The Nautical Sextant

This post is preceded by  “The SOLD KM2 Bubble Sextant”; “C Plath Bubble Horizon Attachment”;“A gummed up AN5851-1 averager”, “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”, “A nautical sextant bubble horizon” and “Sealing A10 vapour pressure bubble chambers.”

A friend recently asked me to refill the bubble unit of his C Plath artificial horizon. Someone had been there before me and mutilated the retaining ring for the top glass and left a leaky bubble chamber, but there was enough ring surviving for me to be able make a repair. Her’s how I did it in (mainly) pictures. Read this in conjunction with the post of 26 June 2012.

Figure 1: remove plate.

Figure 2: Remove bulb carrier.

Figure 3:Displace bulb mount.

Figure 4: Remove bubble unit.

Figure 5: Remove top retaining ring.

To remove the ring you will need to make a little tool from a piece of 10 mm square steel. File the corners off to make a symmetrical octagonal shape until it fits in the octagonal hole in the ring. You don’t have to fit a tommy bar. You could use a 10 mm AF wrench instead.

Figure 6: Remove washer.

Figure 7: Remove another washer, carefully.

Figure 8: Prise out glass, even more carefully.

Figure 9: Rinse and add fluid as necessary.

You can use absolute alcohol if you can get it, gin or vodka. I prefer to use iso-propyl alcohol (isopropanol) because the de-natured ethanol I can get is coloured purple. In an ordinary level tube I bleach it with a drop of household bleach, but am uncertain about its long-term effects in a metal and glass chamber. When you cannot draw any more fluid in, over-fill the chamber and add the glass, remembering to put it with the the concave recess down. A syringe with a 23 or 25 gauge needle is handy for adding the fluid.

Figure 10: Form bubble.

If a bubble isn’t trapped under the glass, left one edge to let in a little air, about this much:

This should be as big as it gets.

Replacing the bubble unit is the reverse of removing it.