beach

Cannery Row and Reeside Ave, Monterey, CA 93940

Nearest landmarks are Backscatter Underwater Video, and US Coast Guard.

  1. Parking is available at the top of the grassy area and on the breakwater.
  2. Traffic flow for the top lot is one way. Enter from Reeside Ave near Backscatter and Breakwater Scuba.
  3. Parking is metered and accepts credit cards or cash. Maximum cost is $10 per day.
  4. Assemble and test your gear.

K-Dock, Monterey

Figueroa St and Del Monte Ave, Monterey, CA 93940

Nearest landmarks are London Bridge Pub and Monterey Harbor Office.

  1. Parking is metered, passes are available from machines or Harbor Masters Office.
  2. Park close to the assembly area.
  3. Assemble and test your gear at the top of the boat ramp.
  4. Certification cards and a separate liability form are required to board the boat.

Lovers Point

17th St and Ocean View Blvd, Pacific Grove, CA 93950

Nearest landmarks are Beach House at Lover Point and Lovers Point Inn.

  1. Street parking is timed but not metered.
  2. Park close to the assembly area, a group of wooden tables and benches.

Cowell Beach

Beach St and Pacific Ave at Municipal Wharf, Santa Cruz, CA 95060

Nearest landmarks are Municipal Wharf and Aquarius Waterfront Eatery.

  1. Parking is metered and limited.
  2. Alternative parking is available on the street, or at Depot Park South Lot, with a walking path.
  3. Assemble and test your gear when you arrive.
  4. Keep valuables locked in your car.

Alternative Parking For Cowell Beach

107 Center St, Santa Cruz, CA 95060

Nearest landmarks are Depot Park Soccer Field and Homeless Garden Project Store..

  1. Drop gear at Cowell Beach in the main parking lot.
  2. Drive up Pacific Ave (later Center St) to the next roundabout.
  3. Secure car and check parking obligations.
  4. Return to Cowell Beach along the walking path.