Thermocouple and Pirani Gauges

One optional, but highly recommended accessory to a double manifold is an active pressure gauge that monitors the pressure in the vacuum line. Since leaks from the line can ruin experiments with air-sensitive compounds, it is critical to be aware of the status of the vacuum at all times. A traditional mercury McLeod gauge can provide accurate readings of pressure, but thermocouple and Pirani gauges can more quickly and... 

A thermal conductivity gauge uses a constant electric current to heat an element whose temperature is a linear function of gas pressure over a limited range. The temperature is typically measured with a thermocouple. In the popular Pirani gauge, a single metal filament is substituted for a thermocouple, and filament resistance is monitored [19]. The range of pressures detected by thermal conductivity gauges is — lO -lO 4 torr, which makes them useful for... 

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. 

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 thermocouple gauge is more straightforward than the Pirani gauge and less complicated electronically. The thermocouple gauge has a thermocouple attached to a filament under constant electrical load, and it measures the temperature at all times. If the filament becomes hotter, it means that there is less air/gas available to conduct heat away from the wire, and therefore there is greater vacuum within the system. 

The use of helium as a probe (or tracer) gas in leak detection is not new. Before the mass spectrometer, it was used with thermocouple and/or Pirani gauges because of the greater thermal conductivity of helium than air. If you list all the attributes of a perfect probe gas, helium obviously does the job ... 

This type of gauge, like the Pirani gauge, uses a filament with a high temperature coefficient of electrical resistance exposed to the gases in the vacuum system. The temperature of the filament, as measured by a small thermocouple connected to a sensitive millivoltmeter and attached to the heated filament, indicates the gas pressure. The range is 0.5 toiT to 0.001 torr. 

Fig. 2. Layout of TVA apparatus [14]. A, Pyrex glass degradation tube B, chromel—alumel thermocouple C, cold water jacket D, removeable lid E, F and G, attachment points for Pirani gauge heads.

Vacuum pressures are measured by a Pirani gauge and a McLeod gauge. The melt temperature is measured from an optical pyrometer through a quartz prism and through a quartz sight glass into an Alundum well which rests on the inside bottom of the crucible. NaK temperatures are determined by thermocouples in capillary wells welded into the NaK pots and quench tank. 

The apparatus is set up as shown in Figure E9-1, p. 529. An all-glass oil diffusion pump should be used so the student can observe how it operates. If either a Pirani or thermocouple gauge is available, it should be connected at I of Figure E9-1. The molecular still used here is a very simple design, is inexpensive, and can be made by almost anyone. Mineral oil should not be used for the oil bath. Its flash point is too close to the distillation temperature to be safe. (An oil bath is used rather than a circular heating mantle so the student can watch the entire operation). It is suggested that two to three spare fuses be taped to the top of the Variacs. 

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