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• Introduction
• Cable
• Connectors
• Search head
• Operations

Introduction

However, although the PI principle is waterproof, the CoverMaster is not ! To be more accurate, the search head and electronics are weather-proof and splash-proof (to IP54), but are not water-immersible.

In the applications that have been envisaged so far, the electronic unit would be kept on a boat, or on the jetty above the water-line, and only the search head would be need to be submerged.
Application Note no. 27 explains that the head lead may be extended in length up to about 30 metres if necessary, so that will not be a serious limit to diving depth.

The problem remains of how to keep the search head and lead dry.
Although the search head - and connector when mated - are splashproof, they will definitely allow water to enter if submerged. If water enters the head, the actual sense windings would not be affected because they are hermetically encapsulated in resin; but the water would migrate through the connector and start to 'wick' along the cable. It has been known for many years that water in the dielectric of the cable from a search head results in the generation of spurious signals in a PI instrument. These arise from the ionic polarisation and subsequent decaying depolarisation with each transmitted current pulse; and, unlike the ionic currents flowing in the water outside the head and magnetically coupled in, these potentials are directly coupled to the receiver circuitry and cannot be discriminated against. Furthermore, the (standard) head lead is sheathed in PVC, which although usually considered as waterproof, does in fact allow water to permeate in with prolonged immersion; normally, if a lead (but not its ends) is likely to be immersed in water, we recommend that the duration should be limited to about 20 minutes, and that deterioration may be expected after about 30 minutes. It has been our experience that after accidental water ingress, both the head and electronics can be dried out and regain full functionality; but that it is essentially impossible to dry out a flooded cable.

Those are the problems; what about some solutions?
It should be appreciated that any thorough or comprehensive solution will require the expertise of specialist marine/subsea engineers. Protovale do not profess to specialise in this field, but there is a high probability that a user with a marine application requirement will already have some contacts within that particular area; and since much of the necessary modifications will need to be done and tested on or near the actual site, such contacts should be utilised wherever possible.

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The results of investigating some subsea applications by discussion with the users involved, yield the following suggestions:

Cable:

A second suggestion is to use ordinary URM76 cable, and waterproof it by threading it through a flexible tube (hose-pipe perhaps?), ideally packed with silicone grease.

A third alternative is to prepare several lengths of ordinary URM76 PVC-sheathed cable, and to treat them as 'consumables' to be discarded and replaced each time one becomes unusable!

Connectors:

At the head end (both CM5 and CM52), the connector used on the standard head lead is a bayonet-locking plastic-bodied 3-pin male cable plug with pin-interface to DIN41524. We use and recommend the Binder series 678, part number 99-0605-00-03; or Amphenol type C91E number T3274-501 is a less-rugged alternative.
Neither of these types are water-immersible, and will therefore need to be sealed.

Search head:

Mount the head inside a hermetically-sealable (and non-metallic!) waterproof housing, with proper underwater cable entering via a pressure-tight gland and wired to a (possibly skeleton) three-pin plug to mate with the head. The housing could be packed with silicone grease or jelly to exclude air and reduce the mobility of any water which may accidentally enter.
Note that cover readings obtained would have to be corrected for the wall-thickness of the housing.

Use a sealed housing as above; but fitted with a true underwater connector instead of a cable gland. The underwater-grade cable would then be fitted with a mating underwater connector.

(For short-duration shallow-depth work) put the head and ordinary connector inside a polythene bag packed with grease, and seal the neck to the underwater cable (or hosepipe or disposable cable!) -- in this case, it may be wise to treat the search head as being a 'consumable' too!

Operations:

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