A decompression calculator is a valuable tool used by divers to determine the safest way to ascend to the surface after a dive. It calculates the amount of time required for a diver to ascend without developing decompression sickness (DCS), also known as “the bends.” This condition occurs when dissolved gases, primarily nitrogen, come out of solution and form bubbles in the body’s tissues and bloodstream. Properly using a decompression calculator helps divers avoid this serious condition.
Purpose and Functionality
The primary purpose of a decompression calculator is to ensure a safe ascent by providing a detailed decompression schedule. This schedule tells divers how long to stop at specific depths during their ascent to allow their bodies to safely release the absorbed nitrogen. The calculator uses established algorithms and tables, such as the Bühlmann decompression algorithm or the U.S. Navy tables, to make these calculations.
Formula for Decompression Calculation
The Bühlmann algorithm is commonly used for decompression calculations. It models the absorption and elimination of inert gases from the body’s tissues. Here is a simplified version of the formula:
Pamb=PN2+PH2O+PO2+PHeP_{\text{amb}} = P_{N_2} + P_{H_2O} + P_{O_2} + P_{He}Pamb=PN2+PH2O+PO2+PHe
Where:
- PambP_{\text{amb}}Pamb is the ambient pressure at depth.
- PN2P_{N_2}PN2 is the partial pressure of nitrogen.
- PH2OP_{H_2O}PH2O is the partial pressure of water vapor.
- PO2P_{O_2}PO2 is the partial pressure of oxygen.
- PHeP_{He}PHe is the partial pressure of helium (if applicable).
For nitrogen decompression calculations: PN2=(FN2×(Pamb−PH2O))P_{N_2} = (F_{N_2} \times (P_{\text{amb}} – P_{H_2O}))PN2=(FN2×(Pamb−PH2O))
Where:
- FN2F_{N_2}FN2 is the fraction of nitrogen in the breathing gas (typically 0.79 for air).
- PambP_{\text{amb}}Pamb is the absolute pressure at depth.
- PH2OP_{H_2O}PH2O is typically a constant value (e.g., 0.0627 atm at body temperature).
Inputs for the Decompression Calculator
To use a decompression calculator, you need the following inputs:
- Depth (D): The depth of the dive in meters or feet.
- Bottom Time (T): The total time spent at depth in minutes.
- Gas Mixture: The composition of the breathing gas (e.g., Air, Nitrox), typically represented as a fraction of gases (e.g., FN2=0.79F_{N_2} = 0.79FN2=0.79 for air).
- Ascent Rate: The rate at which the diver ascends, in meters per minute or feet per minute.
Calculation Steps
- Calculate the Ambient Pressure PambP_{\text{amb}}Pamb: Use the depth to calculate the absolute pressure at depth. Pamb=Surface Pressure+(D/10)P_{\text{amb}} = \text{Surface Pressure} + (D / 10)Pamb=Surface Pressure+(D/10) For depth in meters (or appropriate conversion for feet).
- Calculate the Partial Pressure of Nitrogen PN2P_{N_2}PN2: PN2=(FN2×(Pamb−PH2O))P_{N_2} = (F_{N_2} \times (P_{\text{amb}} – P_{H_2O}))PN2=(FN2×(Pamb−PH2O))
- Determine Tissue Loading and Decompression Stops: Use the Bühlmann ZHL-16 model or a similar model to calculate tissue nitrogen loading and required decompression stops.
- Output the Decompression Schedule: Display the decompression stops and times needed at each stop.
Example Calculation
Let’s assume a dive to 30 meters for 40 minutes using air (21% O2, 79% N2). The surface pressure is 1 atm, and the ascent rate is 10 meters per minute.
- Calculate Ambient Pressure: Pamb=1+(30/10)=4 atmP_{\text{amb}} = 1 + (30 / 10) = 4 \, \text{atm}Pamb=1+(30/10)=4atm
- Calculate Partial Pressure of Nitrogen: PN2=0.79×(4−0.0627)≈3.11 atmP_{N_2} = 0.79 \times (4 – 0.0627) \approx 3.11 \, \text{atm}PN2=0.79×(4−0.0627)≈3.11atm
- Determine Decompression Stops: Use tables or an algorithm for specific decompression stop times and depths.
Relevant Information Table
| Input | Value | Unit |
|---|---|---|
| Depth | 30 | meters |
| Bottom Time | 40 | minutes |
| Nitrogen Fraction | 0.79 | – |
| Ascent Rate | 10 | meters/minute |
| Surface Pressure | 1 | atm |
| Water Vapor Pressure | 0.0627 | atm |
| Ambient Pressure | 4 | atm |
| Nitrogen Pressure | 3.11 | atm |
Conclusion: Benefits and Applications of the Calculator
A decompression calculator is an essential tool for divers, helping them to plan their ascents safely and avoid decompression sickness. By using accurate algorithms and taking into account various factors like depth, bottom time, gas mixture, and ascent rate, the calculator ensures that divers follow a safe decompression schedule. This not only protects their health but also enhances their diving experience by providing peace of mind. Whether you are a recreational diver or a professional, a decompression calculator is a crucial part of your diving toolkit.