[ARGOS++]

Dear Thorium;

(Thank you for so much, - too much flowers.)

In case you set the Atmospheric pressure = “ambient” Pressure (as you did;  –  quite often the usual application.)   –   then you are correct/right!
Or even more simplified:  The “Maximum Vacuum” is simply the maximal difference in pressure between inlet and outlet the pump can reach.   (I hope that’s more “visual” for you.)
For that you may also read how vacuum is measured:   "Vacuum Gauge”

To your Note: For “High Vacuum” or “Ultrahigh Vacuum” pumps (in real: wrong naming!) “Maximum Vacuum” doesn’t makes any sense, because they never should work on/in an ambient pressure as high as Atmospheric pressure, that means you should always use a “Pre”-pump of mechanical nature in combination. Such LPP pumps are working very near their physical Limits!

Another Hint: You should never use an Oil or “Ultra/High Vacuum” pump without a “Cryotrap” with liquid Nitrogen. That’s especially for chem. Labs (for Distillation, Sublimation, Reactions, etc.) where you have to face from very small till countable product decomposition. You may think about the Solubility of such Gases in Oil, etc. and the consequences for the “Ultimate/Absolute Pressure”
For this you may read on:  "Cryotrap”

To your Selection:  You spent too less information, so “nobody” can make the decision for you.
But some Hints anyway:

• –   I would select a pump with a little lower “Ultimate/Absolute Pressure”, because you have to think about that you will only asymptotic come nearer to the “End”-Pressure of your 4 Torrs, and that will take time. (Also a higher “CFM” may be here of help.) Maybe something like 0.5 till 0.1 Torrs is more adequate, also because the pump will not become younger with time and you may not prefer to call every two months for service.
• –   Also to think about if your applications not in real requires a certain range of pressure like 100 till 4 Torrs, because that will be an additional “imposition” in case of an Oil Pump(for example).
• –   If your ambient pressure is the Atmospheric pressure so “Maximum Vacuum” will be very rarely a problem for “only” 4 Torrs.

How knows, at the end your decision will be a compromise in one or the other direction, not only its prize.
For Vacuum pumps you may also inspect:   "Vacuum Pumps”

And for Vacuum Techniques/Engineering:   "Vacuum Engineering”


Good Luck!
                    ARGOS⁺⁺

[thorium]

Thank you veeery much.

So Maximum Vacuum is just an upper limit and it doesn't make any sense for a pump that's going just 2 Torrs?
Is this right? (I want to be sure because this vacuum costs over $4000 which is a really great money!)

And what's CFM?

Thank you again again and again!

[ARGOS++]

Dear Thorium;

If I assume that in most cases with “Vacuum” you are meaning “Pump” instead, then you are half (maybe more!) wrong!  ─   and half right!

Because > 4000$ are a lot of ….:
Please! ─ Re-read/study my both last posts (#13 & #15) at least once again, with much more care!!

I DON’T sale Pumps!,   ─  And I don’t like to be responsible for your decision under such conditions!!
Not advice!, I gave you hints (including Reasons!), maybe worth to think about!
It is impossible to give Advice if I know NOT anything about application, environment, and staff.

About CFM: Many Thanks to Mr. Dr. Eugene Dakin (Eugenedakin) for the following Link:

"Vacuum Pump Sizing (CFM)”

But take very much care that the graph therein is only an Example, and differs quite a lot from Pump to Pump (mostly not only by Type) and from Manufacturer to Manufacturer!!

Sorry! that your last post is telling such worse!

Good Luck!
                    ARGOS⁺⁺

[thorium]

Ok ARGOS++ don't get angry

You're right! I'm not taking advises "just" reccomendations from you! Because reccomending someone something is a heavy burden... I just want to learn this:

So Maximum Vacuum is just an upper limit and it doesn't make any sense for a pump that's going just 2 Torrs?

I've read what you've wrote several times but I can't figure out anything other than this!

Thanx!

[my_kemical_romance]

okay..

Um im having trouble with charges...

I dont understand what they mean and why they are there...

CAn anyone help me...please?

[ARGOS++]

Dear Thorium;

I never had, have, or will ever have in mind to get angry!,   ─  Not in such situations.

