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  1. Plat

    Radiometer rotation speed as function of pressure

    I have considered using drag to measure pressure as you describe, but I think at pressures as low as I’m working with, drag would be negligible compared to the torque produced by even a small motor. I am interested in measuring in the range of 1Pa to 0.001Pa.
  2. Plat

    Radiometer rotation speed as function of pressure

    I am experimenting with using a radiometer as an approximate indicator of pressure in my homemade high vacuum system, running a small turbo pump. I am interested in the relationship between pressure and vane rotation speed, with light intensity being constant. I have only been able to find...
  3. Plat

    Turbo pump - vibration and noise at a certain RPM

    I have picked up a used pfeiffer turbomolecular pump and controller for pretty cheap. It runs like a champ, completely smooth and quiet except for when it’s first starting up. As it approaches 5500 rpm, it starts vibrating somewhat and sometimes produces a momentary scraping noise when the...
  4. Plat

    Calculate the strength of several polycarbonate sheets laminated together

    I have been using simple lacquer thinner to wet the surface of one sheet and then press another against it. My understanding is that this solvent causes the polycarbonate from each piece to fuse together, leaving no foreign material or adhesive. I have done deflection testing with a 20cm beam...
  5. Plat

    Calculate the strength of several polycarbonate sheets laminated together

    Based on dimensions and the materials' Young's modulus, I know how to calculate the expected deflection of a rectangular sheet of material under a given distributed or point load and given various edge support conditions. I have discovered however that multiple thinner sheets when bonded...
  6. Plat

    Calculate the thickness of a plate to withstand a specified uniform pressure

    The full thickness of the sheet will be comprised of two thinner sheets solvent laminated together. Thus, the chamber will have an inner and outer layer of polycarbonate, with the inner and outer layers alternately overlapping each other at the corners so that the force under pressure is never...
  7. Plat

    Calculate the thickness of a plate to withstand a specified uniform pressure

    I agree however I am not an engineer, that's why I'm asking.
  8. Plat

    Calculate the thickness of a plate to withstand a specified uniform pressure

    In exploring the feasibility of constructing a vacuum chamber, I am trying to calculate the thickness of rectangular polycarbonate sheet needed to withstand 100kPa of pressure, given dimensions of 30.5cm by 61cm (12 by 24 inches) and clamped edges. I have found some calculators that will tell...
  9. Plat

    Properties of RF harmonics

    How can the power/amplitude of a particular RF harmonic be calculated? I would assume it is some well-defined fraction of the amplitude of the main frequency? Do RF harmonics from a square-wave drive appear only on the even, odd, or both, multiples of the base frequency? How do high-frequency...
  10. Plat

    High-frequency transformers - how are they different?

    That's very very helpful, thank you. Are there any inherent/unavoidable losses as frequency increases that cannot be 'tuned' away in the transformer's construction? For example, could a transformer that was properly designed to operate at 200kHz be just as efficient as one that was designed to...
  11. Plat

    High-frequency transformers - how are they different?

    How does the core material affect the frequency it can be used at? Is the advantage of ferrite its lower magnetic permeability? And is this an advantage because it results in lower inductance and therefore lower inductive reactance? I don't know how that could make sense because even with a...
  12. Plat

    High-frequency transformers - how are they different?

    I apologize for being somewhat scatter-brained here, I just have too many questions on this topic. I know high-frequency transformers use ferrite cores instead of laminated silicon-steel, but what other differences are there? Does their reactance still follow the standard formula: 2*Pi*F*L ...
  13. Plat

    Mains transformer with 1.4 ohm primary winding?

    I had no idea inductance played such a big role here. Thank you for the explanation and I'll be committing that equation to memory.
  14. Plat

    Mains transformer with 1.4 ohm primary winding?

    That makes perfect sense but I have measured the primary windings of other mains transformers with the same meter and got reasonable values of around 200 ohms. I wonder why this one is different? Just larger as far as I can tell.
  15. Plat

    Mains transformer with 1.4 ohm primary winding?

