speak mix man
bottom mix: The gas mix (air, enriched air, trimix, or heliox) used during the working and/or deepest portion of a dive.
dalton's law: The ideal gas laws, in particular, Dalton's law, which states: "The total pressure exerted by a gas mixture is equal to the sum of the partial pressures of the component gases in the mixture." This has great utility for technical divers and is used frequently in calculating the essential operational characteristics of a given mix. A common form of this equation can be expressed as:
Pg = P x Fg
where Pg is the partial pressure of a gas, g, and is equal to the total pressure, P, of the gas mixture times the proportion or fraction of the gas in the mixture, Fg. Uses of this formula include:
1. Calculating the partial pressure of a gas, for example PO2 or PN2, given the working depth in pressure and the fraction in the mix.
2. Calculating the proper fraction of gas, Fg, to be used in a in a mix, given the working depth and required partial pressure.
3. Calculating the Maximum Operating Depth (MOD) of a mix, from an oxygen toxicity perspective, given the oxygen fraction (FO2), as discussed below.
enriched air: An oxygen-nitrogen mixture with an oxygen content (fraction) greater than that found in air (21%), typically ranging 30-50%; also referred to as nitrox, enriched air nitrox (EAN), and oxygen-enriched air.
equivalent air depth (EAD): Given a planned working depth, the equivalent air depth, or EAD, of an enriched air mix is the depth at which the nitrogen partial pressure would be the same if the dive were conducted on air. The EAD principal states that only this nitrogen partial pressure need to be considered when calculating a decompression. The benefit of EAD is that it allows standard air tables to be used when diving enriched air. An EAD can be calculated as:
imperial: EAD =(1- FO2) x (D/33 + 1) x 33 - 33
.79
relative FO2 working conversion to fsw
in the mix depth (atm)
metric: EAD = (1- FO2) x (D/10 + 1) x 10 - 10
.79
relative FO2 working conversion to msw
in the mix depth (bar)
Example: The EAD of an EAN 36 mix at a working depth of 90 f/28 m is: (0.81 x 3.7 atm) x 33-33 = 65 f/20 m.
equivalent narcotic depth (END): Given a planned working depth, the equivalent narcotic depth, or END, of a specific mix is the depth at which the partial pressure of nitrogen (i.e., the narcosis level or goon-factor) would be similar if the dive were being conducted on air. Typically, the END of a trimix is typically set at 100-150 f/31-46 m, depending on the operation. END (fsw) can be simply calculated as:
imperial: END = ( FN2) x (D/33 + 1) x 33 - 33
.79
relative FN2 working conversion to fsw
in the mix depth (atm)
metric: END = ( FN2) x (D/10 + 1) x 10 - 10
.79
relative FN2 working conversion to msw
in the mix depth (bar)
Example: The END of a trimix 14/50 at a working depth of 300 f/92 m is: 0.45 x 10 atm x 33-33 = 115 f/35 m.
gooned: What you get from breathing air beyond about 200 f/61 m, and running a high END (see above).
heliox: The standard oxygen-helium breathing mixture used in deep commercial and military diving, in which helium is substituted for all of the nitrogen found in air to eliminate narcosis and reduce breathing gas density. For deep dives in excess of 800-1000 f/246-307 m, compression rates (rate of descent) are generally reduced, and narcotic gases such as nitrogen and sometimes hydrogen can be added back into the basic heliox mix to ameliorate High Pressure Nervous Syndrome (HPNS), though this is rarely done in the field.
Following the European convention (see explanation under trimix x/y below), heliox "x"&endash;for example, heliox 12&endash;specifies a mix with 12% O2, balance He. Though not commonly used in technical diving because of the expense, heliox will likely become a standard breathing mix for use in closed-circuit systems which offer a hundredfold improvement over open-circuit in terms of gas utilization efficiency.
intermediate mix, a.k.a. decompression mix: The gas mixes used during the decompression phase of a dive. Today, most gas diving operations follow a planned decompression procedure of switching to one or more enriched air nitrox mixes during decompression, followed by pure O2 (or EAN 80) at the 20 f/6 m and 10 f/3 m stops in order to eliminate any helium as soon as possible, and maximize allowable oxygen partial pressures. Enriched air and/or oxygen are often used for decompression during extended air diving.
maximum operating depth (MOD): The maximum operating depth, or MOD, of a mix is the maximum depth that the mix can be used at and still maintain a safe working oxygen partial pressure. For surface-oriented mix dives, the working PO2 is normally set at about 1.4 atm, which can be sustained for the duration of most scuba-based runs.
