Televac^® AN 3025: Hot Ion Vacuum Gauge Degas Instructions for the MM200

Hot ion vacuum gauges consist of a filament, grid, and collector. The filament is heated to produce electrons, which are accelerated toward the grid by a potential difference. Before reaching the grid, the electrons collide with gas molecules and ionize them. The resulting ions move towards the collector, where they are measured as an electrical current. This ion current is proportional to the gas density and, therefore, the pressure in the vacuum system.

Over time, gas molecules can become absorbed or adsorbed onto the grid, causing contamination that may introduce vacuum measurement errors. Degas mode heats the grid to release these sorbed gases and reduce the effects of contamination.

Two methods can be used to heat the grid during degassing:

1. Resistance heating: Current is passed through the grid to heat it.
2. Electron bombardment heating: A higher potential is applied to the grid, accelerating electrons more strongly than during normal operation and heating the grid through electron bombardment.

Televac^® recommends degassing a hot ion vacuum gauge no more frequently than once a month and at least once every three months. The appropriate frequency depends on the vacuum process and the amount of contamination present in the system.

Applications with higher levels of contamination, or systems operating in the ultra-high vacuum (UHV) range of approximately 10^-10 to 10^-11 Torr, may require more frequent degassing. In cleaner systems or applications with less contamination, degassing may be required less often.

Degassing too frequently can accelerate degradation of the filament and grid. However, allowing excessive contamination to accumulate can affect gauge performance and vacuum measurement accuracy. For UHV systems that are infrequently exposed to atmosphere, degassing may only be required approximately once every six months to one year.

Rather than degassing on a fixed schedule, the hot ion gauge can be compared with a reference vacuum gauge to determine whether its readings have drifted.

Install the reference gauge and the hot ion gauge on the same vacuum system, as physically close to each other as practical, and compare their pressure readings under the same conditions. When evaluating the results, consider the accuracy specification of each gauge. The hot ion gauge in this example has a typical accuracy of ±10%.

Suitable reference gauges may include:

• Another hot ion gauge
• A cold cathode gauge
• A spinning rotor gauge (SRG)

Different gauge technologies may have different accuracy specifications, which should be considered when comparing readings.

• Hot Ion Gauge 1: 1.0*10^-6 Torr
  • With ±10% accuracy: 9.0*10^-7 to 1.1*10^-6 Torr
• Hot Ion Gauge 2: 9.0*10^-7 Torr
  • With ±10% accuracy: 8.1*10^-7 to 9.9*10^-7 Torr

The measurement ranges overlap, so degassing is not indicated based on this comparison.

• Hot Ion Gauge 1: 1.0*10^-6 Torr
  • With ±10% accuracy: 9.0*10^-7 to 1.1*10^-6 Torr
• Hot Ion Gauge 2: 1.0*10^-7 Torr
  • With ±10% accuracy: 9.0*10^-8 to 1.1*10^-7 Torr

The measurement ranges don’t overlap, indicating that the hot ion gauge may need to be degassed.

These instructions apply to the Televac^® MM200 Vacuum Controller when used with Televac^® and ETI^® hot ion vacuum gauges.

1. Connect all applicable vacuum gauges to the vacuum system and connect the gauge cables to the MM200 Vacuum Controller.
2. Power on the MM200.
3. Pump the vacuum system down until the hot ion gauge is reading 1.0*10^-5 Torr or below.
   1. If the gauge is in Auto mode, it will automatically turn on when the rough vacuum gauge reaches 1*10^-2 Torr (10 microns).
   2. If the gauge is in Self mode, it must be turned on manually using the standard turn-on procedure.
4. Press SETUP until the left display shows 0 and the right display shows dGAS.
   1. The left display indicates the amount of time, in minutes, that the gauge will remain in degas mode.
5. Use the left up and down arrows to increase or decrease the degas time.
   1. Televac^® recommends a degas time of 5 minutes.
6. Press SETUP or TEST/OP until the MM200 returns to the measurement screen.
7. The gauge will begin degassing. The remaining degas time, in minutes, will flash on the right display.

The hot ion gauge contains two filaments. The procedure above will degas whichever filament is currently selected.

To change the selected filament:

1. Press SETUP until the left display shows FIL and the right display shows nbr1 or nbr2.
2. Use the left up and down arrows to select:
   1. nbr1 for filament 1
   2. nbr2 for filament 2
3. Press SETUP or TEST/OP until the MM200 returns to the measurement screen.