Mixed-gas diving requirements

Yes. A decompression chamber must be ready for use at the dive location for mixed‑gas diving as required by 1910.426(b).

• The standard is categorical: mixed‑gas diving shall be conducted only when a decompression chamber is ready for use at the dive location. See 1910.426(b).
• ‘‘Ready for use’’ implies the chamber and personnel/supplies necessary for decompression must be on site and able to be operated when needed (employers should document readiness in dive procedures). See 1910.426(a).

A bell must be used for dives deeper than 220 feet of seawater (fsw) or when the dive involves in‑water decompression time greater than 120 minutes, except where heavy gear is worn or the dive is in physically confining spaces, as specified in 1910.426(b)(1)(i).

• If either condition (depth >220 fsw or in‑water decompression >120 minutes) applies, plan for bell use unless one of the two specified exceptions applies.
• The exceptions (heavy gear or physically confining spaces) are explicitly in the standard; employers must document when an exception applies. See 1910.426(b)(1)(i).

A closed bell is required for dives deeper than 300 fsw, except when diving is conducted in physically confining spaces, according to 1910.426(b)(1)(ii).

• Plan for a closed bell for any mixed‑gas dive expected to exceed 300 fsw unless the dive site is a physically confining space that prevents its use.
• Employers should include the closed‑bell requirement and any documented exceptions in written dive procedures. See 1910.426(b)(1)(ii).

Yes. A separate dive team member shall tend each diver in the water as required by 1910.426(c)(1).

• This means one attendant per diver while that diver is in the water; a single tender cannot be assigned to watch multiple divers in the water at the same time.
• Employers should ensure staffing plans reflect one dedicated tender per in‑water diver and document assignments in dive operations procedures. See 1910.426(c)(1).

Yes. A standby diver shall be available while a diver is in the water, as required by 1910.426(c)(2).

• The standby diver must be prepared to enter the water immediately to assist in an emergency.
• Employers should include standby diver procedures and readiness checks in their dive safety program. See 1910.426(c)(2).

A diver must be stationed at the underwater point of entry when diving is conducted in enclosed or physically confining spaces, per 1910.426(c)(3).

• This requirement ensures someone is positioned at the entry point to assist or monitor divers in overhead or confined environments.
• Include the underwater point‑of‑entry attendant role and responsibilities in written dive plans for confined or enclosed space dives. See 1910.426(c)(3).

Each operation must have a primary breathing gas supply sufficient for the planned dive including decompression, and a dive‑location reserve breathing gas supply, as required by 1910.426(c)(4) and 1910.426(c)(5).

• Primary supply: sized to support divers for the entire planned bottom time plus scheduled decompression obligations.
• Reserve supply: an on‑site reserve at the dive location to cover unexpected needs, delays, or emergencies.
• Employers should calculate gas requirements for planned profiles and maintain documented gas‑management procedures. See 1910.426(c)(4) and 1910.426(c)(5).

When heavy gear is worn, the employer must provide an extra breathing gas hose available to the standby diver and an inwater stage for divers in the water, per 1910.426(c)(6)(i) and 1910.426(c)(6)(ii).

• Extra hose: must be capable of supplying breathing gas to the diver and be available to the standby diver for emergency deployment.
• Inwater stage: a platform or method to support divers underwater when heavy gear limits mobility or ascent.
• Employers should specify what constitutes heavy gear, how the extra hose is stored and deployed, and how the inwater stage will be used in written procedures. See 1910.426(c)(6).

An inwater stage must be provided for divers without access to a bell when dives are deeper than 100 fsw or are outside the no‑decompression limits, as required by 1910.426(c)(7).

• If a diver cannot use a bell and the planned profile exceeds 100 fsw or requires decompression, include an inwater stage in the dive plan.
• Ensure procedures cover staging, tending, emergency recovery, and communications related to the inwater stage. See 1910.426(c)(7).

When a closed bell is used, one dive team member in the bell shall be available and tend the diver in the water, as specified by 1910.426(c)(8).

