The following are examples of pressure change when you scuba dive.
As you descend, the volume of gas inside your mask will decrease due to increasing pressure. Add gas to the mask to make the volume consistent by blowing out enough gas through your nose to equalize it.
As you descend, you will add gas to the BCD to control your descent. Also, your exposure suit has gas in it—either in the neoprene material of a wetsuit or in the gas within the suit itself for a dry suit. So, any gas in your exposure suit will compress, making you more negative. You will master how to deal with this and achieve neutral buoyancy using your BCD in your confined water session.
The opposite happens during your ascent due to the decreasing ambient pressure. Gas in both your exposure suit and your BCD will expand, and it needs to be vented to control your ascent. It is important that a diver should never ascend faster than 9 meters/30 feet per min. The reason why slow ascent rates are important will be explained in detail in the “Physiology” unit.
Ear equalization is necessary during descent. The volume of gas in the air space in the middle ear will decrease as the ambient pressure increases. This can cause discomfort and must be equalized (just like being in an aircraft and feeling the pressure in your ears). This will be explained in much more detail in the “Physiology” unit.
As previously explained, lungs need to be equalized by continuous breathing. Not doing so can cause serious injury. Remember to never hold your breath while scuba diving and to continuously breathe.