Alphatron is a type of cold cathode ionization gauge and can also be considered as a radioactive ionization gauge. As the cold cathode and hot cathode types earlier explained, are composition dependent, the transfer characteristics may be obtained relative to air for different gases and the system can be used as a leak detector.

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The device uses alpha particles in order to ionize the gas in the vacuum chamber. The number of ions formed in the chamber is directly proportional to the gas pressure, if the chamber dimensions are shorter than the range of alpha particles. The figure below shows the schematic diagram of an alphatron.

Alphatron Vacuum Gauge

The ions produced by the alpha particles are collected by the collector electrode and a current between 10^-13 and 10^-9 Amperes will flow though the resistor R. The output voltage e0 is measured using a high input impedance output meter. The device has a range between 10³ to 10^-3 Torr.

A quartz reference gauge is a device used to measure vacuum. The working principle is pretty much same to that of a bourdon tube. Here, 2 bourdon tubes are used and a formed into a helix. When a pressure difference between the two occurs, the setup begins to rotate. This rotational deflection is picked up using an optical circuit as show in the figure below.

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The reason for using quartz is that it has good spring characteristics and if the unit is kept at a constant temperature environment, the angular deflection per unit pressure is repeatable. The main disadvantage with the device is that it cannot be used as a vacuum gauge in gases with fluorine content as this erodes quartz.

The rotational deflection is connected into an electronic signal, after it passes through the optical circuit. This electronic circuit is further amplified and then the output is annulled using a servo-control system. The corresponding output is displayed by analogue techniques or counted digitally, which can be directly in pressure units. With a tachogenerator on the servomotor shaft a damping adjustment facility can be provided, if necessary. The device is known to have a resolution of 1 milliTorr for 100 milliTorr full scale reading.

The working of a thermocouple type vacuum gauge is very similar to that of a pirani gauge. The only difference is that the hot wire temperature is measured directly with a thermocouple which is attached to a wire. For different pressures, the temperature is measured by the fine-wire thermocouple, the hating current being initially fixed by the resistance as shown in the figure. This device is usually used for comparison purposes. The sensitivity of such an instrument depends on the pressure and the wire current.

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The figure below shows the working of thermocouple gauges for comparison purposes. Two sets of thermocouples are used to measure temperatures of heater wires in the two chambers and oppose each other. When there is a difference in pressures, there occurs an unbalance which is measured by a potentiometer circuit. Instead of a single thermocouple per wire, a thermopile is often chosen to increase sensitivity. The thermocouple gauge is also composition dependent and needs empirical calibration for the high vacuum range.

A basic pirani gauge consists of a fine wire of tungsten or platinum of about 0.002 cm in diameter. This wire is mounted in a tube and then connected to the system whose vacuum is to be measured. The temperature range is around (7-400) degree Celsius and the heating current is between (10-100) mA.  A bridge circuit is also used for greater accuracy. The pirani gauge is connected as one arm of the bridge circuit. The figure is shown below. Vacuum measurement is usually taken in three ways.

• When the pressure changes, there will be a change in current. For this, the voltage V has to be kept constant.
• The resistance R2 of the gauge is measured, by keeping the gauge current constant.
• The null balance of the bridge circuit is maintained by adjusting the voltage or current. This change is made with the help of a potentiometer and the change brought will be a measure of the pressure produced.

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An additional reference gauge can also be used in the adjacent arm of another pirani gauge, in the bridge circuit. The additional gauge is evacuated and sealed, which helps in the compensaton for variation in ambient temperature. For commercial use, the range of the instrument can be extended from 10^-3 Torr to 1 Torr.

McLeod Gauge is a vacuum gauge that uses the same principle as that of a manometer. By using the pressure dividing technique, its range can be extended from a value of 10^-4 Torr. The basic principle is called the multiple compression technique. It is shown in the figures below. If there are two bulbs A and B connected with the McLeod and test gauges through capillary tubings, the pressure on the right hand side of the test gauge is very small and the capillary connection between T and bulb B very long, then the flow law can be written as

V.dp2/dt = K.(p1-p2)

V- Volume of the bulb

dp2/dt – Pressure Gradient in time between the two elements

K – Flow conductance in the capillary.

As p2 is very small when compared to p1, the flow rate remains practically constant and is proportional to the pressure. This forms the basis of the calibration.

There are many variations of the McLeod Gauge. The basic construction is shown in the figure below.

Working

The gauge is used to compress a small quantity of low pressure gas to produce a readable large pressure. Bulb B of the gauge is attached to capillary aa’. The mercury level in the gauge is lowered up to l1 by lowering the reservoir, thereby allowing a little process fluid to enter B. By raising the reservoir, the gas is now compressed in the capillary aa’ till mercury rises to the zero mark in the side tube and capillary bb’. The capillary bb’ is required to avoid any error due to capillary.

The McLeod gauge is independent of gas composition. If, however, the gas contains condensable material and during compression it condenses, the reading of the gauge is faulty. The gauge is not capable of continuous reading and the scale is of square law type. For linearizing the scale at comparatively higher pressures, a second volume is introduced as shown in the figure below, where the scale shown is linear.

For pressure measurement below atmosphere or vacuum, different gauges are available. Manometers and bell gauges can go up to 0.1 Torr. Diaphragm gauges are usable up to a pressure of 10-3 Torr. For pressure below this value, electrical gauges like Pirani or Ionization Gauges are used. Vacuum measurement is broadly classified into Mechanical Type, Thermal Type, Ionization Type, and Radiation Vacuum Gauge.

• A standard manometric type mechanical gauge that is used down to 10-4 Torr is the McLeod Gauge.
• Thermal type vacuum gauge can be further divided into Pirani Gauge and Thermocouple Type Vacuum Gauge.
• Ionization Gauge can be divided into Hot Cathode Type and Cold Cathode Type.
• One type of radiation gauge called Alphatron is explained.
• Other type of vacuum gauge includes the Quartz Reference Vacuum Gauge.

Thermal Types

The heat conductivity of gases is independent of its pressure, at normal pressure. But, heat conductivity starts falling as the pressure is lowered t 10 Torr and below. The reason behind this is less collision between gas molecules within the wall and also their small number in a specific volume. The energy is carried to the walls of the container due to this collision. Thus, lesser number of molecules will be available to take the heat away from the source.

At low pressures, the heat loss that occurs from a hot wire mounted in a glass or metal tube is due to the following factors.

• Convection
• Conduction through the lead mines
• Radiation
• Conduction in the gas

Out of these, convection is comparatively negligible. A new clear wire has a small surface emissivity and is god for producing high temperature at low gas pressures. Due to oxidation and carbonization, the surface tends to deteriorate. This causes error of the device at low pressure ranges.