Memorandum Background: Results and Discussion:

Transcription

1 Memorandum To: Dr. Abercromby From: Jason Rapp and Matthew Stumbo Date: January 17, 2012 Subject: Vacuum Chamber Basics Technical Memo Background: We explored several aspects of vacuum chambers, in preparation for the more advanced subjects we plan on studying. Beginning with a small plastic chamber (capable of achieving a very low vacuum of only ~36-65 torr) we tested the effects of a vacuum chamber on everyday objects. In the second session we measured the function of the chamber itself, specifically its rate of depressurization and the correlation to the length of the conductance element between the chamber and the roughing pump. Lastly we measured the mass loss effect on Kapton tape exposed to a vacuum for a period of 24 hours. Results and Discussion: With the small plastic vacuum chamber we performed several basic experiments with everyday materials. The basic procedure for the plastic vacuum chamber was constant between each object in the chamber. Begin by placing the object in the chamber, and holding the chamber sealed by hand. Pump air out with the attached syringe. After a few pumps the pressure differential should hold the chamber closed and it no longer needs to be held in place. Pump the syringe times to maximize the pressure differential and observe the effect on the object in the chamber. To depressurize the chamber, loosen the nut that holds the tubing to the apex of the chamber. Initially a mostly deflated balloon was used to observe vacuum effects. In a vacuum it is expected that the remaining air in the balloon will expand the balloon until the pressure differential and tension in the balloon material were in balance, which is what we observed in the tied balloon. In the untied balloon the air could escape as easily as from the rest of the chamber, so it maintained its original shape, without inflating. Marshmallows also exhibit interesting behavior in a vacuum, as you would expect from the large amount of air injected into the marshmallow to puff it up. In the draw stroke of the syringe the marshmallow expands to keep forces balanced. After the draw stroke, the marshmallow slowly shrinks to its regular size as air escapes through the porous surface. Once the vacuum chamber is opened, the air crushes the marshmallow, reducing its size substantially. The stale marshmallow did not

2 Pressure (PSI) The next object of experiment was a suction cup. When the vacuum is pulled, the pressure in the chamber reduces to and meets the partial vacuum pressure between the suction cup and the surface to which the suction cup is stuck. When the pressure in the chamber drops below this equilibrium, the suction cup should fall off the surface to which it was attached. This effect was observed in the experiment. The gummy bear showed little to no evidence of outgassing when placed in the vacuum chamber. We measured a 0.01g drop in mass (in a 3 gram gummy bear), which is most likely measurement error, so it is unlikely that the gummy bear outgassed. Another test was on the effects of the partial vacuum on liquid water. To explain: temperature at which water boils is determined partially by pressure. Boiling occurs when the individual energy of the molecules in the liquid gain enough energy to leave the liquid. When a gas is present, this energy is higher due to pressure balance. When we remove this gas (place the liquid in a vacuum chamber) the boiling point is reduced because the pressure balance is lower. This was tested in water in the small vacuum chamber, to verify this effect. As observed, water heated to roughly 50 degrees Celsius boiled at around 100 torr (11 pumps, with a rolling boil at 14 pumps which corresponds to a pressure of around 80 torr). If the vacuum were stronger by a factor of two (dropped to around torr) the Clausius Clapeyron relation 1 states (as an approximation) ( ) (1) Where L is the specific latent heat, R is the gas constant, and T is the temperature. C is a constant of integration. Using this Relation of syringe pumps to measured pressure 12 approximate form, we find that if we halve the pressure, the boiling 10 temperature will drop roughly 15%. Figure 1 shows the result of placing a pressure gauge in the vacuum chamber, to correlate the pressure in the chamber and the number of pumps on the syringe. Note that the pressure in the chamber asymptotically approaches a non-zero PSI. This is the steady state of the Number of Pumps system based on the rate at which we pump air out, and the rate at which air leaks into the chamber. With a better pump or a better seal, this could be improved It is possible to calculate the pressure in the chamber without using a pressure sensor in the chamber, simply measure the weight of the chamber before and after pulling a vacuum. The Figure 1. Relation of syringe pumps to measured pressure. Measured in the small plastic vacuum chamber, shown is absolute pressure, not gauge pressure.

