Measuring and testing – Fluid pressure gauge – Electrical
Reexamination Certificate
1998-10-28
2001-07-10
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Electrical
Reexamination Certificate
active
06257069
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the measurement of extremely high vacuum using electronic sensing devices, particularly Bayard-Alpert type gauges. The invention relates to the neutralization of x-ray effects in such measuring devices such that the sensitivity and accuracy of the gauge is enhanced.
BACKGROUND OF THE INVENTION
One of the various devices employed for measurement of ultrahigh vacuum, that is, very low gas pressure, is the Bayard-Alpert (BA) type gauge. A BA gauge operates by releasing free electrons into the low pressure space to be measured. The electrons are accelerated and collide with gas molecules present in the space, producing positive ions in the process. The positive ions are collected by an electrically conductive sensing element, an ion collector, in which the ions create a current (the “ion current”). The positively charged ions stimulate an electron flow into the collector from the external measurement circuit. Thus, the “ion current” inside the gauge gives rise to an equal and opposite electron current flowing into the collector from outside the gauge.
The ion current, in ideal conditions, is proportional to the number of ions impinging on the ion collector. Because the number of ions is also proportional to the number of gas molecules present in the space that are ionized by the injected electrons, the ion current is actually an indicator of the quality of the vacuum. The fewer gas molecules that are present, then the fewer ions are created in collisions with the free electrons, and the lower the current is in the ion collector.
As is often the case in other fields, the measurement device can produce effects that alter or limit the quality of the measurement. That is particularly so where the pressure to be measured is ultra-low (an ultrahigh vacuum), approaching a nearly total vacuum. In the case of the BA gauge, one phenomenon that presents a measurement limit is known as the “x-ray effect,” which is a result of the accelerated electrons in the gauge creating not only positive ions in collisions with the gas molecules in the gauge, but also causing x-ray emissions from metals in the gauge. The x-rays cause photoelectrons to be released from conducting surfaces in the gauge. The collector current is directly affected by the photoelectron emission. An “x-ray current” is produced in the collector.
This current induces an error into the measurement, which becomes larger in proportion to the ion current as the measured gas pressures become smaller. Ultimately, the collector current caused by the x-ray effect limits the ability of the device to measure very low gas pressures.
The x-ray current is a composite of two distinct effects. The “forward x-ray effect” is caused by electrons hitting the grid of the BA gauge, producing soft x-rays. Some of these soft x-rays strike the ion collector, causing photoelectrons to be ejected from the collector. The collector current, which is positive when caused by the collection of positive ions, is increased by this electron emission. The electron emission from the collector has a similar effect as the collection of a positive ion. This enhanced forward current is known as the “forward x-ray current.”
A “reverse x-ray effect” is also at work in the gauge. The x-rays created by electrons colliding with the grid strike other elements in the gauge, such as the interior of the gauge envelope. Photoelectrons are released by those elements in some of these collisions and some of these photoelectrons may strike the ion collector. These photoelectrons tend to reduce the current caused by the collection of positive ions created in electron-gas molecule collisions. Put another way, the reverse x-ray effect causes a “reverse x-ray current” in the collector.
Thus, two currents caused by the same process combine to cause error in, and at a sufficiently low pressure, to limit the sensitivity of a BA type gauge. However, the fact that the two x-ray induced currents in the gauge cause opposite currents offers the possibility of balancing the two effects, resulting in the neutralization of the x-ray effect. The result of neutralizing the x-ray effect in a BA gauge would be improved sensitivity to extremely low gas pressures and more accurate measurements of ultrahigh vacuums.
SUMMARY OF THE INVENTION
The present invention is a Bayard-Alpert measuring gauge in which selection of the materials used for the electrode elements in the gauge, the configuration of the electrodes, and the electrical biasing of the electrodes combine to balance the forward and reverse x-ray currents and to maintain that balance for extended periods of time. The result is a BA gauge having improved accuracy and sensitivity at very low gas pressures.
A first aspect of the invention is the selection of a chemically inert conductive metal for both the collector and the shield electrodes in the BA gauge. In a preferred embodiment of the invention, gold is the chosen substance. A gold coating on the interior surface of the gauge forms the shield electrode. The collector electrode may be either formed of or coated with gold. The object is to impart the same photoelectron emission characteristics to both the shield and the collector. The emission of photoelectrons from the collector is the origin of the forward x-ray current. Imparting the same photoelectron emission characteristics to the shield by using the same substance as the collector makes the balancing of the forward and reverse x-ray effects more achievable.
Alternatively, the shield and collector may be made of (or coated with) other materials, where the same material is used for both elements. It is also a workable solution to make the shield of one substance and the collector of a different substance, provided that the substances have similar properties when exposed to electron emissions and soft x-rays.
A second aspect of the invention is the electrical biasing of the gauge electrodes to neutralize the x-ray effect. Using standard bias for the cathode, collector and grid in the preferred embodiment of the invention, the shield electrode is biased with a negative voltage potential in the range from ground potential to about −50 volts, and which has ranged in test gauges between approximately −10 to −30 volts (V). This negative bias enhances the reverse x-ray current and, in the preferred gold-coated gauges, substantially offsets the forward x-ray current, neutralizing the x-ray effect in the gauge.
In some cases a modified collector geometry may be employed to further enhance the collection of photoelectrons emitted from the shield. One such modification involves a trident collector, so-called because it has three branches. The increase in photoelectron gathering enlarges the reverse x-ray current and assists in neutralizing the total x-ray effect in the gauge.
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Brady Howard M.
Drubetsky Emil
Goehner Ronald H.
Kendall Bruce R. F.
Drinker Biddle & Reath LLP
Oen William
The Fredericks Company
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