Figure 1 —

Operating Range for Bayard-Alpert Ionization Gauge Tube

Description

Bayard-Alpert (hot cathode) gauges provide pressure

measurement based on the ionization of gas molecules by

a constant flow of electrons from a filament. The number

of ions created is directly proportional to the density of

molecules. The ions are collected and measured by a

vacuum gauge controller and converted to pressure based

on the sensitivity of the gauge. The Series 274 Bayard-

Alpert gauges consist of 3 types: glass tubulated, nude

gauge with electron bombardment degas (EB), and the

Gas Species Effects

Bayard-Alpert ionization gauges have different relative

sensitivities for different gas species. As a result, pressure

readout provides a direct reading only for the gas for which

the gauge is specified (nitrogen). This is called a readout

of nitrogen equivalent pressure. A simple mathematical

conversion of the direct pressure readout to the pressure

of the non standard gas can be made. This conversion is

made using the relative gauge sensitivity for the specific

gas. These sensitivities are tabulated in reference material

manual for Series 274.

Hot filament type ionization vacuum gauges work best in

given pressure ranges for individual gauge designs. The

following is a brief outline of the considerations relevant to

selection of the proper gauge.

A glance at the chart (Figure 1) will quickly indicate the

gauge or gauges suitable for a given application. The

Torr. Variations in design permit operation to various orders

of vacuum. The nude type 274022, 274023, 274041 and

Operating Principle

negative electrons are emitted at a well-controlled,

selectable rate from a heated filament and are accelerated

toward a positively charged wire grid (anode). Electrons

pass into the space between the grid and a collector

wire at ground. In this space the electrons collide with

gas molecules from the vacuum system, producing

positive ions. The positive ions are then collected by the

grounded collector wire that is located along the axis of the

cylindrical grid. At a constant filament to grid voltage and

electron emission current, the rate that positive ions are

formed is directly proportional to the density of molecules

The strength of the ion current is then indicated on a

microammeter that is calibrated in units of pressure.

The low end of the operating range of a Bayard-Alpert

gauge is determined by the X-ray limit of this type of

gauge. The X-ray limit varies with different gauge designs.

Features & Benefits

• Goodvacuumpressuremeasurementforeconomical

cost

• Operatesonindustrystandardelectrodevoltagesand

wide range of electron emission currents

(10

mA to 10 mA)

• Well-knownperformancecharacteristicsinvacuum

industry

• Ioncollectorelectrodewell-shieldedfromleakage

currents

• Availablewithburn-outresistantfilamentsandstandard

vacuum connections

• Allunitscanbedegassedbyelectronbombardment;

some can be resistance degassed

X-rays are produced when the electrons emitted by the

filament impact the grid and support wires. Because of

the geometry of the Bayard-Alpert gauge, only a small

fraction of the emitted X-rays are intercepted by the ion

collector. When the X-rays strike the collector wire they

cause electrons to be photoelectrically ejected from the

collector. This X-ray current limits the pressures that can

be measured, and is equivalent to a pressure reading in

model. The X-ray limit refers to the lowest pressure

indication that may be obtained in a gauge when all the

output current is due to X-ray induced photoemission and

there is an absence of gas.

The X-ray limit will be increased as a result of hydrocarbon

contamination of the electrodes, since contaminated

surfaces release more secondary electrons under X-ray

bombardment. Such contamination can generally be

removed by thorough degassing of the electrodes.