A Simple Guide to Three Key Numbers on Your Dive Computer: GF Values, PO2 Settings, and Tissues 1–16

Do You Know What GF Values, PO2 Settings, and Tissues 1–16 Mean?

First and foremost, we'd like to extend our sincere thanks to GARMIN for providing us with the newly released Descent Mk2S for testing. The Descent Mk2S is slimmer and more compact than its predecessors, and is positioned as the world's lightest dive computer. Its expanded color options also provide consumers with a complete product lineup to choose from. Whether it's a hardware limitation or a deliberate product differentiation, if you want to use the SubWave sonar of the Descent T1 transmitter to read your scuba tank / cylinder pressure, you'll need to go straight for the Descent Mk2i. This article isn't really an unboxing — instead, we've invited dive instructor Lin Yu-Ping of 揪潛水 to share with us three numbers you'll commonly see on a dive computer, and what they mean when planning a dive: GF values, PO2 settings, and Tissues 1–16.

The GARMIN Descent Mk2S launched in June 2021 and is positioned as the world's lightest dive computer, with expanded color options and a complete product lineup for consumers to choose from.

The Descent Mk2S has a slightly smaller watch face than the Mk2/Mk2i, and does not support the Descent T1 transmitter.

Alright, that's enough of the sponsored segment — let's get to the main point of this article!

We'll break down three numbers that divers often see but seldom fully understand: GF values, PO2 settings, and Tissues 1–16. We'll explain what each one means and how it affects dive planning. Before we begin, it's worth noting that all current diving research is fundamentally based on mathematical models and partial human studies developed by organizations such as NOAA (National Oceanic and Atmospheric Administration). These models cannot account for individual differences such as age or physical condition.

As a result, the algorithms built into dive computers are intentionally set to be highly conservative — after all, no dive computer manufacturer wants to assume that kind of liability. But if you don't understand what these numbers actually mean, blindly chasing the most "conservative" settings doesn't make you a safer diver — it just means you probably shouldn't be diving at all.

Note: This article uses dive computers running the Bühlmann ZHL-16C algorithm as examples. Dive computers using the RGBM (Reduced Gradient Bubble Model) algorithm may not be directly applicable.

Key Points of This Article

This article may challenge some commonly held diving beliefs. Our goal is to introduce the theoretical foundations behind these settings, so you can plan your dives safely, meaningfully, and with reduced risk.

Conservatism — GF Values:

Most divers are familiar with the conservatism settings on their dive computer: High, Medium, and Low. Some dive computers also display GF (Gradient Factor) parameters for reference, and even allow you to set your own GF values. Below are GARMIN's default GF settings for each conservatism level:

High Conservatism: 35/75
Medium Conservatism: 40/85
Low Conservatism: 45/95

Decompression theory is difficult to explain in a quick-reference format, because it involves gas dynamics, the decompression models proposed and continually refined by Workman and Bühlmann, M-Values, tissue half-times, the 16 theoretical tissue compartments in the human body, and much more. If you're interested in diving deeper into this topic, we highly recommend reaching out to dive instructor Lin Yu-Ping of 揪潛水 — he could talk about this stuff all day (and he'd enjoy every minute of it). Alternatively, ask your local dive instructor about enrolling in an enriched air nitrox (EANx) specialty course for a more comprehensive understanding of decompression theory. Since this is a quick-reference guide, allow us to skip straight to the simplest explanation of what these two numbers mean.

Dive Trivia: The M-Value was developed by Scottish physician John Scott Haldane, using a set of theoretical tissue compartments to define a mathematical model outlining the "safe maximum limit" — the upper boundary of allowable nitrogen saturation. This forms the foundation of modern decompression theory. The model was subsequently refined by Bühlmann and NOAA into the dive planning tables we use today.

Left value (GF Low): This affects how frequently you'll need to make decompression stops. The lower you set it, the farther you are from your body's maximum allowable residual nitrogen limit — which means decompression stops will be triggered more easily.

Right value (GF Hi): This affects your no-decompression limit (NDL). Setting it to 75 means your new anchor point is located 75% of the way between 1.58 (the leftmost point of the red line) and 0.79 (the leftmost point of the green line). In other words, setting GF Hi to 75 will cause decompression stops to trigger more readily than setting it to 95, because your personal ceiling (the yellow diagonal line) has been lowered. Simply put, this value influences your NDL during recreational dives at depth — the more conservative the setting, the more likely you are to enter a decompression obligation, which brings its own set of complications.

Drawing a diagonal line (the yellow line) between the two new anchor points defined by GF Low and GF Hi creates your personalized dive ceiling. This line still falls below the M-Value maximum (the red line). Your dive computer then uses this chart to calculate how to execute gradient decompression stops between the yellow and green lines. The diagrams below illustrate the difference in ascent profiles (blue line) and decompression stop execution (pink line) between GF settings of 35/75 and 45/90.

Figure 1: GF set to 35/75 — you are required to make multiple decompression stops.

Figure 2: In comparison, with GF set to 45/90, you only need to make one decompression stop under the same conditions — which can actually reduce other risk factors.

Here we'd like to address a common misconception: many divers blindly pursue the most conservative settings, setting their conservatism to High (GARMIN's 35/75), as shown in Figure 1. You'll notice that compared to Figure 2, this requires an additional decompression stop — and more time spent underwater means more exposure to external risk factors, such as running low on gas, hypothermia from extended bottom time, strong surface currents, or becoming separated from the group. More conservative does not automatically mean safer.

