Michael Let me start this diy-story by presenting a simple glass-to-metal pressure-tight coupling that can be used in this design. Actually, we will need two such couplings, i.e. one at each end of the view glass. You could either machine something (from scratch) similar to what you see in my sketch, or simply find a standard brass fitting suitable for this type of modification. Just remember, when this thing is assembled, the design should provide sufficient flexibility to minimize breakages of the view glass. The view glass could be, let say 5/16" OD [~8 mm]. However, we shall have to decide its overall length at a later stage. Oh, and the coupling should be bored (in the lathe), rather than drilled, because the diameter of the cavity can then be made to fit exactly the outside of the washer ring. A rectangular section ring should be cut from a length of rubber or neoprene tubing of such dimension that when fitted onto the view glass tubing in the position shown - and compressed by the washer and nut - it should form a preliminary seal. That is, any increase in water pressure inside the system will compress the elastomer further, so this is actually a self-tightening seal. A swelling (mushroom) could be made on each end of the view glass so that it would not pass through the backing washer, as this would normally prevent the tubing from being ejected under pressure. However, in this particular design the tubing will be firmly positioned between it's two couplings (i.e. with no chance of being ejected). Therefore, the swelling could very well be omitted. However, I would still recommend you dress the rough cut ends of the final view glass using a blow torch. Should you decide to go for swelling ends, then make sure that all washers and nuts are inserted correctly onto the glass tubing prior to swelling the final end of your view glass. If you forget this, you would simply have to start over again with a new view glass, because there will be no way to get those brass parts over any of your two perfectly made swellings. Sufficiently exact neoprene rings can be made by putting a suitable piece of neoprene (or rubber) tubing onto a proper sized mandrel. Such a mandrel should provide a snug fit with the tubing for best results, and the chuck of the lathe must be clamped onto a section of the mandrel which is covered by the tubing. Use a sharp knife resting on the tool holder while the headstock is rotating at slow speed. With a little practice, rings of reproducible dimensions are easily made. I believe this type of seal (and coupling) could be tested to at least 500 psi [35 bar], but don't you worry too much about such excessive water pressures. The inlet valve of a Papenberg depth gauge should be closed whenever you go deeper than it's maximum range, anyway. Finally, if you do not have access to a small lathe, then simply use a clear flexible plastic laboratory tube or hose in place of the view glass - and fasten this at both ends using proper fittings with hose clamps. Best regards, Jens Laland P.S.: to be continued...
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