a hot-cathode gauge's volume is 100 cm3, 1% of the gas within the gauge is removed every second!54 It has been shown that if the conductivity of the connecting tube is >10 liters/sec, these affects are negligible.

If you are concerned specifically with accuracy, hot-cathode gauges show greater accuracy and reproducibility. However, it must be operated by an experienced technician in controlled conditions with repeated backing and degassing. Otherwise, the cold-cathode gauge will provide greater accuracy without constant attention. This is mostly attributed to the how the electrodes of the hot-cathode gauge do get hot during operation which in turn provides some incidental outgassing. The cold-cathode gauge operates at room temperatures and has no incidental outgassing.55

Electrical Warning: Ion gauges require high voltage, so common sense must be observed. For instance, do not pull on the cord when unplugging a gauge from a wall outlet, but instead pull on the electrical plug. Avoid spilling conducting liquids around the gauges. All pieces should be grounded; and if the unit has a threepronged outlet, do not cut off the ground or bypass it. Be sure to unplug the unit if it is being repaired. Replace worn and/or frayed cords immediately. Cover all bare (exposed) electrical leads with tape, tubing, shrink tubing, or plastic screw caps. Keep flammable liquids or gases away from electrical devices in case of sparks and/or electrical arcs. Because of the high voltages possible with ion gauges, dirt (and even fingerprints) can cause unexpected arcs. Therefore, keep the gauge surface clean. Turn off the gauge and its controller when helium leak testing because a discharge can be created around high-voltage feedthroughs, which in turn can destroy the gauges controller. Likewise, turn off the gauge and controller when checking for leaks on a glass system using a Tesla coil. The discharge can destroy the electrical circuits within the controller. Finally, if your vacuum system is mounted on a metal rack, ground the rack with grounding strap (braided copperplated wire) to plumbing or some other substantial ground. For proper contact, file off any paint or corrosion on the rack and the ground before attaching the grounding strap with a threaded hose clamp.

7.5.20 The Hot-Cathode Ion Gauge

The most common hot-cathode ion gauge (and the most common high-vacuum gauge used) is the Bayard-Alpert gauge. It can read vacuums between 10~3 and 10"10 torr. With special gauges, readings as low as 10"14 torr can be obtained. A diagram of its general structure is shown in Fig. 7.48. The hot-cathode gauge operates by heating a filament, which causes an emission of electrons. These electrons are attracted to a grid which is held to a high (+) potential that attracts the (- ) electrons. As electrons stream toward the grid, they collide with the gas molecules enroute, ripping off an electron and creating positive ions. The positive ions are attracted to the ion collector, collected, and counted. The positive ion current is measured in amperes from the gauge tube, but the hot-cathode gauge controller then interprets this count as vacuum reading. Hot-ion gauges are designed to be

help protect the gauge from condensable vapors. However, it will indicate pressures lower than really exist within your system due to the cryogenic pumping capabilities of the cold trap. In addition, it will also slow the speed required for the system and the gauge to come to equilibrium.

3.On metal systems, you have two choices: Connect the ion gauge by connecting tubing (same tubing size rules apply from Point 2), or use a nude gauge.

4.The ion gauge is gas- (see Table 7.13) and temperature-dependent. Therefore, if your lab has temperature swings and/or you vary the gases within your system, constant recalibration may be required. Table 7.13 (and others like it) can only provide a benchmark for making corrections because your system is not likely to have pure gas samples. In addition, variations between gauges of the same type (but from different manufacturers) can be quite large while the differences between gauges of different designs can be phenomenal. For the most accurate interpretation of your gauges readings, obtain calibration tables from the gauges manufacturer.

5.Never turn a gauge on until the pressure is below 1 um or less. You must have a second gauge for higher pressures (for example, a Pirani or thermocouple gauge).

6.The hot-cathode gauge must be outgassed every time the gauge is

exposed to the atmosphere for an extended period of time, or at pressures near the base pressure of operation (10~5 torr). The outgassing cannot be done at any higher pressure than 10"5 torr, and it can be per-

formed either by providing a current to the grid within the gauge (which can often be supplied by the gauge controller) or by electron bombardment. Note that some gauges can only be resistively heated or electron bombardment degassed. Gauges that require outgassing will read a pressure higher than really exists. The amount of error is dependent on the degree of outgassing required.

Regardless of the outgassing approach used, filament temperatures of about 800°C are required. Initially, a new gauge may need to be outgassed for some 1520 minutes. If a gauge is showing evidence (i.e., dirty electrodes) that outgassing may be required again, some 15 seconds should suffice. During degassing, the ion gauge envelope becomes very hot. Be sure that the gauge is mounted in such a fashion that accidental contact with technicians or flammable materials is not possible.

If a gauge is outgassed at too high a pressure, a layer of metal (from the electrodes) may be deposited outside the gauge envelope (some controllers prevent degassing at too high a pressure). This condition can cause the insulation to become "leaky." A temporary solution can be achieved by grounding the electrical