You have freshly packed the scrubber.

Everything tight. Everything tested.

Und trotzdem bleibt die Frage: 

Is my circuit really CO₂-safe?

Many experienced rebreather divers are familiar with this uneasy feeling. This is precisely why a concept was discussed to ensure greater safety:

The CO₂ prebreathe test

But what is the practical benefit of this test? And why can it give a false sense of security?

What is a CO₂ prebreathe test?

The idea behind the test is simple:

You breathe for about five minutes at the surface on the rebreather - before descending.

The aim: to determine at an early stage whether the CO₂ scrubber is working or whether there is a critical fault.

Ideally, you will notice symptoms such as shortness of breath or discomfort if CO₂ is not absorbed correctly - and abort the dive before it becomes dangerous.

The most comprehensive study on this topic to date was conducted by Deng et al. (2015).

A controlled laboratory experiment was carried out to test how reliable the Prebreathe test actually is.

The study design:

• 20 erfahrene Taucher

• Drei Testbedingungen:

  - Normaler Scrubber

  - Teilweise defekter Scrubber (absichtlich falsch montiert)

  - Scrubber komplett entfernt

• Die Probanden wussten nicht, in welchem Zustand sie sich befanden (Blindtest).

→ Find out if they recognize CO₂ problems within five minutes.

Deng et al. not only recorded the subjective symptoms, but also physiological data:

• EtCO₂ (endtidales CO₂)

• Atemfrequenz und Volumen

• Minutenventilation

Beim teilweise defekten Scrubber stieg der PI CO₂ (CO₂ in der Einatemluft) leicht an. Doch die Taucher kompensierten dies unbewusst durch tieferes Atmen der EtCO₂ blieb stabil.

The problem:

Without rising EtCO₂ there is no alarm pain, no discomfort, no warning.

The body does not respond - even though the system is not functioning safely.

In the "scrubber missing completely" variant, the CO₂ value in the body rose rapidly.

Most of the test subjects noticed this through symptoms such as:

• Atemnot

• Schwindel

• Druckgefühl im Kopf

• Benommenheit oder Angst

But not all of them. Here, too, five out of twenty felt "normal" - despite having very high values.

The researchers found that

Some divers barely reacted physiologically - even though their CO₂ levels rose well above the safe limit.

The study was carried out on the surface - under the best conditions.

But the problems get worse underwater:

• Erhöhte Atemwiderstände

• Steigende Gasdichte

• Physische Belastung

• Ablenkung

A partially defective scrubber that still "works" on the surface can become a deadly trap under water.

Interestingly, the study showed that

An increase in CO₂ alone does not automatically trigger warning symptoms in all people.

Some test subjects did not react at all or even reacted paradoxically:

→ Rising CO₂ values, but falling respiratory rate.

→ No noticeable discomfort - despite measurable hypercapnia.

This may be due to individual tolerances, habituation, training level or psychological factors.

What does this mean for your everyday diving life?

✅ The prebreathe test can reveal gross errors, for example if you have forgotten the scrubber.

❌ But: It gives you no guarantee of CO₂ safety during the dive - especially in the case of insidious errors such as

• Falsch gepacktes Absorbermaterial

• Kanalbildung

• Leichte Undichtigkeiten im Kreislauf

Das sind deine wirklichen Schutzmaßnahmen um CO₂-Vergiftungen zuverlässig zu vermeiden, hilft kein kurzer Atemtest sondern nur Sorgfalt und Wissen:

• Sorgfältiges, standardisiertes Packen

• Regelmäßige Wartung und Checks

• Training zur Erkennung von Hyperkapnie

• Verständnis der eigenen physiologischen Reaktionen

• Kritische Einschätzung der eigenen CO₂-Toleranz

The CO₂ prebreathe test is no substitute for real safety measures.

It is a supplementary check - nothing more.

Don't rely on a ritual that gives you a false sense of security.

The best CO₂ avoidance doesn't start in the rebreather, but in your head - with knowledge, respect and discipline.

Source:

📚 Deng et al. (2015): “Effectiveness of a 5-min CO₂ pre-breathe to detect defective CO₂ scrubber packing”, Diving and Hyperbaric Medicine, Vol. 45(1), S. 16–24.