Question

2. A table in Wikipedia notes that the “High end of lung pressure, exertable without injury by a healthy person for brief times,” is 19.3 kPa.

a. Is this number a gauge pressure or an absolute pressure? How do you know?

b. Suppose you are snorkeling off the coast of the Bahamas, and you want to modify your standard snorkel by adding a length of PVC tubing so that you can go to somewhat greater depths and still breathe through the snorkel. What is the longest length of snorkel you can use (in cm, to three significant figures) and still be able to “clear” the snorkel by forcefully exhaling to expel the water if it accidentally fills with water?

Answer 1

a) It must be gauge pressure because absolute pressure = atmospheric pressure + gauge pressure

atmospheric pressure is 101.325 kpa, so if the value is absolute pressure it must be greater than 101.325 but it is 19.3 kpa less than 101.325 kpa so the given pressure is gauge pressure

b) pressure = density of fluid * acceleration due to gravity * length of the tube

from the above equation, we can find the length of the tube if density and snorkel pressure are given.