odd dinner conversation

[cellio]

The question (not mine): if you were building a thermometer (the kind that lives in your medicine cabinet at home), what range of temperatures would you support? I said the problem was insufficiently specified, but that my baseline would be 96-106 and if there's no appreciable expense in widening it, I'd go in the range of 90-110 or -120, because why not. But the problem was still insufficiently specified; I was assuming digital readout, not a column of mercury in a usually-illegibly-marked tube. In the latter case, you want the minimum useful range, because you've got limited real estate for the markings. If you could have those 10 degrees occupy 80% of the tube and have the rest be compressed that'd be different, I said.

So Dani challenged that -- why assume that the tube is uniform? I said because otherwise you're out of the price range of medicine-cabinet thermometers. This, in turn, led to speculation about how that type of thermometer is manufactured; I argued for a large uniform (hollow) rod that's cut to length with ends then treated (seal at one end, mercury + bulb at other), while he argued for individually molded. (Insert tangent about plastic vs. glass here.) Of course, neither of us actually knows anything about this; we're trying to make intelligent guesses and apply design principles from other fields.

I don't think we're the only people who have weird speculative conversations like this, but I never seem to notice stuff like this coming from other tables in restaurants. On the other hand, we haven't been kicked out of any restaurants for annoying the neighbors either. (On the third hand, it seems to take a lot to produce that result.)

Comments

[siderea]

FWIW, My guess (as a ceramist who researches bricks, not glass, though I did have a few glass classes) would be for individually molded. The treatment of the ends would require the glass to be returned (at least partially) to the viscous state, which could mess up the hollow tube.

Actually, my understanding is that glass tubes have some fascinating properties when heated to a plastic state. For instance, if you take a tube an heat it in the middle, then draw the two solid ends in opposite directi ons, you get... a really amazingly thin tube of glass. This is, in fact, how extremely fine pipettes used to be (and may yet be for all I know) manufactured, I was told by the lab instructor who made us do this. :) You can actuall spot-heat glass pretty well, so heating only the end to a fusable temperature while leaving the rest alone is feasable.

That said, if I wanted a to seal the end of a glass tube while making sure that the tube remained a consistent diameter, I'd either: (1) just have a plug of metal up the center (only works on one end, but works really well) when closing it or (2) heat the end of the glass to the melting point plus some, and pinch it off really fast/abruptly, using momentum to keep the rest of the shape stable or (3) dip the end in molten glass, or more precisely, put a glob of molten glass on the end. That's my favorite to win.

I suspect as an industrial pro cess it would b e possible to get either (2) or (3) to have sufficiently consistent results not to wack out your calibration.

I'm envisioning a mechanical process which goes: tubes cut into lengths; mounted on pipes; a glob of molten glass put on top end and a timed, pr essurized blast of air feeds up through the pipe to form a consistent-volume bulb; pipe removed, mercury+air added; glob of semi-molten glass daubed on top.

I think that will get you a more consistent result than two lateral halves fused together, but IANACE.