Calculating vacuum -- These numbers do not make sense

  • #1
Ripcrow
82
7
TL;DR Summary
I have built a purpose designed pump. I have a sealed container filled with ambient temperature water and have attached a tube to act as a manometer (water ) One end is attached to the water container and the other end is blocked off.
Should be a simple test to see how much vacuum can be created by the pump but it is lifting the water in the manometer over 40 inches on one side. From what I’ve seen when reading a manometer you have to double the length on one side. But this means it’s achieving 80 inches h2o which is impossible also from what I’ve read.

The manometer maxes out at 40 inches and it will continue to pull the water into the container so I’m not sure where the actual maximum is but the numbers don’t seem right either.

The other variable is I’m using a flexible tube as the manometer so it does squash down which would account for some variation. The container I’m pumping from holds about 2 litres of water and the manometer holds about 400 millilitres. Am I reading the manometer correctly?

The pump was designed to create a strong vacuum but the numbers are supposedly impossible.
 
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  • #2
Ripcrow said:
TL;DR Summary: I have built a purpose designed pump. I have a sealed container filled with ambient temperature water and have attached a tube to act as a manometer (water ) One end is attached to the water container and the other end is blocked off.

it’s achieving 80 inches h2o which is impossible
Hg? H2O?
 
  • #3
Bystander said:
Hg? H2O?
H2O.
 
  • #4
Ripcrow said:
H2O.
Think about it.
 
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  • #5
I have thought about it. The only explanation I have is the flexible tube is forcing the water higher in the tube than what it should be.
 
  • #6
40" of a water column is a weak vacuum (about 0.9 atm). 40" of Mercury (Hg) is impossible since 29.92" would be a near perfect vacuum. Hg density at rt is 13.546gm/cc and pure water is about 1.00 gm/cc. The height of a water column under a near perfect vacuum at the closed end would be 29.92 * 13.546 = 405.3" = 33.77'. Of course, the water in such a long column would boil instantly (at rt) and fill any vacuum with water vapor. The lowest pressure possible would be the water vapor pressure at the temperature of the water column.
 
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  • #7
And this is why nobody uses water in a practical manometer.
 
  • #8
So that 40 inches is the correct way to read the manometer or do we still have to double the distance making the actual reading 80 inches.
At 40 inches online converters give the reading as 74 torr and about 9990 pascals. So that’s about .1 ATM. The pump is removing about 1850 mils of the water volume from a total of 2 litres so the volume is close to 90 % removed
 
  • #9
A simple manometer would be a basic clear tube (glass or plastic) with the bottom open end in a pool of
the liquid. The top would have a port that goes to a vacuum pump. Using Hg for example, the tube could be 32" long, and a strong vacuum would leave a bit over 2" of vacuum at the top. The best you can get is 29.92" distance between the Hg pool height and the Hg height inside the column. Hg works well because it allows
a short manometer and will have almost no vapor pressure at room temp (rt). For water you'd need something about 34 foot tall, and it still would not be accurate because of the boiling mentioned. You are not interpreting the vacuum levels correctly. One can reference compared to a perfect vacuum OR a negative value compared to 1 bar (=1 atm nominal). For lab work, pressure levels compared to a perfect vacuum (0 atm = 0 torr = 0 mbar) are normally used. If you have a crude water manometer, 40" in a single tube would only be 0.9 atm referenced to a full vacuum, or -0.1 atm referenced to the atmosphere.
 
  • #10
I don't recommend you play with mercury as the vapors are toxic and you need to be further along in chemistry/physics. Help from a teacher or someone much more knowledgeable is also crucial.
 
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  • #11
The volume of water does not matter at all. Any diameter clear tube would give the same liquid
height at the same vacuum pressure. Sucking water into commercial mechanical vacuum pumps will destroy them.
 
  • #12
Need to sign off. Research manometers on the web and fully understand how they work before trying
to build one. As mentioned, even a 8.5 foot manometer will only be long enough to show 0.75 atm absolute
pressure or -0.25 atm relative to the atmosphere if water is used. So, kind of pointless for all but very weak
vacuums. Good luck.
 
