# Get an intuition for pressure values

I am currently working with pressure differences in the milibar range. However I do not have a good feeling for pressure differences. In every day life:

• Where does a pressure of say 100 mbar over atmospheric pressure occur?
• When I blow up my mouth, what is the pressure I get?
• When I blow up an air balloon, what is the pressure inside?
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Your mouth or balloon blowing are very bad examples, the reason is because there is no obvious reference.

A McDonalds straw is a better example. Say that it's 7 inches long. You know the density, you know gravity.

$$P=\rho g h$$

In this case, the full 7 inches will get you about 17 mbar. From my everyday experience and intuition, I believe that I can maintain a pressure in my mouth or blowing up a balloon several times what is required to suck up a (watery) drink the full length of a straw.

100 mbar is likely to overwhelm your mouth, but it might just be "athletic". In order to test it, you'll only need a few pipes from an aquarium store, a tub, and maybe some clamps (unless you have more specific equipment). Looking at some anecdotal information online, I see some people reporting in the neighborhood of 2 psi (137 mbar), corresponding with reports that "it felt like my eyes were about to pop out of my head".

Remember, safety first!

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One millibar ($\text{mbar}$) is aproximately the pressure you feel on your hand when you hold a penny.

A penny has mass $m = 0.0025\;\text{kg}$ and diameter $D = 0.019\;\text{m}$ making the pressure

$$P = \frac{m\;g}{\pi D^2/4} = 86\;\text{Pa}$$

and $100\;\text{Pa} = 1\;\text{mbar}$.

So if you stack 116 pennies you will get $100\;\text{mbar}$.

You could try with other coins, or household items, as long as you know their weight, or mass, and the contact area. A marble, for example, is not a good idea because the contact area depends on the load applied.

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That's a great example for general physics instruction. I lol at the prospect of 116 stacked pennies, but still very helpful. An instructor could even have students feel some pressure on their hand w/ the pennies and experience the same pressure in their mouth using a straw. I feel like that kind of learning would be helpful. – Alan Rominger Jul 14 '11 at 3:04

A different way to get a feeling for pressure differences is a vacuum cleaner. Sometimes the possible vacuum is printed in the manual. A good one should provide a vacuum of 100-200 mbar below atmospheric pressure if you put your hand in front of the tube. With a piece of cardboard closing the tube you can get a feeling for the force that this small pressure difference can create.

To get 100 mbar over atmospheric pressure you can submerge a ballon or other container in water. In a depth of 1 m you have a hydrostatic pressure of 100 mbar.

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