Pressure measurement ionization gauges

Hot-Cathode Ionization Gauges. For pressures below approximately lO " Pa, it is not possible, except under carehiUy controlled conditions, to detect the minute forces that result from the coUision of gas molecules with a soHd wall. The operation of the ion gauge is based on ionisa tion of gas molecules as a result of coUisions with electrons. These ions are then subsequendy collected by an ion collector. Ionisa tion gauges, used almost exclusively for pressure measurement in high, very high, ultrahigh, and extreme ultrahigh vacuums, measure molecular density or particle dux, not pressure itself. 

If the pressure in the system is measured by an ionization gauge, the pumping speed SG of this gauge must be added to the pumping speed of the pump. In some cases it is necessary to take into account also the pumping speed Sw = —Fw w hich is due to the adsorption on the system walls and can even differentiate to some extent between the individual components. At the beginning of an experiment F0 = St and Fa0 = StP t/Pt. 

Total pressure, required for detailed interpretation of the mass spectra, is determined with an ionization gauge (S). The gas inlet system (A, B, C) is used for calibration purposes. The relation between measured total pressure and the ion current of an injected specific gas permits calibration of the mass spectrometer in absolute partial pressure units or amps/torr. 

Fig. 22. Thermobalance for vapor pressure measurements. Schematic drawing of experimental equipment. A-Knudsen cell B-cold trap C-Ionization gauge D-Balance and housing E-Diffusion pumps F-Thermostatically controlled reaction chamber...

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg ... 

When the sputter-ion pumps are installed one should ensure that the magnetic fields will not interfere with the operation of other devices (ionization vacuum gauges, partial pressure measurement units, etc.). Mounting devices for the sputter-ion pumps may not short circuit the inductance flow and thus weaken the air gap inductance and pumping speed. 

It is not enough merely to reach a low pressure it must also be possible to measure it. Ionization gauges are almost always used for this purpose. In these the residual gas is ionized, collected at an electrode, and the resulting current measured. The current varies linearly with the gas pressure down to about 10 11 torr. If the ions are separated by mass —making the gauge a mass spectrometer —then the partial pressures of various gases in the vacuum chamber can be determined. 

Pressure-Measuring Device. Accurate pressure measurements are made with a manometer if needed. In most instruments the carrier gas, hydrogen, and air (for flame ionization detector) are provided with separate gauges. 

With both molecular beams in operation, the pressure in the main chamber is 1 x 10 Torr. The maximum rise in pressure due to the secondary beam source is 3 x 10 Torr for HCl, while for the other gases it is 1 x 10 Torr or less. For the scattering box arrangement with a pressure of 2 x 10 Torr of reactant gas in the main chamber, the pressure in the oven chamber is 3 x 10 Torr of which 1 x 10 Torr is attributable to the reactant gas. The quoted pressures are uncorrected ionization gauge measurements. 

Ionization gauges measure the ion current produced by the passage of electrons from a heated wire to an anode maintained at a positive potential. The positive ions produced in the gas by collision with these electrons are attracted to a grid maintained at a negative potential. The ion cuiTent is a function of the gas pressure. 

The emitter, or heated wire, of an ionization gauge will be permanently damaged if it is exposed to gas pressures greater than 10 toiT. Some means of measuring pressures of more than 10 torr should be provided. 

---