"Considerations in the Selection of Underwater Connectors" Every underwater device which uses, produces, or transmits electrical power or signal needs a reliable method of electrical transmission. Typically that means a cable and a connector. The connector is so often the "weak link" that the choice of connector is a critical element in designing a reliable underwater electrical system. The most important part of a car is what's under the hood, not the body paint. Similarly, the most important parts of the connector are the internal contacts (pins and sockets). All other parts of the connector serve only to properly mate a pin and a socket. Therefore, connector selection is really the choice of a connector that will properly mate the pins and the sockets that you need, under the conditions that are present. Top Considerations-Contacts The main factors to consider in connector selection are the contact details. The voltage determines the distance necessary between contacts (i.e. from one pin to another and from a pin to the shell); so first verify that the connectors under consideration will handle your peak voltage. (Note: peak VAC values are well above the RMS values usually given for AC power, so ensure that the connectors will handle the peak values.) The electrical current (amperage) determines the size of the conductors. Check that the contacts and cable conductors are fully adequate to carry the current. A good practice is to choose conductors that the maximum current to be carried is no more than 80% of the capacity of the conductor. Inadequate pin engagement is an important but little-known problem. Simply stated, the pin must engage the socket deep enough to provide adequate current-carrying capacity. The rounded tip of the pin is electrically non-functional, and the larger the pin, the larger the end radius, so larger pins need to go deeper into the socket because they lose more linear engagement distance to their larger tip radius. Be cautious with connectors which need a longer body for a larger pin--this may be a sign of inadequate pin engagement. Gold plating thickness is another detail. Most connector makers state that their contacts are plated "per MIL-G-45204", but this specification covers a wide range of plating thickness, from a heavy, thick plate down to a "flash" plating only a few microns thick. For applications requiring good telemetry with a minimum of noise, or if a longer system life is desired, ask for a thicker gold plate. Class 1 (50 microns thick) is good for most applications. Top Considerations-Connector Bodies Obviously, even when the contacts are right for the voltage and current, have adequate engagement and are properly plated, the connector body is critical to the connector's ability to reliably mate those contacts. Connectors with low pin densities and large pins (if molded in a flexible material) usually don't need a positive indexing method. These connectors typically rely on a larger pin (or a non-conductive polarizing pin) for indexing. While not ideal, this method is adequate for many applications. However, for high pin densities and/or small (<14 AWG) pins, a positive indexing method is required. This typically is achieved by a key/keyway system, often in conjunction with a coupling ring (engaging nut) that draws the connectors together and apart. This can be an excellent system, providing that several important design features are present. Proper engagement sequence is the most important feature to look for. This simply means that the shell's key must engage the mating keyway before the coupling ring threads engage, and that the coupling ring engages before the pins enter the sockets. If this sequence is not maintained, severe damage to the connector can result. Try this simple test on a sample connector: purposely mismate the two halves, and see if you can engage the coupling ring threads. If you can, so can your customer, and therein lies potential for serious problems. While you're doing it, check for "scoop", the ability of one connector to impact and damage the pins in the other. Top The material and size of the key are also important. Rubber keys are inferior to keys made of metal or hard plastic because they can bend (thereby allowing for misalignment) and can wear over time. And when it comes to keys, size really does matter. Big is too big; keys that extend more than about 30 degrees of arc do a less effective job of indexing, so avoid those "monster" keys! Some connector manufacturers reduce their manufacturing costs and avoid the use of costly o-rings in connector shells by molding the contacts directly into the shell, usually in a rubber material. This method depends on a face-to-face seal between rubber surfaces, so great care must be taken not to overtighten the connector pair; overtightening may cause leakage and possible internal connector damage. If you consider use of this type of connector, make sure that it has a positive stop, ideally provided by metal faces. Pin density will be reduced: due to the flexibility to the molding material, larger pins (and more inter-pin space) are required. Top Considerations-Environment/Application Even when the contacts are right and the connector has good design features, the connector must be appropriate for the intended use and environment. The connector materials must be able to withstand the environment conditions without degradation. For example, extended exposure to sunlight (ultraviolet energy) will cause damage to neoprene, and many steels will corrode in seawater. So check that the connector will fully withstand the environment. The connector must not adversely affect the application, either. For example, all ferrous materials (steel, etc) should be avoided in cases where the connector's magnetic signature might affect the system. In extreme cases, even the nickel used under gold plating could have an effect and should be reviewed. The physical size of the connector, its weight, ease of use (and appropriateness for the application), durability, submergence (depth) rating, field-repair-ability, etc. should all be assessed. You might also consider oil-filled cables or connectors. Ease of installation and use is especially important, so realistically appraise the technical ability of those personnel who will actually install or use the connector. If their skills are lower, a more "user-friendly" connector may be a better choice. And, if possible, train operators in the basics of proper connector use: use only a little lubricant (use of too much grease, a wide-spread problem, interferes with sealing); avoid overtightening, note acceptable cable bending radii; provide grounding wires for steel connectors in aluminum bulkheads, etc. Top Considerations-Commercial When the contacts are right, proper design features are all present, and the connector is a good choice for your environment and application, then it's time to look at commercial factors. Price is an important element and should be analyzed carefully. However, all of the cost factors need to be considered; price is only one part of overall cost. For example, if the connector is a "consumable" (if, say, it's part of a throw-away sensor, long-term durability is not important. However, if it is part of a system that forms a durable capital investment (e.g. a workhorse ROV or a 3-chip camera), durability and ease to use are critical elements. Product or system downtime is a cost factor, too. If a ROV and support ship (and their crews) are idled at sea because of a connector failure on your product, any price savings that you realized on that connector will be quickly forgotten! Remember: if you're supplying a product, the connector's part of that product--your customer's connector problems reflect badly on you, not on the connector maker. Your best strategy is to provide the optimum connector for your customer's needs, not just the cheapest one available. That strategy will upgrade your product, satisfy your customer, and, in the long run, improve the overall marketplace. Top