Note that oxygen limits are sometimes extended as high as 1.6 atm or more for limited duration runs and for decompression. The MOD in feet of seawater (fsw) for a specific mix can be calculated by using a variation of Dalton's law, PO2 = P x FO2, where P is absolute pressure in atmospheres. Rearranging terms, the formula becomes:
imperial: MOD = (max PO2 -1) x 33
FO2
metric:: MOD = (max PO2 -1) x 10
FO2
Example: The MOD of air is found by setting the PO2 to 1.4 atm, FO2 to .21, and solving for depth; 1.4 = ((D/33)+1) x .21, or D = 186 f/57 m. At a PO2 of 1.6 atm, the MOD for air is 218 f/67 m. Hence, rounding up, 190-220 f/58-67 m represent the reliable limits of air, depending on the duration, from a CNS toxicity perspective.
mix: Mix, sometimes called special mix, or simply gas, applies to diving gases other than air, and includes trimixes of oxygen, helium, and nitrogen, mixtures of oxygen with other inert gases (helium, neon, nitrogen), and to oxygen-enriched air mixes. The mixtures delivered by closed-circuit rebreathers are special mixes as well. Commercial divers universally refer to oxygen-helium mixtures&endash;heliox&endash;as "mixed gas," so the term is not quite right for technical diving.
oxtox: An oxygen toxicity incident (see below).
oxygen tolerance units (OTUs): A unit used to quantify the exposure to oxygen for the purposes of monitoring tolerance to whole-body toxicity. One OTU is about equal to an exposure of 1 minute to 1 atmosphere of oxygen, but is slightly more when above 1 atm., and less when below, the threshold being 0.5 atm. OTUs can be calculated as:
OTU = t x ((PO2 - .5) /.5).83
where t is the duration of the exposure in minutes.
oxygen toxicity: Oxygen is toxic to the human body at elevated partial pressures above 0.5 atm. The severity of oxygen's toxic effect depends on the dose measured in partial pressure, and the duration of the exposure. Of primary concern to technical divers is central nervous system (CNS) toxicity which can occur at partial pressures of about 1.4 atm and above, and result in convulsions that resemble grand mal epilectic seizures.
A second form of toxicity is whole-body toxicity resulting from very long exposures to oxygen partial pressures above 0.5 atm, and is characterized by chest pain, temporary lung impairment, and other effects. Whole body toxicity becomes an issue during saturation diving and long multi-day diving operations and is generally not a problem for technical divers.
travel mix: A gas mix, typically carried in stage bottles, used on descent prior to beginning the working portion of a dive. Travel mix can be used to provide decompression advantages in conjunction with special tables, and/or to avoid problems of hypoxia (too little oxygen) near the surface when using a bottom mix with a low oxygen fraction (FO2).
trimix x/y: An oxygen-helium-nitrogen mixture generally used for dives in excess of about 190 f/58 m, in which the oxygen fraction (FO2), typically less than that found in air, is set to provide a safe oxygen partial pressure at the planned working depth, and helium is substituted for a portion of the nitrogen to reduce narcosis and decrease gas density. Although in US commercial diving circles, the inert gas fraction is generally specified first , we prefer the European convention of specifying the oxygen fraction (the percentage of oxygen in a mix, FO2) first, followed by that of helium, as oxygen safety and awareness is a key issue in most technical diving operations. In this convention, trimix x/y is a mix containing x% O2, y% helium, the balance being nitrogen.
wah wah: What you hear when you are very, very deep on air. Just say nah nah.
x-pert: An individual who used to be a spert. There's a lot of them around in diving.