Question

For a laboratory experiment on effusion and low pressure Gas flow, with the goals of measuring the rate of effusion through small apertures, determining average velocities of gases and examine the flow of gas in through a pipe at low pressures , in the viscous and molecular flow.

What are the error sources and possible improvements when undertaking the laboratory protocol?

Answer 1

Generally, efusion of gases at low presure can be studied by the use of apparatus, which consists of the following components such as

1. High vacuum providing system

2. Bulb containing desired gas under study with an initial pressure of 0.1-0.2 torr

3. A pinhole orifice with 0.1 mm diameter

4. Vacuum gauge to measure pressure of the gas in the bulb

5. Expansion system for filling gas in bulb at desired initial pressure

Now, as you know about the experimental apparatus setup, let move forward to your question -

Error Sources :

1. Apparatus not left under vacuum during the entire experimental period.

2. Pressure below 10 before the experiment is not ideal. At least 10 is required before the startup of the experiment.

3. Need of Liquid Nitrogen

4. When one of the valve is in closed position, open all the valves just to ensure that the pump has removed all the residual gas from the chamber. This might be a source of error if some of the residual gas stays inside.

5. The pump should be running even after the experiment. So switching it off leads to errors.

6. Impure gaseous samples.

7. Do not expose to high pressures above 200 , as it leads to oxidation of oil presnt in the pump.

Improvements :

These are the two improvements to avoid all the above mentioned error sources -

1. Admitting Pure Gas Samples :-

The manifold should be completely pumped out and pressure constant at a low value when it is not being pumped. The pressure has to be stabilized to a constant value and allow the gas to effuse through the pinhole and meas and frequently record the pressure as a function of time. For a heavy gas, a small pinhole, every 5-10 seconds might be sufficient near the end of a run . For a light gas, a larger pinhole, and a high initial pressure, every two or three seconds might be appropriate. Record the pressure until it falls to at least 25% of its initial value or until it no longer changes with time.

2. Pumping Out Manifold when Pump is Hot :-

If the pressure is less than 100 microns, the gas can be pumped out through the hot pump by opening two valves and one in shutting position. If the manifold has gas at a pressure of 100 microns or higher that needs to be pumped out, it should be done so directly with the mechanical pump with the effusion pump isolated for a short time.