Gauge, vacuum Pirani

The adsorption of carbon dioxide or oxygen on praseodymium samples was measured by a constant-volume method using a calibrated Pirani vacuum gauge. Praseodymium oxide was heated in oxygen (4 kPa) at 775°C for 1 h, then evacuated at 750°C for 0.5 h just before the measurement. The sample of praseodymium oxychloride was prepared from praseodymium chloride by heating under oxygen flow... 

Figure 1. Schematic of apparatus A, calibrated variable-volume mercury burette B, reference volume C, main chamber D, mixing pump E, adsorption chamber F, reference chamber G, constant temperature baths H, mercury manometer J, cold-cathode gauge P, Pirani vacuum gauge R, mercury reservoir...

Figure 2 Vacuum pump, Pirani vacuum gauge, and hygrometer in low-humidity room for gravimetric moisture test. 

Thermal Conductivity Vacuum Gauges. A very widely applied gauge of this type is the Pirani gauge. Such gauges consist of a wire (Pt, W or Ni, d = 5-20 pm / 5 cm) mounted axially in a cylindrical tube (d 2 cm). The wire is heated by an electric current to approximately 100°C above the ambient temperature and heat loss occurs by three mechanisms, as indicated in Figure 5.3. 

It would take us beyond the scope of this book to mention aU the vacuum gauges that are manufactured. They are fully described in the various monographs on high-vacuum technique [49]. In the 10 to 10 " range the Pirani gauge is much used and in the 10" to 10 range the ionization manometer. Combinations of these two instruments into a single unit are avaUable. 

Answer by Author About 300 panels measuring 30X40 in. were constructed and tested for vacuum stability when used in refrigerator doors. Each panel was equipped with a Pirani-type vacuum gauge. The vacuum history of all these panels was followed for about four years and several of them for 10 years. To our knowledge, there were no instances of vacuum failure. 

Pirani gauge (vacuum technology) See Vacuum gauge, Pirani. 

Thermistor gauge (vacuum technology) A form of the Pirani gauge (see Vacuum gauge, Pirani) in which the resistor element is a semiconductor material rather than a metal. 

Vacuum gauge, Pirani A vacuum gauge that uses the resistance of a heated resistor element, which changes due to gas cooling, in a Wheatstone bridge arrangement, as an indicator of the gas pressme (density). 

With the rotary and diffusion pumps in tandem, aided by a liquid-nitrogen trap, a vacuum of 10 Torr became readily attainable between the wars by degrees, as oils and vacuum greases improved, this was inched up towards 10 Torr (a hundred-billionth of atmospheric pressure), but there it stuck. These low pressures were beyond the range of the McLeod gauge and even beyond the Pirani gauge based on heat conduction from a hot filament (limit Torr), and it was necessary to... 

These measure the change in thermal conductivity of a gas due to variations in pressure—usually in the range 0.75 torr (100 N/m2) to 7.5 x 10"4 torr (0.1 N/m2). At low pressures the relation between pressure and thermal conductivity of a gas is linear and can be predicted from the kinetic theory of gases. A coiled wire filament is heated by a current and forms one arm of a Wheatstone bridge network (Fig. 6.21). Any increase in vacuum will reduce the conduction of heat away from the filament and thus the temperature of the filament will rise so altering its electrical resistance. Temperature variations in the filament are monitored by means of a thermocouple placed at the centre of the coil. A similar filament which is maintained at standard conditions is inserted in another arm of the bridge as a reference. This type of sensor is often termed a Pirani gauge. 

Fig. 6.21. Thermal conductivity vacuum-sensing (Pirani) gauge (a) Pirani transducer (6) typical bridge circuit for Pirani gauge...

Gauges. Because there is no way to measure and/or distinguish molecular vacuum environment except in terms of its use, readings related to gas-phase concentration are provided by diaphragm, McCleod, thermocouple, Pirani gauges, and hot and cold cathode ionization gauges (manometers). 

The Pirani gauge uses the principle that (usually) the hotter a wire gets the greater its electrical resistance. Therefore, if the resistance of a wire is going up, it must be getting hotter. This relationship implies that less air/gas is available to conduct heat away from the wire, and therefore a higher vacuum is being achieved. 

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