Bubble illumination of Mk V and AN 5851 bubble sextants

9 12 2008

A recent posting to the Sextants@yahoogroups.com asked about renewing the illumination of  the US Navy Mk V bubble chamber. I think I am correct in saying that this is identical to the AN 5851 chamber illumination. A little oddly, there was no attempt to have electrical illumination. Instead, self-luminous radium paint was used. Now radium has a half life of nearly 1600 years, so plainly it is not the radium that has decayed, but the luminescent compound mixed with it. Replacing the paint with a modern photoluminescent paint is not difficult, but  some minimal commonsense precautions should be taken to avoid swallowing or breathing in the old paint.

The bubble chamber, incidentally, is one of the easier ones to service, as the manufacturer sensibly provided a plug in the form of a taper pin to allow refilling. The top and bottom of the inside of chamber need to be scrupulously clean before refilling and this is not so simple.

Let us assume, however, that you have a bubble in a chamber that is free from debris. The first step is to remove the chamber together with the eyepiece prism by removing the three screws shown in the following photograph:


Removal of the eyepiece prism is completed by inverting the bubble chamber and removing two more screws, as shown in the next photograph:


The radium paint is contained beneath the black washer-like disc that is secured by four tiny countersunk screws. Removing these allows it to be carefully prised from its seat. The paint can then be removed from the underside of the washer and from the underside of the bubble chamber bottom glass by softening  and dissolving the paint with acetone. Certainly you should take every precaution not to inhale or swallow any. How you dispose of the paint is up to you. The level of radioactivity is low and there is only a small quantity, so disposing in the sea is unlikely to do any harm to anyone.  A local university physics department or the radio therapy unit of a hospital might be prepared to advise.

Replace the paint on the underside of the washer with, first, a layer of white paint and then a layer of modern photoluminescent paint. A layer of the latter (but not the white paint) can also be applied to the bottom glass beneath the washer – the thicker the layer of the luminescent paint, the better the illumination. Re-assembly is the reverse of dis-assembly.

To charge the photoluminescent paint, shine a bright light, say, a 40 watt bulb, onto the index prism for 5 minutes or so. How long the light lasts depends on how long you charge the paint for and on the grade of the paint.

You might wonder whether overpainting the radium paint with a paint containing copper-doped zinc sulphide would renew the radioluminescence. The answer seems to be “No” because the alpha particles emitted by the radium penetrate poorly and the ZnS.Cu needs to be mixed intimately with the radium.

Post me a comment if you would like me to deal with cleaning and refilling the bubble chamber. It is not as simple as replacing the paint, but needs no very special tools, notwithstanding the instructions given in the original overhaul manual.




4 responses

15 12 2008
John Rae

I have a good working markIX and two Mark V (B) one of which has a wonky bubble. I would love more information on servicing the bubble.

16 12 2011
Bob Hauser

On this particular 5851-1 Bendix, there are three small incandescent bulbs situated about the instrument. Each of these miniature bulbs screws into a small casing with a knurled thumb wheel which in turn screws into its appointed socket, the whole works being energized by the two D cells in the sheet metal canister whose cable is plugged into the matching power jack of the sextant. One bulb illuminates the snap cage for the stop watch, a second shines on the averager counter read-out…but the third is mounted immediately adjacent to the index prism in such a way that it can only have been meant to render the bubble visible or so it would appear. This mounting near the prism is definitely not an aftermarket or improvisation. It definitely looks like it grew there the same as the other two; so perhaps at some point during the war, Bendix must have added that third light as a backup figuring that the radium paint would or at least could deteriorate substantially before war’s end. I don’t know and perhaps that third bulb is for some entirely different purpose.
Any event, none of those bulbs lights up when the switch on the canister is thrown yet the bulbs themselves do shine as new when 3 volts is directly applied to them and a multimeter shows that a full 3 v appears at the plug at the end of the cable so that could only mean that the electrical wiring on the 5851 itself has to be faulty and is it right in assuming that all three of the lights are connected in parallel? (I have no schematic) At this stage, I don’t know whether it would profit to pursue fixing the wiring since the bulbs are still good or to proceed with dismounting the bubble chamber and, as described above, using photoluminescent paint (phosphorescent presumably). Trouble with doing this latter is that in order to recharge the luminescent paint, wouldn’t that necessitate repeatedly having to remove the bubble chamber from the instrument to expose it to light?

16 12 2011

Thank you for the comments, Bob. The third bulb lights the main degrees scale and the micrometer drum, but not the bubble. There is no reason, however, why you could not experiment with a small reflector, say a piece of polished aluminium sheet, so as to direct some of the light into the prism to illuminate the bubble. This after all is the way many amateur astronomers illuminate their finder telescope cross wires. You would need to find some way of altering the angle with the angle of the prism and of varying the light intensity. All in all, carefully removing the radium paint using acetone and replacing it with photoluminescent paint is easier and does work. You charge it by shining a bright light through the eyepiece.

If I can, I will post the relevant pages on the lighting, including a circuit diagram, from the official manuals and I will also send them to you separately. The lamps are indeed all connected in parllel, so that if none work, it suggests that the socket, the switch or the wires connecting them, are at fault. The two wires you see attached to the switch unit in Figure 4 of the existing post lead to the third lamp mentioned above.

16 12 2011
Bob Hauser

Roger your last, Bill, and thank you

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