But as I told: I can not feel responsible in any kind for decisions somebody else is doing in situations I don’t know anything about expect 3 values of an imaginary Vacuum Pump!
That’s for everybody else also impossible.

NO!,   ─  Your last statement in its whole is wrong!,   ─  But it is an “upper” Limit!
I feel sad that till yet I was not able to empower you to do an adequate decision for your situation!

Let’s have an additional try with an Example:
(Maybe it’s wise to draw  “A Picture” with a pen and name it correctly.)

• a.)   Let it be a nice springday with an atmospheric pressure of 762 Torrs ( = 30.0 “Mercury) and with a Vacuum Pump that has left a little bit its best Conditions (“Ultimate/Absolute Pressure” = 1.0 Torr; “Maximum Vacuum” = 29.89 “Mercury;  CFM = ~5.0 L/min) with an actual “Maximum Vacuum” of only 28.92 “Mercury.
  There maybe different reasons for that:
  • ─  The Vacuum Pump got aged.
  • ─  The manufacturer had a Lack in the production of your Vacuum Pump.
  • ─  Or the Oil-Vacuum Pump colected solvents or other gases by an earlier Application,   somebody has not use a “Cryotrap”, but that you can get “corrected” by a Service who will exchange the Oil!
  • ─  And so on!
• b.)   Draw near the right border of the paper a “flask” symbolising your Application you like to evacuate and name the pressure therein with p_A and draw next left your Vacuum Pump and connect its inlet with your “flask”, and name the pressure on the outlet of the Vacuum Pump with p_O.
• c.)   For the first calculation let assume this Vacuum Pump has to work at the spring day so we can set p_O = 30.00 “Mercury. What will be the lowest pressure p_A you can get in your “flask”?
  It’s p_A = p_O  - “Maximum Vacuum” =  30.00 – 28.92  = 1.08 “Mercury, but 1.08 “Mercury corresponds to ~26 Torrs, and that’s never below 4 Torrs!
• d.)   But with the Data from the Manufacturer you will get 30.00 – 29.89  = 0.11 “Mercury = ~ 0.5 Torrs.
  So it’s easy possible with the “new” Vacuum Pump to meet its “Ultimate/Absolute Pressure”!
• e.)   What’s with your “aged” Vacuum Pump, have you to draw it away?
  Usually not, because the “Ultimate/Absolute Pressure” of the Vacuum Pump must not (but can) underlay Degradation in the same Magnitude, so you have at least two possibilities left:
  • 1.)   You may call for Service and hope for repair. At least for an Oil-Vacuum Pump you know there is a good chance. 
  • 2.)   For this Solution you have to connect (“draw”) another flask with the outlet of your Vacuum Pump and label it “Buffer”, but keep for its pressure therein still p_O.  Now draw a “cheap”  “Pre”-Vacuum Pump with a Maximum Vacuum of only ~10 – 15 “Mercury, but with a much higher CFM as your Vacuum Pump, and connect its inlet with your Buffer and label the pressure at its outlet with p_Atm. We can now start our calculation, because now   p_Atm  = 30 “Mercury:
    What’s the pressure in the Buffer?   p_O  = 30 – (>10)  = < 20 “Mercury.
    And whit’s now your p_A?    p_A =  p_O - Maximum Vacuum =  < 20 - 28.92 = < 0.0 “Mercury. In these cases the Ultimate/Absolute Pressure will be the limiting factor who still may rest at 2 – 3 Torrs.  But I can not forecast how long it will take to get there. 

Quintessence: 
For every Vacuum Pump who has to “deal” with the Atmospheric Pressure at its outlet “Maximum Vacuum” is an important limiting factor,  but such pumps are usually designed by the manufacturer (Let’s hope!) that in good conditions the Vacuum Pump will easily reach its “Ultimate/Absolute Pressure”.

WOW!,   ─   Finally it has got a very long Explanation, but I hope it has clarified a Lot, and empowers you to made a better decision for your Situation.

Good Luck!
                    ARGOS⁺⁺

P.S.:  I could be possible that I’m a few days “Out of Business”, because of 40.4°C Fever.  Sorry!.
.