Guys, i have noticed something related to PSI

(i might be wrong but let’s discuss it)

When they speak about pressing ur cheese @ 40 psi (final press on traditional cheddar) this doesn’t mean to apply 40 pounds on every square inch of our mold surface ..

if i have 6” circle mold this is equal to 28 square inch, if the above is right, then i will have to put 1120 pounds on the top of my press to get 40 PSI, and this is absolutely not correct..

what i just found out, is that when they mention psi , they mean the hydraulic meter pressure which u read it on the pressure meter

Don’t know about your recipes, but I’ve never seen one that calls for psi.  They all call for pounds of weight, and that is NOT psi.

When you find a recipe that states a pressure in psi, it means one of two things.

1. The psi refers to the pressure applied to a hydraulic (liquid operated) or pneumatic (air operated) cylinder that is used to apply pressure to the cheese.  Without knowing the bore size (inner diameter) of the cylinder, there is no way of knowing what the force is that is applied to the cheese.  For example, if you apply 40 psi to a cylinder with a 2 inch bore, it will apply 4 time as much force as 40 psi in a cylinder with a 1 inch bore.

2. The author of the recipe means to state a force (weight) in pounds and refers to psi by mistake.

As I had stated in an earlier thread, many folks misunderstand the distinction between PSI and force (which can be weight.)  In a press that is driven by a cylnder, the total force applied (equivalent to how much weight you would use) is equal to the pressure of the fluid used (which can be liquid or gas) in psi times the surface area of the inner bore of the cylinder in square inches.  To determine the psi applied to the cheese, you take that force applied by the cylinder, and divide it by the area of the follower in your press.

I think that most of the home cheesemaking books indicate the weight to use when pressing, and they are designed for a 2 lb, 4 in (approx) diameter wheel.  There has been a prior discussion regarding how to scale up for larger wheels, and to caluculate the psi to use in a cylinder driven press you use the reverse of the procedure I described above.

Tom, i understood what u r trying to explain, but some times scientific books or sites of cheese making they refer to PSI as i said in my fist post, and i would go (as i said above) to your point number 1

Rich here are examples of PSI in some sites, (search for PSI inside the pages)

http://www.dairyfoodsconsulting.com/recipes_cheddar.shtml

http://www.foodsci.uoguelph.ca/cheese/sectionf.htm

Nabil,  I emailed Peter Dixon, the author of the recipe you first cited to inquire about the psi controversy.  He was referring to the pressure on a hydraulic cylinder, and not on the surface of the cheese.  He mentioned that to make his cheddar, he hangs a 90 lb weight from an 18” lever arm to apply sufficient pressure to his cheese.  I don’t know the math to figure the actual psi, maybe Tom can.  Its not the 1,100 lbs you thought it might be, but its more than 90 lbs for sure.

I think a part of our problem is that we are using different terms and coming from different frames of reference.  I know that if you make a 4” cheese, you will not use the same weight to press as you would with a 12” cheese.  We are merely attempting to come up with a means of calculating accurately the amount of weight or pressure required for each different size.

nice info and as i was expected it in my first post above it is the hydrolic gauge pipe

he hangs a 90 lb weight from an 18” lever arm, to calculate this we need two things:

1- how much is the mold size (to figure out how much pressure)
2- the pivot length, to be more clear, such devices (lever) contains two arms , long and short and point joint.. we need now how much is the short lever arm so we can calculate the force   see this http://en.wikipedia.org/wiki/Lever

cheers