  • #13
The manometer is sealed to Atmosphere as all pumps have a maximum suction lift of 34 ft due to the atmosphere not being able to lift water any higher. There is a major discrepancy in the numbers. Online converters give 40 inch H2O as 74 torr or .1 of atmosphere.
 
  • #14
Ripcrow said:
I have thought about it.
I don't think you did. I don't think you have properly acknowledged the difference between Water and Mercury.
 
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  • #15
Ripcrow said:
So that 40 inches is the correct way to read the manometer or do we still have to double the distance making the actual reading 80 inches.
Your setup isn't clear to me ("sealed to atmosphere"? Is that opened to atmosphere or sealed from atmosphere?), but the general answer for a manometer is that the reading is the difference between low and high sides. For a u-tube you can put graduations on one side and double the distance from neutral. If there's a large reservoir with a level that doesn't move much you'll have to zero that and use the actual height.

Ripcrow said:
The manometer is sealed to Atmosphere as all pumps have a maximum suction lift of 34 ft due to the atmosphere not being able to lift water any higher.
That doesn't make sense, if read as sealed from atmosphere. If one side is closed the water won't move predictably if there is an air pocket or not at all until a vacuum is pulled (a barometer). Please post a diagram of the setup.

Ripcrow said:
There is a major discrepancy in the numbers. Online converters give 40 inch H2O as 74 torr or .1 of atmosphere.
For a normal manometer I don't see a problem with those numbers but your setup isn't clear to me. In addition to the diagram please describe what you think the problem is/ what you think you should see.
 
  • #16
Rocket scientist 333 said:
And this is why nobody uses water in a practical manometer.
Depends on the application. For low speed airflow(HVAC, low speed aero) it's in a good range.
 
  • #17
FWIW, our lab used a big, centrifugal, water-sealed vac-pump for 'bench' supply: filtration, desiccators, rotary evaporation etc. Partly because it would tolerate eg accidental water ingress. We also had portable 'mechanical' pumps for eg vac. ovens. Must be said, none of our needs reached 'serious' vacuum levels...
 
  • #18
russ_watters said:
Your setup isn't clear to me ("sealed to atmosphere"? Is that opened to atmosphere or sealed from atmosphere?), but the general answer for a manometer is that the reading is the difference between low and high sides. For a u-tube you can put graduations on one side and double the distance from neutral. If there's a large reservoir with a level that doesn't move much you'll have to zero that and use the actual height.


That doesn't make sense, if read as sealed from atmosphere. If one side is closed the water won't move predictably if there is an air pocket or not at all until a vacuum is pulled (a barometer). Please post a diagram of the setup.


For a normal manometer I don't see a problem with those numbers but your setup isn't clear to me. In addition to the diagram please describe what you think the problem is/ what you think you should see.
 
  • #19
I have a tank that is sealed and completely filled with water. . Half filled the tube with water and sealed one end of it and connected the other to the sealed tank with a u tube arrangement. The sealing of one end of the tube stops atmosphere from pushing down on the water forcing it through the tube when lower pressure is available. A pump will lift water 34 ft when atmosphere can act on the water source. When I start the pump it draws water from the bottom of the tank and will pump out 90% of the water in the tank and the water in the manometer will rise 40 inches on one side while the other side is lowered 40 inches. As the water in the tube is lifted on one side by the vacuum created by the pump the water on the other side is lowered against the vacuum created by sealing the tube against atmospheric pressure. Both sides of the tube are acting based on vacuum.
As the tank is emptied to about 90% of original volume the expectation would be that the total vacuum would be 10% of atmosphere less water vapour created by the boiling of water at lowered pressure. If I’m reading the manometer correctly at 40 inches it makes sense but if we have to double the distance then 80 inches of water doesn’t make sense. The water temp is about 20 degrees celcius so it should start boiling at 59% atm or 240 inch h20.
Asking how the manometer should be read correctly. My manometer is maxing out so will have to put a gauge on the system anyway but would like to know if it’s worth doing or do I need to redesign the pumping system.
 
