Filaments

All gauges listed in this brochure have either tungsten or

thoria coated iridium cathodes. The relative advantages

and disadvantages of each are tabulated in Figure 2.

Sensitivity

The sensitivity is defined as follows:

This definition makes the sensitivity relatively independent

of the electron current and dependent only on gauge

construction. In practice, it is often necessary to determine

the lowest pressure that can be read with a given gauge

tube and controller. To determine this, the lowest readable

current sensitivity of the electrometer portion of the

controller must be known.

Knowing the sensitivity of the gauge tube from the

manufacturer’s data, the relationship for ionization current

vs. pressure can be determined.

Figure 2 —

Filament Selection Guide

Current measured in amperes. Pressure in Torr.

Degassing

To reduce the outgassing in a gauge to a negligible level,

the process of degassing is employed. For attainment

of the lowest pressure levels the glass envelope gauges

should be baked at 400°C for 1 hour.

The electrodes are degassed by heating to a temperature

of 900°C nominally for approximately 15 minutes after

baking.

The electrode heating is accomplished by either electron

low voltage through the grid. In general, gauges equipped

withbuiltupgrids;i.e.,squirrelcageorgridsweldedat

multiple intersecting components, are degassed only by

electron bombardment. The collector may be degassed

only by the electron bombardment method. Grids

consisting of a helix or double helix, in which both ends of

the helix have external connections, are degassed by the

heavy current method. All gauges with helical grids may

also be degassed by means of electron bombardment.

The 274022, 274023, 274041, 274042, 274050, 274057,

and 274058 gauges utilize a squirrel cage grid and must

use electron bombardment for degassing. To provide

274043, and 274053 gauges were developed.

Electrical Leakage

The accuracy of pressure measurement, which depends

on the measurement of currents as low as nanoamperes

and below, is affected by leakage. These paths may be

classified as either internal or external.

Internal and external leakage is held to the lowest possible

value in designs such as the tubulated Bayard-Alpert. The

collector terminal of the gauge is at the opposite end of the

envelope from the grid and cathode terminals.

Designs which bring the collector out as one of a series of

pins in a base or header are more susceptible to leakage

problems than the tubulated design.

Internal leakage usually results from the evaporation

of tungsten or thoria vapor from the cathode. This is

controlled by means of a shield where the collector lead

passes through the stem or header insulator. Great care

must be exercised in insulating and shielding the lead to

the collector terminal on the gauge from the controller.

phenomenon

tungsten Filament

thoria-Iridium Filament

Accidental Exposure

to Atmosphere

Do not Use

High Tolerance

High Oxygen

Partial Pressure

Easily Oxidized

Relatively Immune

to Oxidation

Chemical Reaction with

Gas to be Measured

Higher Filament Temperature,

High Reaction Rate

Lower Filament Temperature,

Lower Reaction Rate

Corrosive Gas Tolerance

Higher

Lower