What Is Cracking Effort — and How Do I Know If Mine Is Good?
Dive GearRegulatorsAll Levels
The Short Answer
Cracking effort is the minimum inhalation force required to open the second stage demand valve — the moment the regulator begins delivering gas. A well-tuned second stage cracks with almost no effort. A poorly adjusted or worn second stage requires noticeable suction before gas flows. You can assess yours without any specialist equipment.
What Cracking Effort Actually Measures
Inside your second stage, a lever mechanism connects a flexible diaphragm to a valve that controls gas flow. When you inhale, the slight negative pressure you create flexes the diaphragm inward. That flex moves the lever, which opens the valve, and gas flows. When you stop inhaling, the diaphragm returns, the lever resets, and the valve closes.
Cracking effort is measured in centimetres of water column (cmH₂O) — a unit that describes the pressure differential required to open the valve. Industry standards and manufacturer specifications typically express it this way. For a diver, the practical experience is simpler: cracking effort is how hard you have to pull before the gas starts flowing.
A regulator with high cracking effort makes you do a moment of work before each breath begins. At depth, breathing multiple times per minute under load, that moment of work accumulates into fatigue, elevated CO₂, and reduced gas efficiency. The ideal cracking effort is the lowest that does not cause freeflow.
How the Demand Valve Works
Second Stage Demand Valve — Cracking Sequence
What Good, Acceptable, and Poor Look Like
Cracking effort is specified in centimetres of water column (cmH₂O). The lower the number, the less effort is required to initiate gas flow. Most quality second stages are set between 1.0 and 2.5 cmH₂O. Outside that range in either direction, performance degrades.
Cracking Effort
Assessment
What It Means
Below 1.0 cmH₂O
Too low
Risk of freeflow — valve opens without demand. Requires service adjustment.
1.0 – 2.5 cmH₂O
Optimal
Gas flows with minimal inhalation effort. Consistent and efficient breathing at all depths.
2.5 – 3.5 cmH₂O
Acceptable
Slight additional effort at the start of each breath. Noticeable but not dangerous in recreational diving.
Above 3.5 cmH₂O
Poor
Elevated work of breathing. Fatigue at depth, CO₂ retention, reduced gas efficiency. Service required.
Above 5.0 cmH₂O
Unacceptable
Significantly impaired breathing. Do not dive this regulator. Workshop assessment and service essential.
How to Assess Yours Without Specialist Equipment
You cannot measure cracking effort precisely without a regulator test bench. What you can do is perform a structured qualitative assessment that will tell you whether your second stage is functioning within an acceptable range — and whether it warrants professional attention.
The Five-Step Self-Assessment
Performed with the regulator connected to a cylinder at full pressure · At the surface · Before any dive
1
Purge test
Press the purge button. Gas should flow immediately and stop cleanly when released. Any delay, gurgling, or continued flow after release indicates a seating or spring problem that affects cracking effort.
2
The first-breath test
Hold the second stage at the same depth it would sit in your mouth. Take one breath slowly and deliberately. Gas should begin flowing almost as soon as inhalation begins — before you have generated any noticeable suction. If you feel a distinct pull before gas arrives, cracking effort is elevated.
Note: repeat this test after the regulator has been submerged. Some degradation of cracking effort only becomes apparent when the diaphragm has been wetted.
3
The consistency check
Take ten slow breaths. The first breath should feel identical to the tenth. If the effort required increases progressively across the sequence, the intermediate pressure from the first stage is dropping under sustained demand — a first stage issue rather than a second stage one, but still affecting your breathing experience.
4
The orientation test
Hold the second stage mouthpiece-down (as it sits when you are horizontal at depth) and take a breath. Then hold it mouthpiece-up and repeat. A well-set regulator should breathe identically in both orientations. A significant difference — particularly easier mouthpiece-down — can indicate the Venturi lever setting rather than cracking effort, but is worth noting.
5
The comparison baseline
Breathe through a hire regulator at your dive centre — ideally one that has just been serviced. Note how the initiation of gas flow feels. Your own regulator should feel at least as easy to initiate as this baseline. If it feels meaningfully stiffer, it needs service.
The Venturi Lever — Related but Different
Most second stages have a Venturi lever or switch — sometimes labelled + / − or Dive / Pre-dive. This lever adjusts the internal gas flow path to either assist or resist the Venturi effect, which is the tendency of flowing gas to draw more gas behind it.
In the + or Dive position, the Venturi effect assists delivery once the valve has cracked — making breathing feel easier and more effortless once gas is flowing. In the − or Pre-dive position, the Venturi assist is reduced to prevent freeflow at the surface when the regulator is not in your mouth.
The Venturi lever does not change cracking effort — the force required to open the valve. It changes how gas flows once the valve is already open. If your regulator feels hard to initiate regardless of Venturi lever position, the issue is cracking effort, not the Venturi setting. If it flows freely once breathing starts but the first pull of each breath is effortful, the cracking effort is set too high.
What to Tell Your Service Technician
If you suspect elevated cracking effort, describe it precisely:
→At what point in the breath does gas begin — immediately, or after a noticeable initial pull?
→Does the effort feel consistent throughout a sequence of breaths, or does it worsen as cylinder pressure drops?
→Does the issue persist in both Venturi lever positions?
→Did it feel different before the last service — and when was the last service?
→Is the regulator exposed to salt water? Salt contamination is the most common cause of elevated cracking effort in regularly serviced regulators.
⚠️
An annual service interval is not a manufacturer recommendation — it is a safety requirement. Cracking effort drifts over time as springs fatigue, O-rings harden, and salt contamination accumulates on valve seats. A regulator that breathed well twelve months ago may have elevated cracking effort today. The only way to know is a bench test during service. Do not rely on subjective assessment alone for a piece of life-support equipment.
Cracking effort is one specification of the second stage. The full picture — how the two-stage pressure reduction works, what the demand valve does, why balanced first stages matter, and what EN 250A certifies — is covered in the Regulator Gear Science.