Author Topic: PSI  (Read 675 times)

Offline artguy

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PSI
« on: January 14, 2014, 10:41:14 PM »
Ok so I stumbled upon a thread with some awesome conversion charts and formulas for converting physical pounds to psi for pneumatic presses.  I cannot for the life of me find the  thread again!  I want to be able to convert lbs to psi.  My cylinder is 2" bore with 9" stroke and my mold is 6.5"x7".  Any help? 

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Online smolt1

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Re: PSI
« Reply #1 on: January 14, 2014, 11:21:19 PM »
There are 2 parts to this.

The cylinder converts the pressure( PSI ) in the tank to a force( LBS ) on the plunger . The plunger then presses( with force in LBS ) on the mold which is distributed over the surface creating a pressure ( PSI ) on the curds in the mold.

The formula for the mold is:   pressure ( in PSI ) = force ( in LBS ) / area( in square inches ) of surface of mold.

Offline jwalker

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Re: PSI
« Reply #2 on: January 15, 2014, 05:03:54 AM »
A 2" cylinder has an area of approximately 3.14 square inches , length of stroke doesn't matter.

So just multiply the air pressure used , by the bore size (3.14") , and you have total force exerted on cheese.

100 psi x 3.14 = 314 lbs of total downward force on the cheese (when using 100 psi).

A 7" mold has a surface area of approximately 38.5 square inches.

Total force (314 pounds) divided by 38.5 = approximately 8.155 pounds of force exerted on each square inch of cheese , or 8.155 PSI.

If you figure out the force exerted at 100 psi air pressure , then the rest is easy , using 50 psi air pressure , would give you half that ,  25 psi would be one quarter of that and so on..........

Area calculator is  (a = pi * r2) .

No..........I'm not a professional CheeseMaker , but I play one on TV.

Offline artguy

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Re: PSI
« Reply #3 on: January 15, 2014, 10:53:58 AM »
Can you give me an example if per say I am reading a recipe and it says  use 20 lbs of physical weights? Trying to wrap my brain around the process of converting physical pounds to determine what I should set the regulator to given the volume of the mold.


Online smolt1

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Re: PSI
« Reply #4 on: January 15, 2014, 11:38:52 AM »
In many cheese making books it says press with 20 lbs without saying what size mold. In those recipes the assumption is that you are making a 1 or 2 gallon batch and the mold size is 4 or 4 1/2 inch diameter mold. So that recipe would NOT give you the correct PRESSURE if you made a larger batch and used a larger diameter mold.

With your cylinder( 100 PSI gives you 314 lbs ) to get 20 lbs ( of weight not pressure )  you would need  100 x 20/314 =6.37 psi on the pressure gauge.

correction: "NOT" added
« Last Edit: January 15, 2014, 12:57:25 PM by smolt1 »

Offline artguy

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Re: PSI
« Reply #5 on: January 15, 2014, 12:22:31 PM »
Makes sense.  Thanks!

Offline jwalker

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Re: PSI
« Reply #6 on: January 15, 2014, 12:23:29 PM »
 ;D Yep , what Smolt said !

But this might be an easier way of remembering and adjusting it:

Every 1 psi of air pressure , gives you 3.14 physical pounds of downpressure (with your 2 inch cyl.) , so 20pounds , divided by 3.14 equals 6.36 , or close enough to what smolt said.

Take any physical weight you are planning on using , and divide by 3.14 and you will get the needed psi to achieve that weight.

No..........I'm not a professional CheeseMaker , but I play one on TV.

Offline artguy

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Re: PSI
« Reply #7 on: January 15, 2014, 12:27:52 PM »
but dont I have to take into consideration the diameter of the mold?



Offline jwalker

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Re: PSI
« Reply #8 on: January 15, 2014, 12:50:24 PM »
but dont I have to take into consideration the diameter of the mold?

Yes , but only if you want to know the PSI exerted on the cheese , that last calculation only gives you the total weight exerted and psi needed to achieve it.

A 7" cheese has about 38.5 square inches , 3.14 divide by 38.5 = .08 

so every 1 psi of air you use will give you .08 psi on a 7" cheese.

It sounds complicated . :-X

No..........I'm not a professional CheeseMaker , but I play one on TV.

Offline artguy

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Re: PSI
« Reply #9 on: January 15, 2014, 01:05:04 PM »
so how do I accommodate?  Do I have to multiply by a factor of .02 and add it to the original weight?


Offline artguy

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Re: PSI
« Reply #10 on: January 15, 2014, 01:07:40 PM »
so if the recipe calls for 20 lbs of weight and I am using a 7" wide mold with a 2" bore. I will need 6.5 psi?

Offline artguy

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Re: PSI
« Reply #11 on: January 15, 2014, 01:18:31 PM »
or do I multiply by .92 and add that as a correction factor? 

Offline artguy

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Re: PSI
« Reply #12 on: January 15, 2014, 02:08:58 PM »
Could someone double check my math hear?  This is based on surface area of 4" and 7" molds on a press with a 2" bore


Offline artguy

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Re: PSI
« Reply #13 on: January 15, 2014, 02:10:18 PM »
lets try this file instead