Sealing A10 vapour pressure bubble chambers

13 10 2010

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” ,  “AN5851-1 : jammed shades carrousel” ,  “A Byrd sextant restored” ,  “Update on Byrd Aircraft Sextant” and “A nautical sextant bubble horizon”

The popular WW II A10 bubble sextant, in common with nearly every bubble sextant of the period, was prone to leakage of fluid from the bubble units. The fluid was xylene and the seals between the bubble chamber and the top and bottom glasses were lead washers sitting in  grooves of semi-circular section

Early units were filled through a tapered hole below the Lucite light pipe, the hole being closed with a brass taper pin, another potential source of leakage, that was cut off flush with the outer surface of the chamber, making it not only nearly invisible, but also very troublesome (though not impossible) to remove and replace.

Recently, I was asked to refill three A10 bubble units for a new friend and when I received them I found that they had a modified filling system that makes refilling them somewhat easier (Figure 1). A tapped hole has been made nearly all the way through the front wall, terminating in a smaller hole at the entry into the chamber. The hole is closed by a screw, the end of which squeezes a lead ball into the smaller hole to seal it. There is a secondary seal in the form of a lead washer that sits in a shallow counterbore underneath the screw head.

Figure 1 : Sketch of chamber closure

Providing that the glasses are clean and their seals are still intact, something that becomes obvious as soon as the unit has been re-filled with xylene and sealed, refilling is much simplified. Using a glass syringe (plastic ones are rapidly attacked by xylene) and needle, the bubble chamber is filled with xylene and the control worked back and forth until no more bubbles issue from the control unit  into the bubble chamber, refilling as necessary. It is then sealed temporarily with the screw and holding the unit with the control unit down, it is flicked briskly towards the floor several times. This will usually force  more air out of the control chamber into the bubble chamber, which is then refilled to the brim of the filling hole, repeating the flicking process until no more air is present.

I found it rather fiddly to remove the lead ball from the filling hole and impossible in one case, so I omitted it, so as not to cause problems for the next person to refill the unit. In any case, lead shot is now frowned upon in New Zealand and I could not obtain any. However, if the seal beneath the screw head looked suspect, I replaced it with a new one made from thin sheet lead. This washer does not of course have to be round, though the hole in it does, and I made this using a punch. As a back up, I painted on several coats of shellac, which is not soluble in xylene, over the screw head and surrounding metal.

If you cannot remove the lead ball, you will have to remove at least the bottom glass, and it is probably better to remove the top one as well so you can give it a through cleaning, as my experience is that the old seals nearly always leak.  017  O rings made of Viton make very satisfactory replacements. They are resistant to attack by xylene, unlike common O rings made of Neoprene rubber. Neoprene rings can of course be used in the A10-A air-chamber bubble units to reseal the glasses, the air chamber and the diaphragm, as, according to the handbook, these were filled with alcohol.

If on operating the control of the A10 unit a stream of bubbles issues from the side of a glass, there is a leak and it will have to be re-sealed. A well-sealed unit gives a faint click as the control is turned to maximum. A bubble may not appear if it forms inside the control unit and you will have to give it a quick flick to force xylene into it and the bubble out into the bubble chamber. You can in some units avoid having to do this by forming the bubble with the control unit downwards, when a stream of bubbles flows up out of the control unit. In a well-functioning unit, only a tiny bubble is left if the control is turned fully anti-clockwise and this disappears after a little while as the vapour is reabsorbed into the fluid. If this does not happen, air may have been dissolved in the xylene (or there may be a leak) and you will have to add a little more xylene through the filling hole. Tighten the screw down hard and paint over with several coats of shellac (Figure 2).

Figure 2 : Later A10 bubble unit

 Needless to say, older units can be converted to this way of filling by carefully drilling and tapping an M3 hole through the front wall. The lead ball is optional and if you cannot find some lead shot 2.5 mm in diameter (the core size of an M3 screw), a steel one may do just as well, as in the A12 unit. However, the availability of Viton O rings means that the unit can now be refilled easily, though somewhat messily, by removing the bottom glass and then filling and refilling until all bubbles have been removed.




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