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Ionization Gauge – Hot Cathode Type

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Ionization Gauge is a device that is used to measure vacuum. In the hot cathode type, a column of gas is introduced into which, a potential difference V is applied with free electron in the space. This causes the electron with a charge e to acquire a kinetic energy Ve. If the pressure range of the gas in the column goes below a certain limit, called the critical pressure, then corresponding to a voltage larger than the critical voltage Vc, the energy Ve may be high enough to initiate ionization, and positive ions will be produced when the electrons collide with the gas molecules.

The value of Vc is smallest for cesium (3.88V) and largest for helium (24.58V), among monoatomic gases or vapours. For diatomic gases like N2, H2 and so on, it is roughly about 15V. This is known as the ionization potential and at this potential the pressure is also important.

At very low pressures, during the intervals of time for transit from the cathode to the plate in a vacuum chamber, more than one collision is unlikely for an electron. Then for a fixed accelerating potential V>Vc, the number of positive ions formed would vary linearly with the value of pressure. Thus, a determination of the rate of production of positive ions for a given electron current should give a measure of the pressure.

Working

The construction of a hot cathode type ionization gauge consists of a basic vacuum triode. The figure of an external control type hot cathode gauge is shown below.

External Type Ionisation Gauge
                   External Type Ionisation Gauge

The grid is maintained at a large positive potential with respect to the cathode and the plate. The plate is at a negative potential with respect to the cathode. This method is also known as the external control type ionization gauge as the positive ion collector is external to the electron collector grid with reference to the cathode. The positive ions available between the grid and the cathode will be drawn by the cathode, and those between the grid and the plate will be collected by the plate.

The internal control type is shown below. Here the grid is the positive ion collector and the plate is the electron collector.

Internal Type Ionisation Gauge
Internal Type Ionisation Gauge

One of the most popularly used hot filament gauges for industrial applications is the Bayard – Alpert type filament gauge. It consists of a helical grid with a potential of +150 volts. This huge potential attracts the electrons and thus causes gas ionization. At -30 volts, the gas ions are attracted to the central ion collector, thus producing an ion current of 100mA/Torr. This value is then fed to the electronic systems to be amplified and displayed.

The hot cathode ionization gauge is useful in measuring the total pressure of all the gases present in the system. The biggest advantage of this device is its very small response time. This is because of the devices small inertia. The device is used for pressure measurement between the ranges of 10-8 to 10-3 Torr with an output current varying between 10-9 to 10-4 A.  But this range depends on the gas, other things remaining constant.

Where the pressure is higher than 10-3 Torr, the positive ions make a greater impact on the cathode to heat it up and ultimately destroy it. At pressure ranges below 10-8 Torr, in external control type, the electrons impact over the grid and radiates soft x-rays, which results in the production of electrons from the plates as secondary emission. These electrons produced will be of the same order as that of the positive ion current in the plate circuit and thus neutralizes this current. Thus the internal control type is known to be a better option to measure pressure as low as 10-9 Torr.

When the cathode remains at very temperatures (say 3000 deg C), the gaseous matters present inside may reset with the filament or with themselves particularly at different pressure stages. This may causes the device to produce wrong outputs and may also affect the cathode life. During extreme conditions of high temperatures and low pressures, the presence of any gases inside the device, will be forcefully released, thus causing the pressure to increase. Thus, the electrodes have to be properly treated before use. This can be done only by passing high currents through the electrodes, especially the filament and the grid and by high frequency heating of the plate. To overcome these problems, the cold cathode type ionization gauge is also used by many.

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