To study the variation of resistance of a hot filament as a function of pressure
Pirani Gauge, Barometer, rotary pump with a needle valve, vacuum meter.
A Pirani gauge is nothing but a filament of an electrical bulb fused to a glass tube with provision for evacuation. It is one of the common pressure indicators used in high-vacuum systems. The Pirani gauge exploits the fact that the rate of heat conduction between two surfaces by gas molecules is proportional to the pressure, provided that the mean free path of the molecules is larger than the separation of the surfaces. The gauge consists of an electrically heated wire surrounded but not touched by a sleeve. This arrangement is mounted within the vacuum chamber or pumping line. The temperature of the wire is determined from its resistance and hence the rate of heat loss by conduction through the gas to the sleeve is computed. The typical measuring range of Pirani gauge is from 100 to 10-3 mbar, which is ideal for measuring the pressures attainable using a rotary pump.
1. Make the necessary vacuum and electrical connections as shown in Fig.1a and Fig.1b.
2. Before starting the experiment, note down the atmospheric pressure ‘P’ from the Fortin’s barometer, which is also the pressure inside the glass tube.
3. First find the resistance of the filament at atmospheric pressure by null deflection method using the thumb wheel set up. Its operation is given below. Fig.1b, is a simple Wheatstone’s network P = Q = 400 Ω. S is the actual thumbwheel (variable arm) and RF is the Pirani gauge filament. Connect the DC 12V power supply to the terminals provided. Adjust the thumbwheel for null deflection in the galvanometer and read off the units of the thumbwheel which gives directly the resistance of the filament RF at that particular pressure.
4. Close the needle valve fully.
5. Switch on the rotary pump and wait for sometime to get enough forevacuum. Now switch on the vacuum meter.
6. Adjust the needle valve for the pressure in the system to be about 0.5 Torr. Now find the resistance of the filament as described earlier in part 3. Similarly adjust the needle valve for various pressures in small steps from say 0.5 Torr to 10-2 Torr or even less (till the ultimate vacuum of about 10-3 Torr is obtained) and each case
Find the resistance of the filament ensuring that the pressure inside the system (glass tube) is constant during the measurement time.
Pressure, P (Torr)
Draw a graph between RF and ln P
i) Why do you plot a graph between RF and ln P rather than RF and P.
ii) How does the resistance vary at pressures lower than 10-3 mbar. Can you use Pirani Gauge for measuring vacuum produced by a diffusion pump?