Macor. Costs the earth, and it's hard to tap holes in without chipping. Boron nitride is much nicer, but also expensive.
I'd just make the temperature control loop a bit overdamped, so there's no need for overshoot recovery. Two resistors and one cap!
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Really... they are only 77 kB on my machine... George H.
An airflow-driven turbine is easily made nonmagnetic. So a bunch of my pneumatic tools qualify as having nonmagnetic motors. But, as you say, hand-cranked by grad students will do just fine.
Soapstone, pyrophyllite, and slate are the less expensive machinable ceramics (because they're 'just rock').
But if you want the temperature in a gas cell to be constant, use the convection-oven approach, and stir the gas. For extra credit, you could filter it through a heat-exchange medium (wad of copper wool, or tube packed with BBs, or even just metal baffles).
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Don't you have to machine them "green" and subsequently heat them in a furn ace to make them hard, which produces significant changes of shape and dime nsions?
I used a Warren stirred-air bath to control the temperature of my (silica) reaction vessel for my Ph.D. project. The shaft that spun the fan that stir red the air had a hollow shaft that let me feed light through the reaction vessel and monitor the chemical reaction (decomposition of nitrosyl bromide ) going on inside it.
The whole thing was thoroughly coaxial, with a cylindrical baffle that rout ed the air stream past the reaction vessel, turned it around at the end and routed it back outside the baffled.
The main heat source was resistance wires wrapped around the outer wall of the furnace/air-bath but there was a second - fast-acting - resistance heat er wound onto the baffle. Like a mug, I wound my helical temperature sensor ( several metres of thin platinum wire) onto a hexagonal cage that I could slip into the furnace, and the AC heating current transformer coupled into the temeprature sensor output and had to be filtered out.
-- Bill Sloman, Sydney
That's true of pyrophyllite, which I've used quite a lot - we needed accurate but disposable spacers, so never cared to heat-treat it. Carbide tooling cuts it fine, but for best finish, we used grinding, with guides and sandpaper. Cleanup is a nuisance, though; mineral dust is not a good thing around the metal-cutting tools. Paintbrush and vacuum cleaner were always on hand, smocks were hung nearby, and we kept all of that work in a separate cubby from anything else.
I once tried making a frost sensor using water sealed in a small tube with a pair of electrodes. It wasn't super-accurate. The problem was that corrosion products of the elctrodes changed the freezing point of the water.
John
yeah, use insulated electrodes and sense capacitance.
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