The Nautical Sextant

This post is preceded by “Bubble Illumination of Mk V and AN 5851 Bubble Sextants”

Mark V is the US Navy designation for the AN5851 sextant. Early models were fitted with vapour pressure bubbles, which are hard, but not impossible to refill. This post will deal with only the somewhat easier air bubble models, which are distinguished by having a cylindrical air chamber projecting from the top left hand side of the chamber and, because I want at least to start to answer a request for details before Christmas intervenes, I’m sticking to the simplest case. This is where there is a bubble of some sort present and the fluid and glasses appear clear. If there are particles floating around in the fluid, or it has completely dried up leaving a powdery film inside the chamber, a full overhaul will be needed.

At the front of the chamber, you will see projecting a peg about 2 mm in diameter. This is a tapered pin that occupies a tapered hole through which the chamber was filled. To remove it, remove the chamber and eye lens assembly as described in the previous post. Then grasp the end of the pin in a pair of pliers and pull directly in line with it while twisting. It should come out without too much trouble, unless you pull and twist out of line, when it may break off and leave you in deep trouble, the solution for which will be covered in a future post.

Once the pin is out, you can refill the chamber with xylene using a hypodermic syringe and needle, if you can locate a glass syringe. You can use a disposable plastic one, but the xylene attacks the plastic and the syringe rapidly becomes very stiff to operate. An alternative is to use a glass pipette with the end drawn out to a fine tube of about 1 mm diameter. This is what I now use. Some decorating shops stock xylene in bulk and may be prepared to sell you 500 ml or so. It will last you a long time. An alternative suggested, with which I have no experience, is Zippo or Ronsonol cigarette lighter fluid. You should not use alcohol, as the glasses  have been sealed with shellac, which is soluble in alcohol.

When the chamber is full, temporarily replace the taper pin with a pushing and screwing motion,  hold the chamber with the air chamber downwards and press repeatedly on the control knob until no more air comes into the bubble chamber. This displaces air from the bellows into the bubble chamber. Then refill the bubble chamber . At this point you can  replace the taper pin using a firm pushing and screwing motion. Do not tap it in with a hammer. It may be necessary to experiment a little to get the right amount of fluid. It should be possible to remove the bubble altogether, so that sights using the natural horizon can be taken. If the bubble cannot be entirely removed, remove the taper pin and add a little more xylene.

To empty the air out of the bubble chamber to reduce the size of the bubble, hold with the air chamber upwards while pressing repeatedly on the bubble control knob until you have a bubble of the desired size. Pressing forces air into the air chamber through a tiny capillary tube and when you release, xylene passes into the bubble chamber. To get a larger bubble, hold with the air chamber downwards while operating the control. 

If you cannot get a bubble at all, the air chamber may be completely full of xylene and you will have to remove a little, the capillary passage between the bubble and air chambers may be blocked by debris( clear with a piece of fine wire), or the hole in the ball that seals the bellows unit to the bubble chamber may be misaligned (see next post).

Do let me know if you find this post useful and ask if something is unclear. Consider, too,  buying yourself a gift of The Nautical Sextant…