    I don't understand how this is possible. This is a rather large transformer in a stereo system amplifier. The transformer is a simple step-down with 120V mains primary to a center-tapped secondary providing both 38V and 76V. According to ohm's law, at 120V this thing should draw about 85 amps...
  16. Plat

    Low pressure barometer

    Never heard of that device. Looks perfect for this but I wouldn't want to mess with the mercury and it's unfortunately too expensive for my little hobby. Here is that video. Doesn't address the issues I have had with the barometer but shows it in use.
  17. Plat

    Low pressure barometer

    I have found the source of the out-gassing I originally mentioned to be from porosity in the plastic where I had sealed the top of the tube. it's amazing how much gas can appear to come from nowhere when it is expanding ten thousand times or more. I decided in this case that a barometer would...
  18. Plat

    Low pressure barometer

    Did a couple more tests. My original testing was with a clear plastic tub. First I used a glass column that had a small hole near the top(from a previous experiment), which I patched with resin. Using this, the oil actually dropped on its own, instantly! But having the patched hole there was...
  19. Plat

    Low pressure barometer

    The bubble's size does respond to changes in pressure. The tube is 7.9mm id(5/16 in), stands 12cm tall, and I have marked 1cm increments in height from the bottom. I can probably discern 2mm changes in level. The oil surface shows a noticeable concave meniscus. With the size of the bubble...
  20. Plat

    Low pressure barometer

    Has anyone ever done this before? For additional testing I have placed a radiometer with the glass envelope punctured into the chamber that the barometer is t-ed into. The Crookes Radiometer will not start to spin until the pressure is below about 300Pa, which is what my barometer reads when...
  21. Plat

    Low pressure barometer

    I see that but I am removing/introducing air bubbles and standing the barometer upright while under vacuum the whole time. The pressure which the barometer is experiencing does not change before or after introducing the bubble. I just tip it sideways so the bottom edge of the tube reaches the...
  22. Plat

    Low pressure barometer

    That makes sense, but I am removing all air pockets when it is already under vacuum. I will try leaving no bubbles in the column and see if the fluid will drop given enough time. That's what I see happening when I allow a small bubble to enter the bottom of the column. It rises and as soon as...
  23. Plat

    Low pressure barometer

    The pump is rated down to 5Pa, but I will only get close because of imperfect sealing. I put a container of cold water into the system to test whether it would boil, which indicates approximately the pressures I am reaching, being sub 1kPa. The only role the vapor pressure of the oil plays is...
  24. Plat

    How do you run vacuum pumps in series?

    True but I figured for his application, turbomolecular pumps were not on the table.
  25. Plat

    How do you run vacuum pumps in series?

    The first thing that comes to mind is running one pump in a vacuum chamber that was evacuated by another pump. But then you have no cooling airflow so it wouldn't run long. Also this probably wouldn't reduce your ultimate pressure because those pumps already operate at the lower limit of what...
  26. Plat

    Low pressure barometer

    I'm not sure what is going on. I have tried it t-ed in to the chamber with boiling water approaching 0*c and the temp dropping as it boils, so I know I am getting into the sub 1kPa area. The column does track the the vapor pressure of the water at the measured temperature, but reads about 25%...
  27. Plat

    Low pressure barometer

    The oli is a well-known brand, so I think there must be some contamination, water perhaps. I have gently heated the oil for a while to drive off any foreign substances and it as behaving much better actually. The column height is more stable but still slowly creeps down however. I have also...
  28. Plat

    Low pressure barometer

    I am working on a barometer that will operate t-ed in to an experimental vacuum chamber. The fluid column is composed of vacuum oil so it doesn't boil. I have calculated that given the oil's density of 0.92g/cm3, each cm of oil height indicates 90 pascals of pressure. The oil has a vapor...
  29. Plat

    I Measuring vacuum w/ hot filament - calculation (pirani gauge)

    Inadvertently, I have found another interesting thing about gas at low pressure. There looks to be a specific pressure below which thermal conductivity of air decreases suddenly and dramatically. In the chamber, I was running a small fan with a piece of tissue paper taped to the front, so I...
  30. Plat

    I Measuring vacuum w/ hot filament - calculation (pirani gauge)

    This is what I came up with. When I mentioned earlier the sudden change between 20Pa and 7.9kPa, that looks to be due to a mismatch in data between using the sponge and not. For this data, I used the rough vacuum gauge, boiling ethylene glycol, water, and 70% by volume isopropyl alcohol...
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