Nude Gauge

11 04 2009

While disassembling the vacuum chamber last night, I removed the probe for the vacuum gauge that came with the chamber. Now, I assumed the chamber came with a pirani gauge because the readout for the guage only went down to 10e-3 torr. But looking at the probe, I’m pretty sure this is a nude ion gauge:


I think this is it’s pinout.

The Core

11 04 2009

I’m preparing to have the core fabricated. I have a number of considerations to consider.


Welding. We have to weld the lids to the chassis.

Maybe TIG welding will work. My concern is that heat affected zone will damage the SC coils inside. We have ~2.5 mm from the surface to the coils. Laser welding has a much smaller heat affected zone. TODO: get a quote for laser welding from EB Industries. Can anyone comment of the viability of TIG welding for this sitation?

Surfacing. The product that comes back from prometal has a rough surface which we need to machine so that the lid mate well.

Previously I tried wet sanding. This worked decently. However, I wasn’t able to get the deeper surface imperfections, it took a lot of sanding. The outer ring of the torus half saw more material removed than the inner ring, which means that the inner rings mate very tightly, but the outer ring has about a 0.25 mm gap.

There is a surfacing machine here in the shop. It’s large enough to accommodate the lids, but not large enough to accommodate the chassis. The surfacing machine uses a magnetic vise, so the work piece must be magnetic. The sample parts we ordered from prometal are magnetic, however the next parts will be made with a less magnetic stainless steel alloy (the chassis should not be magnetic).

We may need to take surfacing into consideration for the design of the part. ie, we may need to include some extra material on the prometal part, so that after we surface it, we have a perfect half torus.

UPDATE: Stuart told me about Lapping which seems to be an advanced for of wet-sanding.

CAD problems

I’m using BRL-CAD to generate my parts. Lately I’ve been getting this error when I try to export to STL: class_lu_vs_s: loop transits plane of shell/face?  I can’t proceed until I overcome this bug.

Even when the STL export works, it takes forever to render an STL with the resolution I need for production (I’m talking days here). This is really cramping my flow.

Permeability of the Core

We are building a superconducting core. There will be liquid nitrogen at atmospheric pressures inside the core (and connected to outside of chamber via a fluid feedthrough). The core can’t be so permeable as to leek nitrogen into the vacuum which would poison the reaction. Speaking of pressure differentials, the core must withstand the pressure from the inside. To calculate this pressure, I think we need to know the internal surface area of the core.

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