  • #20
If the side of the manometer away from the tank is closed, then there is no way for it to reliably read pressure, since the air pressure on that side will vary. The numbers you are reading from it are meaningless.
 
  • #21
Ripcrow said:
I have a tank that is sealed and completely filled with water.
You have already been asked to provide a diagram of this set up. Your verbal description makes very little sense. Your diagram should follow the same 'rules' as other diagrams of liquid systems so look at some other examples on Google. I suggest using the 'Images' option.
 
  • #22
russ_watters said:
If the side of the manometer away from the tank is closed, then there is no way for it to reliably read pressure, since the air pressure on that side will vary. The numbers you are reading from it are meaningless.
Seems to be something. Sealed manometers read pressure independent of external pressure
 

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  • #23
sophiecentaur said:
You have already been asked to provide a diagram of this set up. Your verbal description makes very little sense. Your diagram should follow the same 'rules' as other diagrams of liquid systems so look at some other examples on Google. I suggest using the 'Images' option.
 

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  • #24
Ripcrow said:
Seems to be something. Sealed manometers read pressure independent of external pressure
Maybe, but you still need to know the pressure on at least one side in order to interpret the reading. Think about it and label the diagram with the heights and pressures. Can you make them add up?

Note: when people say "provide a diagram", the diagram needs to include descriptive labels/numbers.

Also, where did you get that diagram and what do they have to say about it?
 
  • #25
Both sides are sealed at sea level atmosphere (14.7psi ). I’m measuring vacuum created. So the starting pressure is 1 atm and the finish pressure is 40 inches h20 on both sides
 
  • #26
Ripcrow said:
Both sides are sealed at sea level atmosphere (14.7psi ). I’m measuring vacuum created. So the starting pressure is 1 atm and the finish pressure is 40 inches h20 on both sides
Why did you select that particular diagram for an attachment? What does the "26.4cm" represent? The pressure inside the closed portion is given by the total height of the space - not just a part of it. The manometer should start with the tube full. As the test pressure is reduced below AP, the gap at the top will show the test pressure. Are you sure you have understood enough about manometers? Or perhaps you should post a relevant diagram. Does it use water or mercury, btw?
If you are using a partially filled (i.e. with air)space then that air will expand (Boyle's Law) and modify your reading.
But, as I have already pointed out, without a proper diagram of your equipment we can't really help of comment. If you want help then you must make a bit of an effort. Not some random copy and paste from google.
 
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  • #27
Ripcrow said:
Both sides are sealed at sea level atmosphere (14.7psi ). I’m measuring vacuum created. So the starting pressure is 1 atm and the finish pressure is 40 inches h20 on both sides
It's not going to work with both sides sealed. You have to know the pressure on one side to find the pressure on the other.
 
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  • #28
russ_watters said:
It's not going to work with both sides sealed. You have to know the pressure on one side to find the pressure on the other.
There's a rabbit hole somewhere that he has missed / ignored.
 
  • #29
There are lots of manometers that are sealed both sides.
 
  • #30
Ripcrow said:
There are lots of manometers that are sealed both sides.
That’s ok but there has to be a known reference pressure that isn’t AP. You have not given a diagram to explain how you do without AP.
Is this ‘thing’ a secret or is it just a vague idea in your head? Help us please.
 
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  • #31
Fixed I think. Bought a gauge that went as low as -100 kpa and it went well past that and hard up against the stopper at around where the -130 kpa graduation would have been if had been marked. Must have gone well past that level though as when I repeated the test the gauge only got as low as roughly -110 so I’m thinking the gauge was damaged in the initial test or it found a way to admit a small amount of air into the tank , perhaps through the pump seals. Either way it has proven to be more than is required. Thanks for all your input.
 
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