Wednesday, June 4, 2008

Gauging With Intent

We are going to have to read pressures both for experimental purposes and to decide when to turn pumps off and on. Of course first start with minimalist education available on page 7 of this [pdf]. The first critical piece of data is that only capacitance gauges are gas independent. The second critical bit is that the output is linear with pressure and they can cover a 10,000 to 1 pressure range. i.e. 1 millivolt to 10 Volt output.

The Adixen Capacitance Vacuum Gauges come in four models.

1100 to 1e-1 torr
110 to 1e-2 torr
11 to 1e-3 torr
1.1 to 1e-4 torr

Two is probably the minimum for a system. One (1100 torr to 1e-1 torr) to monitor the backing pump inlet pressure. Plus one other chosen for the chamber operating pressure. The 110 to 1e-1 torr model would be good for most current fusor experiments. The minimum output to realize the inherent accuracy of the instruments is around 20 to 50 mV.

Here are some more capacitance gauges.

MKS 722 which has some really nice specs but only comes in a KF 16 flange. Which means an adapter. It covers the 1 torr to 1000 torr full scale range. Around $900 with flange mount. Add about $100 for a flange adapter.

MKS 626A can be ordered with a CF40 (2 3/4") flange. Full scale ranges from .1 torr to 1,000 torr. Around $950 with flange mount.

What if you want to go to lower pressures? I'll go into that in another post.

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