Vacuum Techniques

In this experiment you will learn about vacuum technology and how to use it for measurements

Experimental procedure

Determine the atmospheric pressure measuring the force needed to open vacuum system

  1. Build the illustrated system:
  2. Consider the available set of masses and give thought to the favorable cross section to have in your vacuum system.

    Figure 1a
    Figure 1b
  3. Hold the weights' holder and press it against the O-ring.
  4. Pump the air from the system, making sure that there is no leak from any of the connectors.
  5. Once the system is under reduced pressure, close the valve connecting the system to the pump.
  6. Let nitrogen flow slowly through the needle valve that is directly connected to the vacuum system until the weight drops. Take a note of the pressure the system was, a moment before the weight dropped.
    • For better results - calculate the expected pressure in which the weight will disconnect before performing the experiment.
    • Based on this first measurement, what are the leading errors in this experiment? Estimate their values and write them down.
  7. Repeat this for several weights.
  8. Fill in the proposed table and plot (in the right units) pressure versus total weight. find the atmospheric pressure and find the O-ring sealing area.
  9. Total Weight Pressure
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Measurement of unknown volume with known volume

Build the illustrated system in the next figure, but before building it measure all the pipes' dimensions so you can estimate your system volume via direct measurements. Make sure that both Pirani and the Piezo pressure gauges are connected and make your system as large as possible (think why the system should be as large as possible).

Pipe number Diameter Length Calculated Volume
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Total: xxx xxx ---
Figure 2
  1. After constructing the illustrated system and closing the valves, the system should be at atmospheric pressure (write this pressure as the initial pressure).
  2. With the two parts of the system still separated, open the pump valve and evacuate the unknown volume. close the valve back again (note that the pressure in the cylinder is still Pinitial).
  3. Open the valve separating the two system parts (the known and the unknown volumes) and wait until it stabilizes. write down the pressure, Pfinal, and its error.
  4. Close the separating valve and pump down the unknown volume as before and repeat the process.
  5. fill the proposed table with the correct units.
  6. Pinitial Pfinal
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  7. Plot Pinitial versus Pfinal and evaluate the unknown volume and compare this result to the direct measurement of the system you have made in the beginning of the experiment.
  8. Do not take your system apart. You will use it in the next section.

Measurement of pumping speed of the rotary pump in a fixed volume.

  1. After you have measured the unknown volume, you will use the same system from the previous part. Make sure you are using the largest volume system configuration. Allow nitrogen to enter the vacuum system and fill it until reaching atmospheric pressure Do not pass the 1 atmosphere limit!.
  2. Open the pump's valve and pump nitrogen out of the system. Every 2-3 seconds close the valve between your system and the rotary pump and measure the dependence of pressure versus time.
    • Note: both pressure gauges are connected and give some readout. Considering the gauges working pressure ranges and the dependence of the pumping speed on the pressure, is one of them more correct for validating the manufacturer declared value? which one? keep tabs on both readouts and clarify this.
    Time Pressure
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  3. Build a graph that will allow you to calculate the pumping speed, S. Compare your result to the manufacturer's value.

Measurement of pumping speed of the rotary pump in a fixed pressure.

  1. Let nitrogen gas flow to your system until the pressure is 1 atm. Do not pass the 1 atmosphere limit!
  2. Disconnect the known volume cylinder and, place instead of it, the mass flow meter (MFM). Make sure that the needle valve of the MFM is closed.
  3. Drain the system by pumping it.
  4. Open gently the needle valve of the MFM and allow air to flow in and wait for the reading to stabilized.
  5. Measure the pressure in your system and the gas flow rate that enters your system.
    • Be careful to take readings only from an instrument that is within range!
  6. Find the pumping speed and compare it to the previous section. Compare your result again to the manufacturer's value.

Measurement of conductivity of different tubes and calculation of the air viscosity

At your disposal, there are 6 tubes in different lengths and inner diameter.

  1. Build the illustrated system with one of the tubes:
  2. Figure 3
  3. Pump the system and allow air to flow in the system via the needle valve connected to the MFM.
  4. Note the MFM reading and both of the pressure gauges readings. Make sure that you are in the linear range of all the gauges.
  5. For each tube, determine the conductivity. Which equation would you use? find how it depends on gas pressure.
    • Did you succeeded in finding the conductivity for every tube?
    • If not, why not?
  6. Compare your results for conductivity of the tubes with the use of equation (14) using the air viscosity value from literature.
  7. Using all of the results from the different tubes, calculate the air viscosity, and validate the tube's diameter fourth power law.
  8. Compare the calculated air viscosity with the experimental value.
Tube Diameter Tube Length SMass Flow Meter PGauge 1 PGauge 2
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