• That bell‑based team member is explicitly required to be present in the bell and ready to tend the diver, in addition to other tenders topside as applicable.
• Employers should include the bell‑tending role and related emergency duties in written team assignments and procedures. See 1910.426(c)(8).

A diver‑carried reserve breathing gas supply must be provided for each diver except where heavy gear is worn or physical space does not permit; it is required for dives deeper than 100 fsw or when the diver is prevented by the dive area configuration from directly ascending to the surface, as stated in 1910.426(c)(9), 1910.426(c)(9)(i), and 1910.426(c)(9)(ii).

• Required when: diving deeper than 100 fsw or diving profiles that require decompression beyond no‑decompression limits, and when the diver cannot directly ascend due to the dive area's configuration.
• Exceptions: not required if heavy gear is worn or physical space prevents carrying a reserve; employers must document and justify exceptions in dive procedures. See 1910.426(c)(9).

Yes, but only where the standard explicitly permits it: a bell is not required for dives deeper than 220 fsw when heavy gear is worn or when diving in physically confining spaces, and a closed bell is not required for dives deeper than 300 fsw when diving is in physically confining spaces, as set out in 1910.426(b)(1)(i) and 1910.426(b)(1)(ii).

• These are narrow, specific exceptions in the standard; employers should not broadly rely on them without clear documentation.
• When invoking an exception, document why the exception applies and include alternate safety measures in dive procedures. See 1910.426(b)(1).

Yes. Employers engaged in mixed‑gas diving shall comply with the listed requirements unless the standard explicitly specifies otherwise, as stated in 1910.426(a).

• Compliance is mandatory for employers who perform mixed‑gas diving operations; where the standard provides an exception, that exception is explicit.
• Employers should incorporate all applicable provisions of 1910.426 into their written dive safety plans. See 1910.426(a).

The standard requires an extra breathing gas hose capable of supplying breathing gas to the diver in the water and that it be available to the standby diver when heavy gear is worn, per 1910.426(c)(6)(i).

• ‘‘Available to the standby diver’’ means the standby diver must have immediate access to the hose so they can rapidly provide gas to the in‑water diver in an emergency.
• Employers should document how the hose is stored, deployed, and tested to ensure readiness in dive procedures. See 1910.426(c)(6)(i).

The dive team member in the bell counts as the person tending the diver when a closed bell is used; 1910.426(c)(8) requires that one dive team member in the bell be available to tend the diver in the water.

• This requirement ensures direct tending from within the bell during bell operations.
• Employers should still consider additional topside tenders or support personnel as necessary based on the dive profile and other provisions of 1910.426. See 1910.426(c)(8).

No. The standard requires a separate dive team member to tend each diver in the water, so one team member may not simultaneously tend multiple in‑water divers, per 1910.426(c)(1).

• Operational planning must provide enough trained tenders so every in‑water diver has a dedicated attendant.
• Employers should include tender assignment practices in written dive procedures to ensure compliance with 1910.426(c)(1).

Employers may rely on the specific exceptions in the standard where equipment requirements (for example bell or closed bell use) are impracticable because the dive is in a physically confining space, but they must follow any alternative measures required or implied by the standard and document the situation, per 1910.426(b)(1)(i), 1910.426(b)(1)(ii), and 1910.426(c).

• The standard explicitly recognizes that bell or closed‑bell use may be excepted where physical space does not permit; employers should instead provide equivalent safety measures suitable for the confined environment.
• Document the confined‑space limitation, any compensating controls (extra tenders, reserve gas, inwater stage, emergency plans), and include them in written dive procedures. See 1910.426(b) and 1910.426(c).

It means environments that block a direct ascent—such as overhead obstructions, tunnels, wreck interiors, or other confined configurations—trigger the requirement for a diver‑carried reserve breathing gas supply unless an explicit exception applies, per 1910.426(c)(9)(ii).

• Practical examples include diving inside wrecks, caves, or under structures where a diver cannot make an immediate vertical ascent to the surface.
• In those situations, provide a diver‑carried reserve unless heavy gear or physical space makes that impossible; document the analysis and any alternative controls in dive procedures. See 1910.426(c)(9)(ii).