Report by dive guide, Daniele Zanoni
The human ear is capable of hearing sounds in the range of 20Hz and 20KHz. In the inner ear, inside the cochlea, there are several microscopic hair cells. These cells respond to mechanical sound vibrations by sending electrical impulses to the auditory nerve. The brain then processes the information and lets us "hear" the sound. Different cells are responsible for different frequencies. Over time, a part of these cells may get damaged or broken. If the damage is extensive, which means involving a large number of hair cells, hearing loss will result. The high frequency area of the inner ear is the one that is most often damaged by loud sound. In order to understand what loud means, we have to set up a scale and a threshold after which the noise/sound will become dangerous. Sound level is measured in decibels (dB). This number will tell you, in a way, how bigger the sound is compared to the threshold of perception of an average human (0 dB) and it is expressed in a logarithmic scale.
When the sound gets louder than 85dB, your ears might be damaged according to the length of exposure and the intensity of the sound. That means it will take longer to damage your ears at 85dB than at 130dB. When you blow-dry the dust cap of your regulator's first stage by exposing it to the air coming out of your diving cylinder, you will create a noise which can be as high as 90-130dB. At 90dB you are likely to damage your ears if you are exposed for more than eight hours per day. Things change drastically when the sound is louder. Between 110 and 130dB ear damage will occur only after 2 to 15 minutes per day. On a typical dive boat, the drying of the dust cap can happen for more than 2 minutes a day since 15 to 20 people will do that. The total exposure will be longer than 4 minutes and in the long run hearing loss will occur.
When you open the tanks to dry the dust cap, the pressure of the air coming out will drop from the tank pressure to ambient pressure (1 ATA). Typically, after a dive, a tank will have around 50 bar pressure left. When you open the valve of the tank, the pressure will drop from 50 to 1 bar. Since there is a relation between the pressure and the temperature of the gas, the temperature in this case will drop very quickly. If the remaining gas inside the tank is at a higher pressure, suppose 80-100 bar, the changes in pressure, and hence in temperature, will be greater. A tank valve open at full blast for a few seconds may have the temperature to drop below 0°C. At this point, if the air in the tank is not completely dry (which is 0% humid), the moisture will freeze the tank mechanism and when you close it, you risk breaking the mechanism itself.
When you blow-dry your dust cup, the inlet of the first stage is so close to the cap that the water that is blown away from the dust cap will likely enter your first stage. Since we are talking about salt water in the majority of the cases, the inside part of the first stage can be corroded in the long run.
In case of a din connector, do not keep your dust cap attached to your first stage but keep it in a dry place (i.e. the pocket of your trousers or a dry box) so you will not need to dry it. In case of a yoke connector, put your thumb on the inlet of the first stage and use your mouth to blow the water away from the dust cap. Better, keep a piece of tissue in a handy dry place and use that to remove the water.
I hope this article will help a lot of people understanding the danger of creating a high pitch noise and a temperature drop by opening the cylinder when the first stage is not connected to the valve. Have you ever wondered why technical divers drain tanks at a very slow pace when they need a new gas? Now, I guess, you know it.