Subsurface Bug: Garmin TTS FIT Import Problem

Introduction to the TTS FIT Import Issue

We're diving into a specific bug report concerning the Subsurface dive logging software, version 6.0.5494-cicd-release, running on Fedora 43. The issue centers around the import of FIT files, particularly those containing technical dives from a Garmin Descent mk2i dive computer. Users have observed that the Time To Surface (TTS) values displayed after importing these files appear to be "very strange", suggesting a potential misinterpretation or calculation error within Subsurface when processing this specific data. This article will explore the details of the reported problem, the potential implications for technical divers, and the steps being taken to address it.

Understanding Time To Surface (TTS) in Dive Planning

Time To Surface (TTS) is a critical metric in dive planning and execution, especially for technical diving. It represents the minimum time a diver must spend ascending from their current depth to the surface, including necessary decompression stops, to avoid decompression sickness (DCS). Accurate TTS calculations are paramount for safety, and any discrepancies can lead to dangerous situations. In the context of dive computers and logging software like Subsurface, the TTS is derived from the dive profile, including depth, time at depth, and previously completed decompression obligations. When a FIT file is imported from a device like the Garmin Descent mk2i, Subsurface relies on this data to reconstruct the dive and present key information, including TTS, to the user. The observation of "strange values" for TTS in this specific scenario raises immediate concerns about the reliability of dive planning based on the imported data.

The Garmin Descent mk2i and FIT File Data

The Garmin Descent mk2i is a sophisticated dive computer known for its advanced features, including air integration, multi-gas support, and technical diving capabilities. Like many modern dive computers, it records dive data in the FIT (Flexible and Interoperable Data Transfer) file format. FIT files are designed to be a standardized, efficient, and interoperable way to store sensor data, including GPS, heart rate, and, crucially for divers, detailed dive profiles. The accuracy and completeness of the data recorded by the Descent mk2i are generally well-regarded. However, the interpretation of this data by third-party software, such as Subsurface, can sometimes reveal nuances or specific data structures that require careful handling. The reported issue suggests that Subsurface might not be correctly interpreting a particular aspect of the TTS data as exported by the Descent mk2i in its FIT files, particularly for technical dives which often involve more complex decompression profiles than recreational dives.

The Specific Issue: Strange TTS Values

The core of the problem lies in the unexpectedly strange values observed for Time To Surface (TTS) after importing a FIT file from the Garmin Descent mk2i into Subsurface. While the exact nature of "strange" isn't detailed, it implies that the displayed TTS is either unrealistically short, excessively long, or exhibits illogical jumps or inconsistencies. For technical dives, which often push the boundaries of no-decompression limits and involve multiple, precisely timed decompression stops, an accurate TTS calculation is non-negotiable. If Subsurface is miscalculating or misinterpreting the TTS data from the FIT file, it could lead to a diver planning a dive based on flawed information. This could manifest as skipping necessary decompression stops, leading to an increased risk of DCS, or conversely, planning excessive conservatism that is inefficient and tiring. The visual representation of this issue, as hinted by the accompanying image (though not directly displayed here), would likely show unusual patterns or numbers in the TTS-related sections of the dive profile within Subsurface.

Reproducing the Bug: Steps and Context

To effectively diagnose and fix the TTS import bug, the steps to reproduce it are crucial. The user has outlined a clear procedure: import a FIT file containing a Tec dive from the Garmin Descent mk2i. Subsequently, the user is instructed to check the TTS value for the deco ceiling. This implies that the issue might be specifically related to how Subsurface calculates or displays TTS when decompression stops are involved, rather than a general TTS calculation. The presence of a "deco ceiling" in the check further narrows down the potential source of error. It suggests that the algorithm within Subsurface responsible for interpreting decompression obligations and projecting the remaining time to surface might be encountering an anomaly when processing the specific data structure or values related to deco stops as provided by the Garmin FIT file. Understanding the exact data points within the FIT file that correspond to these deco ceilings and the associated TTS calculations is key to pinpointing the root cause.

