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?

 A: 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!
A: 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.
A: 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. 
