Decompression Theory: M-Values and No-Decompression Limits

Decompression Theory — Topics

1 — Half Times 2 — Compartments 3 — M-Values 4 — Surface Interval Credit 5 — Navy Tables vs RDP Dive Computers

Watch Will Welbourn explain M-values — the maximum nitrogen a compartment can safely hold on surfacing — and show how the controlling compartment changes between deep and shallow dives.

Watch the half times and compartments videos first M-values only make sense in context of half times and compartments. Watch those two videos before this one.

What Is an M-Value?

Every compartment can hold more nitrogen than is safe to surface with. The M-value (maximum value) is the maximum nitrogen pressure a compartment can contain when surfacing without an unacceptable risk of bubbles forming and causing DCS.

M-values were determined through test dives showing what nitrogen levels did and did not produce Doppler-detectable bubbles in real divers. They are calculated for surfacing at sea level — which is why special procedures apply when diving at altitudes above 300 metres.

The relationship between half time and M-value Fast compartments (short half times) have high M-values — they can tolerate more dissolved nitrogen on surfacing.
Slow compartments (long half times) have low M-values — they tolerate far less.

The Controlling Compartment

As a diver descends and time passes, nitrogen loads into all compartments simultaneously — but at different rates. The first compartment to reach its M-value ends the dive (or makes it a decompression dive). That compartment is called the controlling compartment.

The controlling compartment is not always the same one — it depends on depth and dive time:

Deep dives — fast compartments load rapidly and reach their M-values first. Short no-decompression limits result.
Shallow dives — the depth is insufficient for fast compartments to reach their M-values. Slow compartments gradually accumulate nitrogen and become the controlling compartment. Longer no-decompression limits result.

M-values and no-decompression limits The NDL for any given depth is essentially the time until the controlling compartment reaches its M-value. Staying within NDLs means no compartment exceeds its M-value — allowing the diver to surface safely at any time. This is the definition of no-stop diving.

What Happens If You Exceed an M-Value?

A compartment can physically hold more nitrogen than its M-value — but surfacing in that state risks bubble formation and DCS. The diver must complete decompression stops at depth to allow nitrogen levels to drop below M-values before surfacing.

Note: even within table limits, there is always some residual statistical risk. The actual risk of DCS within RDP limits is approximately 0.04% — very low, but not zero. This is why conservative diving within limits is always recommended.

Altitude diving M-values are calculated for surfacing at sea level. At altitude, ambient pressure is lower — which effectively reduces the pressure gradient a diver can tolerate. Standard recreational tables cannot be used above 300 metres without modification. Special altitude diving procedures or a computer with altitude compensation must be used.

Spencer Limits

The M-values used in the RDP are sometimes called Spencer limits, after Dr. Michael Spencer who first proposed them based on Doppler ultrasound data. They are lower (more conservative) than the M-values used in the original US Navy tables — reflecting the fact that silent bubbles were forming at Navy table limits even without apparent DCS symptoms.

Decompression Theory — Topics

1 — Half Times 2 — Compartments 3 — M-Values 4 — Surface Interval Credit 5 — Navy Tables vs RDP Dive Computers

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