Technical Details: Subsurface, Platform, and Dive Computer

Let's break down the technical components involved in this bug report. The Subsurface version is 6.0.5494-cicd-release, indicating a recent build of the software. The platform is Linux, specifically Fedora 43. This combination is important because software behavior can sometimes vary across different operating systems and versions. The dive computer model is the Garmin Descent mk2i, a device known for its advanced dive logging capabilities. The FIT file format is the bridge between the dive computer and the logging software. The problem arises when Subsurface attempts to parse and interpret the TTS data within the FIT file generated by this specific Garmin model, especially for technical dives. Potential areas of investigation include:

• FIT File Parsing Libraries: Is Subsurface using an up-to-date and robust library for parsing FIT files, and are there any known issues with its handling of specific Garmin data fields?
• Decompression Algorithm Discrepancies: While Subsurface has its own decompression algorithms, it often relies on data provided by the dive computer. Is there a mismatch in how the Garmin Descent mk2i calculates or encodes decompression stops and TTS, and how Subsurface interprets it?
• Data Field Interpretation: Certain fields within a FIT file might have different interpretations or expected value ranges. The TTS or related decompression data might be formatted in a way that Subsurface's current parsing logic doesn't anticipate.

Potential Impact on Technical Divers

The implications of incorrect Time To Surface (TTS) calculations for technical divers cannot be overstated. Technical diving, by its very nature, involves dives that exceed the limits of recreational diving, often requiring complex gas mixtures, extended decompression stops, and meticulous planning. The primary goal is to manage the risk of decompression sickness (DCS), which can have severe consequences, including paralysis or death. If a dive logging software like Subsurface displays inaccurate TTS values derived from a Garmin Descent mk2i's FIT file import, it undermines the diver's ability to plan and execute their decompression safely. A diver might unknowingly plan to ascend too quickly, missing crucial decompression stops, or alternatively, spend an unnecessarily long time at decompression stops, leading to increased nitrogen loading or oxygen toxicity risks. This issue, therefore, has direct safety implications and requires prompt attention to ensure that technical divers can rely on the data presented in Subsurface for their planning.

The Importance of Accurate Decompression Data

Accurate decompression data is the bedrock of safe technical diving. This includes not only the calculation of required decompression stops but also the estimated Time To Surface (TTS). The TTS provides a real-time or post-dive overview of the decompression commitment, allowing divers to assess their situation and make informed decisions. When importing dive logs, especially from sophisticated devices like the Garmin Descent mk2i, divers expect the software to faithfully represent the data recorded by their computer. Any deviation, such as the reported "strange values" for TTS in Subsurface when importing FIT files of technical dives, can introduce uncertainty and risk. This problem highlights the critical need for dive logging software to have robust and accurate parsing capabilities for various dive computer formats, ensuring that decompression algorithms are correctly interpreted and presented. The integrity of the dive log directly impacts the safety and confidence of the diver for future dives.

Conclusion and Next Steps

This bug report regarding the Time To Surface (TTS) calculation when importing FIT files from a Garmin Descent mk2i into Subsurface highlights a critical area for investigation. The observed "strange values" for TTS, particularly in technical dives involving deco ceilings, pose a potential safety concern for technical divers. The development team will need to scrutinize the FIT file parsing routines within Subsurface, paying close attention to how decompression-related data is interpreted from Garmin devices. Comparing the data exported directly from the Garmin device with how it's processed and displayed in Subsurface will be a key step. Ensuring that the decompression algorithms used by Subsurface correctly interpret the data provided by the Garmin Descent mk2i is paramount. As the Subsurface community relies on accurate data for safe diving, resolving this issue promptly will be a priority. For further information on dive logging best practices and safety, you can consult resources like the Global Underwater Explorers (GUE) website.