3 mass lost is the weight of the air missing, from which you can calculate the pressure in the chamber. Measuring this mass lost, the mass difference was 0.17g. The next experiment conducted involved the determination of the pump speed of a vacuum system at the vacuum chamber. Two trials were run to evaluate the effect of different values of conductance. The first trial utilized a long pipe and the second used a short pipe. To run the trial, the pipe was connected to the pump and the chamber and the pump was turned on. The pressure at the chamber and at the pump was recorded at 5 seconds intervals until the pressures stabilized. the measured values of pressure, conductance, and the manufacturer s value for the pump speed were used to determine the pumping speed at the chamber. Figure 2 shows the conductance for both the short and long pipes as a function of time. As the pressure decreases the conductance also decreases. There is a slight bump in the graph where the flow transitions from viscous to molecular. Once the flow becomes molecular then the conductance remains constant. The conductance and pump speed at the pump were then used to calculate the pump speed at the chamber. Figure 3 shows the pump speed for both the long and short pipes. At first the pump speed drops off sharply. There is a bump in the graph where the flow transitions from viscous to molecular. The pump speed eventually becomes constant as the system reaches equilibrium. Figure 2. Conduction as a function of time. The conductance of the pipe decreases as pressure decreases. Figure 3. Pump speed at chamber as a function of time. The pump speed decreases exponentially, at a rate dependent on pipe length. The conductance is high in the beginning because the pressure is higher. The conductance decreases with the pressure because it becomes harder to evacuate the chamber as the pressure decreases. This same relationship holds true for the pump rate at the chamber. As the pressure decreases it becomes harder for the pump to pull air out of the chamber. There were some other notable relationships in this experiment. The longer pipe had a lower conductance and a lower pump speed at the chamber. Both of these can be explained by the fact that the longer pipe has more inefficiencies than the shorter pipe. The curves and extra length in the pipe reduces the conductance and makes it harder for the pump to pump out air. This results in a higher final pressure in the chamber and a lower chamber pump speed with the long pipe.

4 Table 1 Kapton mass loss results for first lab section Initial Mass (g) Final Mass (g) % Mass Lost Piano (Clean) % Piano (Dirty) % Film (Clean) % Film (Dirty) % The last experiment conducted dealt with mass loss due to outgassing. Layers of Kapton were placed in a vacuum chamber for 24 hours to allow for material outgassing. Two different types of Kapton were used, Piano Kapton and film Kapton. One sample of each type was kept clean and another sample was made dirty. The clean samples of Kapton were handled with extreme care to ensure that they were not contaminated in any way. The dirty samples were made dirty through several means, including but not limited to: handled with bare hands, rubbed across one s face, rubbed on the ground. The mass of each of the four samples was then weighed and placed into one of two vacuum chambers (one for the clean samples and the other for the dirty samples). The chamber was then evacuated of air and the samples were heated to approximatley 125 degrees Celsius. After 24 hours the samples were weighed again to determine the mass lost. The early lab section had the expected results. Over time both types of Kapton outgassed material and lost mass. Our lab section did not have typical results. All types of Kapton lost an insignificant amount of mass. Table 1 has the results for first lab section with the percent mass lost for each sample. Table 1 shows that, for the first lab section, the dirty Kapton outgassed a higher percentage of the initial mass than the clean Kapton. However the dirty Piano Kapton did not outgass a very significant amount. There are several reasons why this may have happened which will be discussed later with the results from the second lab section. It is not too surprising that the dirty Kapton outgassed more material than the clean Kapton. The dirty Kapton had other substances on the surface that have a lower activation energy. It is easier for these substances to leave the surface of the Kapton than what is inherently in clean Kapton because the residence time is smaller. The second lab section had unexpected results due to improper lab setup procedure. All the Kapton samples lost negligible amounts of mass. The main reason for this is temperature. The residence time of a substance is very sensitive to temperature. The samples of Kapton for the second lab section were not heated to the proper temperature. This yielded a higher residence time and less material was outgassed. The primary drivers for the cost of a vacuum valve are the precision to which it s crafted. Lower precision means there are more gaps in the seals which introduce leakage into the system. A gate valve is used to isolate parts of a vacuum chamber from other parts. Gate valves are frequently used to isolate sensitive equipment, such as cryopumps; or for isolating sections of the chamber, allowing movement of objects within the chamber. Angle valves are preferred in situations where having low conductance is good or at least acceptable, such as

5 depressurizing the chamber or near the roughing pump. Since angle valves are smaller than gate valves they are also often used when compactness is valued. Figures 4 and 5 show the schematic layout for the two chambers used in the lab. Figure 4. Vacuum chamber schematic for Kermit (Big green vacuum chamber). Figure 5. Vacuum chamber schematic for the student chamber (Silver twins). The silver twins (Fig. 5) don t use gate valves, as they only produce a medium vacuum, and have no need of isolating a cryopump. Kermit, however, uses a gate valve to isolate the cryopump from the atmosphere, and only opens when the chamber is under vacuum. Conclusions The lab was a good introduction the basics of vacuum chamber functionality, and working with vacuum chambers as lab tools. Several vacuum chamber metrics were measured, such as the conductance loss in the pumping system and pumping rates for the chamber and roughing pump. Also basic mass loss in kapton film through outgassing was measured, along with performing some basic experiments with everyday objects exposed to a partial vacuum. Error did not play a major part of the lab, as our conclusions were primarily qualitative. Sources of error include measured dimensions of the vacuum chamber, and systemic error in the convectron gauges. Lacking the error in the pressure measurement makes it impossible to propagate that error in our results. References: 1 Wark, K., "Generalized Thermodynamic Relationships", Thermodynamics (5th ed.). New York, NY: McGraw-Hill, Inc, 1986