I’ve got an aircraft bubble sextant with a dried out bubble chamber, so I need to refill it.
Basically it’s a very precice bulls eye level
The manual (from 75 years ago) says to refill with "Xylene CP (chemically pure xylene) ,commercial or purified grades are not sufficiently pure”
Why would Xylene be used for this instead of something else, and why would the purity matter?
And how pure is modern Xylene that you’d buy at homedepot etc?
Organic solvents are used in bubble spirit levels because of their low viscosity and surface tension - which means that the bubble doesn’t get ‘stuck’ to the inside of the vial, and it moves quickly and responsively when the level changes… They’re also more suitable than, say, water, because of the range of temperatures at which they are liquid, and their non-expansion if they do happen to freeze.
I imagine the instructions you have may just be that specific in an attempt to make the user seek out the right liquid, rather than one that’s merely ‘good enough’ - I expect topping off with something else (i.e. the wrong liquid) as a mixture is probably a not desired, and the easiest way to avoid this is to be very specific about the right liquid.
Given the chamber is prone to drying out, and my limited experience with xylene is that it does seem to be able to dry out from within what one would ordinarily hope were well sealed containers, they may well be worried about the contaminants forming a film inside the chamber as it dries. Given the sorts of things xylene will cheerfully dissolve such contaminants may well include stuff that does not just coat the inside, but might also later oxidise or polymerise and form a difficult to get rid of film (ie not just easily washed out with more xylene.)
You might want to try this stuff: Airpath Compass Fluid ( 1/2 Pint & Quart ) | Aircraft Spruce
Or at least call them and ask what the substance is. They’re a pretty knowledgeable company; you’re not talking to “Peggy” in Bangalore.
Lots of Mil-spec fluids in common use in the WWII era are still used in aviation. This is *probably *the same stuff made to the same standards.
In addition to what’s already been said, they designed it to use this particular grade of xylene. They checked it under rigorous conditions, that the sextant won’t, i dunno, send you off a cliff, or something. They didn’t try any other grades of xylene so vigorously because, it just wasn’t worth their while to check a bunch of other possibilities to the same rigor.
Some people would be pissed off at me for my little screed above. I say this is just the way the world works. Others say that I’m believing everything I read, and should stop being confined by corporate bigwigs, you can use any old xylene, works just fine …
Listen, if you use any old xylene, or use gasoline, or use alcohol, or water, and it works fine, you get to come back and say I don’t know anything. If it sends you off a cliff, then you don’t. And you can’t. How important is this issue to you?
Warning: xylene can be any of three isomers of xylene, in any mixture, and they all have slightly different properties. Xylene - Wikipedia This is why they say, “purified xylene”, what they may mean to say is, “We only tested this isomer.” But your average sailor didn’t keep a copy of an organic chemistry book handy. At least, not until now, with Google capable phones and Wikipedia.
I do think you are missing the point. As pointed out - you need xylene or a similar low viscosity fluid to allow the bubble to work properly. It won’t send you off a cliff if you use water, but it will almost certainly reduce the accuracy of your readings.
The trouble with “grades” of material is that the industrial grades contain uncontrolled contaminants. You don’t know what is in there. Only that there is allowed to be up to a certain spec “other gunk.” Typically these specifications come about after hard experience. Someone sometime in the past has used industrial xylene and had a problem. Maybe your batch won’t have that particular problem. Maybe it will be worse. There is no way of knowing, because there is no way of testing for it.
Thanks Arkon. Doing some reading there it’s pretty clear that the o-xylene isomer would probably not pass Mil-spec. It’d be thickening at too high a temperature to be usable under all possible conditions. And yes, The point of Mil-spec is to rigorously test one particular arrangement of whatever then demand the use of exactly that. They and we know exactly zero about what substitutes might be acceptable. Unless they’re officially published.
Having smelled compass fluid a time or three over the years I’d bet the link I provided is to the right stuff.
One thing none of us have mentioned yet: Since the OP’s level is refillable we know it has some sort of elastomeric seals. Whatever those are, the fluid needs to be compatible with them. Just as alcohol-infused gasoline will trash the fuel system seals in older cars, using an inappropriate fluid will trash the OP’s level’s seals.
As hard as it is to find the right replacement fluid, it’ll be even harder to find the right replacement seals if they’re wrecked.
In a surprising coincidence, on Saturday I happened into a nautical antique and tchotchke shop. A customer was in deep discussion with a clerk about two old nautical sextants set out on the counter. They were all brass and quite weathered; probably dating to the early 20th century. Both folks knew what they were talking about.
Who knew sextant collecting was a hobby?
Late add:
Here’s the relevant Mil-Spec: MIL-L-5020 C LIQUID COMPASS AIRCRAFT then click the orange [Download file] button. Interestingly, the text never describes what the stuff is chemically; it just describes the physical parameters it must have.
I think you have to assume that the xylene is actually xylenes. Without an indication of what the isomer actually is, I don’t see how any other assumption can be made.
I’ll also agree that the stuff you can buy at Home Depot is probably nowhere near clean enough. I’d think a reagent grade supply would probably be as clean as needed. There are ways to further purify even the commercially-supplied solvent (here) but it’s long, tedious, potentially quite dangerous, and not really the kind of thing you want to do in your garage.