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  • Tony Land

Connection types explained



There are many different connection styles used within the rebreather industry. KISS Rebreathers offers a number of different choices to suit diving styles, existing equipment configurations and personal taste. Many times the choices can be confusing and not well explained. The purpose of this article is to illustrate the different choices, review the pros and cons, and help explain why you might want to choose one style over another.


KISS Rebreathers offers several different computer and monitoring choices on all rebreathers. Mainly the Shearwater Petrel or Shearwater NERD is used for PO2 monitoring, although on request, other brands and models may be configured. For any of these devices, you have the following options of connection method.


These options are:

  • Fischer connection

  • Hardwired

  • 4-pin connection, also known as Subconn or wet-mate (AK Industries)

Edit: After I wrote this article, it was pointed out to me the manufacturer for the 4 and 5 pin connections was AK Industries, and not Subconn. We at KISS had referred to them by the name Subfconn. A quick check revealed that some manufacturers use the Subconn-made plugs and connections, while Shearwater research, who's connectors KISS Rebreathers uses, are made by AK Industries. The 2 connections are similar if not identical in design.


Fischer


The Fischer plug refers to the series 103 connection made by Fischer Connectors of Atlanta Georgia. It offers convenience, yet is the most unreliable and hard to maintain connection of all the choices offered.

A male Fischer plug
A male Fischer plug

rebreather fischer cable.
A 9-inch Fischer cable with sealing gland and molex connections.

A 28-inch Fischer cable.


To begin, the Fischer connection is not rated by the manufacturer for underwater use. It is rated for ‘splash resistance’ or ‘moisture resistance.’ The connection’s origins date back to the early 2000’s when these were used with Jurgensen Marine products, and the VR3 computers from Delta-P. As companies like Shearwater Research, and Liquivision began, they continued the use of the Fischer connection as a means of convenience. If the computer was a plug-and-play swap, it was an easier sell to a prospective customer, than something which required a new cable installation.


A Shearwater Petrel EXT with a Fischer cable.
A Shearwater Petrel EXT with a Fischer cable.

The biggest selling point of the Fischer cable is it’s convenience. It allows the user to disconnect the computer from the rebreather easily, which is one of the reasons it remains so popular. Unlike the 4-pin plug, which terminates on a two foot length of wire, the Fischer plug is directly on the side of the dive computer. This allows the dive computer to be removed, and used for open-circuit diving as well. A good example of use would be by dive instructor Drew McArthur. Drew is a rebreather instructor at Divetech Grand Cayman. There are days where he will conduct a rebreather instruction dive, and then an open circuit dive on the same day. His Fischer equipped computer allows him to use the same computer on all dives, and more accurate tissue loading calculations.


The biggest downside is the reliability. The Fischer cable has an o-ring which is not user replaceable. Those who are very careful with their Fischer connection, and maintain it well can enjoy years of trouble free use. However good maintenance does require excellent attention to detail, and even inadvertent accidents can render the cable or the plug unusable.


Fischer cables are available in 28 inch lengths for use with a wrist mounted computer, or 9-inch lengths for use with a NERD or NERD2. Custom length cables are available upon request.


Pros:

  • Easily detachable

  • Least expensive option

  • Good market for pre-owned Fischer computers

Cons:

  • Reliability

  • Maintenance





Hardwired


Hardwired devices are what they sound like - the dive computer or monitoring device is semi-permanently hardwired to the rebreather head. We say semi-permanently, as the cable may be removed from the head using a 15mm hex wrench, but it’s not as quick and easy as having a unit with a plug.

Hardwired Shearwater Petrel
A hardwired Shearwater Petrel showing the sealing gland and 3 molex connections. Note this cannot be disconnected without tools.

Rebreather Heads Up Display.
A hardwired Heads-Up-Display showing sealing gland and molex connections.

The main benefit in having a hardwired device is the increased reliability. There is no connection to become inadvertently disconnected, or to break.


Hardwired does have a downside however, and that is something we call corrosion creep. Saltwater humidity, saliva residue, caustic material all can contribute to premature corrosion of the wires that connect to the rebreather’s oxygen sensors. Over time, this corrosion can ‘creep’ up the length of the cable. While the cable Shearwater uses is flow-stop, meaning water cannot traverse the length of the cable, slow moving corrosion can work it’s way up the wires. This corrosion makes repair of the cables extraordinarily difficult and expensive. Often the cable needs to be completely replaced by a service center.


Corrosion creep can be mitigated by careful and proper rebreather maintenance, however it can still happen. This is why other manufacturers like Dive Rite, and Sub Gravity have made their sensor wiring harnesses a user-replaceable component, and recommend the user change them on a regular interval.


Pros:

  • Reliable

  • No connection to fail

Cons:

  • Cannot be disconnected

  • Corrosion can seep up the cables


4-pin connectors


Subconn 4-pin male and female connections.
4-pin male and female connections.

These are the most reliable of the options. They are known by a variety of names, including 4-pin, 5-pin, Subconn, or wet-mate connections.


Subconn is a manufacturer of these-style connectors, which have been used widely in the ROV industry for decades. In addition, they have been widely used by a variety of rebreather manufacturers for the past 10 years. While some manufacturers like Innerspace Systems Corporation still use the Subconn manufactured connections, Shearwater Research who's connections are on KISS rebreathers, use the AK Industries line of diving connectors.


4-pin connections allow your handset or HUD to be disconnected, and stored in hand-luggage, (or even used as a stand-alone computer, if you don't mind a length of cable tied to your arm.)


4-pin connectors also mitigate the ‘corrosion creep’ issue we spoke about with hardwired connections. While the same issue may occur in the head wiring, the corrosion will be limited to the length of cable that connects to the rebreather head only, and cannot affect the remainder of the wiring connected to the monitoring device. If corrosion creep occurs on the 4-pin pigtail, this can be removed with simple hand tools, and replaced with a spare. This will be a much less expensive repair, than having to send your entire device off somewhere, and it also allows for field repairs on those big trips to Galapagos, Truk or Bikini.


A Shearwater Petrel with a 4-pin Subconn connection.
A Shearwater Petrel with a 4-pin connection. Note how the plug is 28 inches down the length of wire, unlike the Fischer plug, which is tangent to the computer.
A Shearwater NERD with a 4-pin Subconn connection.
A Shearwater NERD with a 4-pin connection.

It’s important to note that Shearwater sells Petrels and NERD’s with both 4-pin and 5-pin connections. These connectors are obviously incompatible with each other. In addition to not being able to physically mate up, their data transmission protocols are different. 5-pin plugs are used with digital data transmission, which is commonly known as CanBUS or DiveCAN systems. CanBUS is a digital communication protocol that was developed in the 1980's for the automobile industry. The protocol was adapted for use in the diving industry and renamed DiveCAN by Shearwater Research. DiveCAN is widely used on rebreathers such as the rEvo, Optima, Prism 2, and many others.


The 4-pin cables are used for analog data transmission which is used on KISS rebreathers. We make a point to mention this, as occasionally we see 5-pin devices on the used market, and these devices will not work with KISS systems.

Subconn 5-pin and 4-pin male connectors.
A digital rendering of AK Industries connections. The 5-pin is on the left, and the 4-pin on the right. KISS rebreathers uses the 4-pin style.

Pros:

  • Most reliable

  • Can be disconnected for secure storage and travel

Cons:

  • Most expensive option

  • Cable between computer and plug limits the computer’s use in open-circuit diving




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