The Pirani Gauge

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. 

The accuracy of a Pirani gauge is typically +20%, although an individual (clean) gauge properly used over a two-year period may show a sensitivity drift of only 2%.53 

One immediate complication of the Pirani gauge is ambient temperature As the room temperature gets hotter, the filament gets hotter, making the gauge read a false better vacuum To solve this problem, a dummy filament is included in the Pirani gauge. The dummy filament is evacuated and sealed off (at a lower vacuum than what is likely to be used with the Pirani gauge) and is used as a standard to help calibrate a zero point. 

An electrical diagram for a Pirani gauge is shown in Fig. 7.47, where V and D comprise the Pirani tube. D is the dummy filament tube that is sealed off, and V is the tube that is exposed to the vacuum system. The filaments in the V tube are connected to a bridge circuit called a Wheatstone bridge with two resistance units called Rj and R2. Power from the power supply passes across the Wheatstone bridge and is adjusted to the proper setting by R3, whose value is read on the mil- 

To set a Pirani gauge, the vacuum on V is set to a pressure lower than what the gauge can normally read. Next Mj is set on its zero point by adjusting the resistance of R2. Thereafter Mj will give proper readings as the pressure is raised to the range of the gauge. 

---

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... 

It is vital to maintain clean contacts on the connection fitting of the Pirani gauge, because it is very sensitive to resistivity changes due to tarnishing or dust. 

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. 

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 advantages of the thermocouple gauge are fairly consistent with the four stated for the Pirani gauge with a few exceptions ... 

The disadvantages of the thermocouple gauge are somewhat different from those of the Pirani gauge ... 

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. 

The thermal conductivity of a gas is the quantity which is measured in the Pirani gauge (page 125) and in the detector of a gas-phase chromatography column (page 171). The thermal conductivity is related to the heat capacity of the gas, which measures the amount of energy that can be absorbed per molecule to the velocity of the molecules, which is a measure of the number of collisions with the heated surface per unit time and pressure and to the pressure of the gas. 

A thermal volatilization analysis of polyvinylacetate degradation has been reported by Gardner and McNeill [200] (Fig. 54). Two maxima are observed at 322 and 435°C. The Pirani gauge situated after the 0°C trap responds to all volatile products, while the gauge situated after the —196°C trap responds only to non-condensable gases. Infrared analysis has shown the presence of carbon monoxide and methane in this last fraction. The... 

When pressures get below about 10" torr (0.013 Pa), McLeod gauges are not practical. There are several types of gauges that can be used. Two of the most common gauges that cover the range from 10 to 1 O torr are (1) the Pirani gauge, and... 

The Pirani gauge was discussed as a device for measuring low pressures. Discuss another type of gauge that can be used. 

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. 

The concept of hot wire anemometry is similar to that of thermal mass flow meters as well as the Pirani gauge to measure pressure a fine wire is placed in a flow stream and then heated electrically. The heat transfer rate from the fluid to the wire equals the rate heat is generated by the wire. 

Valve C is next closed, and A is opened slightly to allow a slow flow through the traps. To hasten pump-down of the system, the traps may be heated slightly with a gas flame. Next, inlet valve A is closed, and C is opened. The Pirani gauges are turned on, and the system is evacuated to below 1 jjL. Then valves C and E are closed. The valves on the trap in use should be open those on the other trap are closed. 

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. 

---