Dive Trivia: Some dive computers include a deep stop, which involves pausing at half the maximum depth for a period of time. This is based on the RGBM (Reduced Gradient Bubble Model) decompression theory and aims to help control the size of nitrogen bubbles forming in the body.

Oxygen Exposure — PO2 Settings:

You've probably heard something like "Setting your PO2 to 1.6 puts you at immediate risk of oxygen toxicity!" But according to NOAA guidelines for divers performing heavy work, a partial pressure of oxygen (PO2) of 1.6 is only considered to potentially cause oxygen toxicity after a single continuous exposure of 45 minutes, or a cumulative exposure of 150 minutes over 24 hours. In recreational diving using 32% enriched air nitrox (EANx), you would need to be at 40 m depth just to reach a PO2 of 1.6 — and realistically, how many recreational divers on a single tank can sustain 45 minutes at 40 m? The bottom line: within the realm of recreational diving, setting your oxygen partial pressure to either 1.4 or 1.6 is perfectly safe.

Let's say you're planning a deep dive using 32% nitrox with a PO2 of 1.4, giving a maximum operating depth (MOD) of 34 m. But if you want to explore a wreck at 40 m, you can adjust your PO2 setting to 1.6, raising your MOD to 40 m. Even if you do reach 40 m, the time limits mentioned above (45 minutes single exposure, 150 minutes cumulative over 24 hours) must be exceeded before oxygen toxicity becomes a realistic concern.

But is 1.6 really the absolute human limit? In hyperbaric oxygen therapy — such as in a recompression chamber — the basic Table 5 treatment protocol involves a PO2 as high as 2.8! This is why it's so important to understand the full theoretical picture rather than relying on hearsay.

Source: Hyperbaric and Undersea Medical Society, Republic of China (Taiwan)

Tissues 1–16:

All dive computers in the GARMIN Descent series feature a Tissues 1–16 chart — a genuinely useful display that many divers overlook. Based on the Bühlmann ZHL-16C decompression model, it shows the nitrogen saturation levels across 16 theoretical tissue compartments in the body. (Bühlmann hypothesized that the body consists of 16 distinct tissue types, ranging from fast tissues to slow tissues. Fast tissues absorb and off-gas nitrogen quickly; slow tissues do so more gradually.) The left side of the chart shows the residual nitrogen status in fast tissues, and the right side shows slow tissues.

After a single dive, you'll typically see a higher curve on the left (fast tissues), while the slow tissues on the right remain relatively flat because they haven't had enough time to absorb significant nitrogen. However, after multiple dives, you may find the fast tissue curve has leveled off while the slow tissue curve remains elevated.

Fast tissues on the left; slow tissues on the right.

You can use this chart to monitor your body's residual nitrogen levels — and it's worth remembering that your dive computer doesn't stop calculating when you surface. It continues tracking your off-gassing even after you're out of the water. So what does this mean in practice? Most people are familiar with the pre-flight surface interval rule — dive computers typically recommend waiting 12 or 24 hours after your last dive before flying. But take our test dive as an example: we used a single scuba tank / cylinder for 54 minutes, with an average depth of just 5 m and a maximum depth of 7.3 m. One hour after surfacing, the residual nitrogen levels across all 16 tissue compartments were virtually identical to those of someone who hadn't dived at all. Theoretically, at that point, you could board a flight without concern.

For ease of use, dive computers simply apply a standard 12- or 24-hour pre-flight surface interval for all divers.

After a single 54-minute dive with an average depth of 5 m and a maximum depth of 7.3 m, the Tissues 1–16 residual nitrogen readings one hour after surfacing were virtually the same as those of a person who had not dived at all.

Dive Trivia: Within the realm of no-decompression recreational diving, your body's residual nitrogen levels are low enough that you can ascend to the surface without needing to follow a formal decompression procedure. This means a safety stop is not the same as a decompression stop — it is not performed to off-gas nitrogen in order to meet decompression requirements before surfacing. Consider this: at the surface, the ambient nitrogen partial pressure is only 0.79, whereas at 5 m depth it's 1.185 — so you actually off-gas nitrogen faster at the surface than at 5 m. That said, safety stops do offer a benefit: if you've been diving at depth, making a safety stop helps moderate your average ascent rate and can reduce the discomfort of reverse squeeze during ascent. Of course, if you encounter an emergency — such as a gas leak, feeling unwell, a lost dive buddy, or deteriorating surface conditions — the safety stop can be skipped to reduce additional risk factors. (If you're on a decompression obligation, however, you must follow the procedures indicated by your dive computer.)

Once again, many thanks to GARMIN Descent Mk2S for giving us the opportunity to test and share this dive computer. All models in the GARMIN Descent series provide audible and vibration alerts for events such as ascent rate warnings, PO2, CNS/OTU, NDL, decompression, and gas switching. You can also customize up to 40 audible or vibration alerts based on depth or time — features that can significantly enhance diving safety.

As always: every dive computer uses an algorithmic model to estimate values. In this article, we've shared a straightforward overview of common dive computer parameter settings based on theoretical foundations and computational models. If you still hold the view that "everyone's body is different, so the most conservative setting is always the safest," then honestly — not diving at all is the safest option. If you're interested in decompression theory, we highly recommend reaching out to dive instructor Lin Yu-Ping of 揪潛水 or asking your local dive instructor about taking an enriched air nitrox (EANx) specialty course. This article is written from a recreational diver's perspective as a brief, simplified overview. In technical diving, divers adjust their parameters far more flexibly when planning decompression dives — if that world interests you, it's well worth exploring!

Thank you to the GARMIN Descent dive computer series for supporting this article.

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