Uwatec PDIS User manual

TABLE OF CONTENTS

1. Introduction to PDIS (Proﬁle-Dependent Intermediate Stop).............................1

2. How does PDIS work?............................................................................................3

3. Special considerations when diving with more than one

gas mixture (PMG) ..................................................................................................3

4. Diving with PDIS .....................................................................................................3

4.1 PDIS during NO STOP diving ...................................................................4

4.2 PDIS during DECOMPRESSION diving ....................................................5

4.3 PDIS during dives with MB levels .............................................................6

Glossary

PDIS: Proﬁle-Dependent Intermediate Stop. This refers to the concept of

introducing an intermediate stop during an ascent.

PDI stop:

This refers to the actual stop itself and the depth at which it is to be carried out.

1. Introduction to PDIS (Proﬁle-Dependent Intermediate Stop)

The main purpose of a dive computer is to track your nitrogen uptake and recommend a

safe ascent procedure. Diving within the so called no-stop limits means that at the end of

the dive you can ascend directly to the surface, albeit at a safe ascent rate, while for dives

outside of the no-stop limit (so-called decompression dives), you must perform stops at

certain depths below the surface and allow time for excess nitrogen to be expelled from your

body before ﬁnishing the dive.

In both cases, it can be beneﬁcial to stop for a few minutes at an intermediate depth between

the maximum attained depth during the dive and the surface or, in case of a decompression

dive, the ﬁrst (deepest) decompression stop.

An intermediate stop of this kind is beneﬁcial as soon as the ambient pressure at that depth

is low enough to ensure that your body is predominantly offgassing nitrogen, even if under a

very small pressure gradient. In such a situation, you can still cruise along the reef and enjoy

the dive, while your body gets a chance to slowly release nitrogen.

In recent times, so called “deep” stops have been introduced in some dive computers and

tables, deﬁned as half the distance from the maximum reached depth and the surface (or

the lowest decompression stop). Spending 2 or 15 minutes at 30m/100ft would result in the

same deep stop of 15m/50ft.

English

With PDIS, as the name suggests, Galileo interprets your dive proﬁle and suggests an

intermediate stop that is a function of your nitrogen uptake so far. The PDI stop will therefore

change through the course of the dive to reﬂect the continuosly changing situation in your

body. Along the same lines, PDIS will account for the accumulated nitrogen from previous

dives, hence PDIS is also repetitive-dive dependent. Conventional deep stops completely

ignore these facts.

The following ﬁgure quantiﬁes the extent of PDIS and illustrates its dependence on cumulative

nitrogen uptake for two sample dive proﬁles. This ﬁgure also demonstrates the conceptual

difference between PDIS and the rather rudimental “deep” stops.

Speciﬁcally, the ﬁgure compares two dive proﬁles to a maximum depth of 40m/132ft

but otherwise very different. Proﬁle 1 stays at 40m/132ft for 7 minutes, then ascends to

30m/100ft for 3 minutes, followed by 12 minutes at 20m/65ft. Proﬁle 2 stays less than two

minutes at 40m/132ft, then ascends to 21m/69ft and stays there for 33 minutes. Both dive

proﬁles are no-stop dives to the limit of entering decompression. The solid line with triangles

represents the PDIS depth as displayed on the computer screen during the course of the

dive for proﬁle 1, the dotted line with triangles represents the PDIS depth as displayed on

the computer screen during the course of proﬁle 2. One can see that the displayed PDIS

depth increases as more nitrogen is accumulated in the body, but does so very differently

in the two dives due to the different exposure in the two proﬁles. The PDI stop is carried

out at 25 minutes for proﬁle 1 and at 37 minutes for proﬁle 2, followed by the safety stop

at 5m/15ft.

The solid line with open circles on the other hand represents the depth that would be

displayed by a computer following the conventional deep stop method, and it would be

the same for the two dive proﬁles. Deep stops completely ignore any facts about the dives

themselves aside for the max depth.

2. How does PDIS work?

The mathematical decompression model in

Galileo, called ZH-L8 ADT MB PMG, tracks

your decompression status by dividing

your body into 8 so-called compartments

and mathematically following the uptake

and release of nitrogen in each with the

appropriate laws of physics. The various

compartments simulate parts of your body

such as central nervous system, muscles,

bones, skin and so on.

The depth of the PDI stop is calculated as

that at which the leading compartment for

the decompression calculation switches

from ongassing to offgassing, and the

diver is advised to perform a 2-minute

stop above the displayed depth (this is the

opposite of a decompression stop, where

you are asked to remain just beneath the

displayed depth). During this intermediate

stop, the body is not taking up any more

nitrogen in the leading compartment, but

rather releasing nitrogen (though under

a very small pressure gradient). This,

combined with the relatively high ambient

pressure, inhibits bubble growth.

It should be noted that the two fastest

compartments, with 5 and 10 minutes

half times respectively, are not considered

for the determination of the PDI stop

depth. This is due to the fact that these

compartments are only “leading” for very

short dives, for which an intermediate stop

is not required at all.

FNOTE:

The PDI stop is not a mandatory

stop and it is NOT a substitute for the

3-5 minute safety stop at 5m/15ft.

!WARNING

Even when performing a PDI stop, you still

MUST perform a safety stop at 5m/15ft for 3 to

5 minutes. Performing a 3 to 5 minute stop at

5m/15ft at the end of any dive is still the best

thing you can do for yourself!

3. Special considerations

when diving with more

than one gas mixture

(PMG)

Switching to a higher oxygen concentration

mix during the dive inﬂuences the PDI stop.

This needs to be accounted for, in line with

the predictive nature of multigas handling in

ZH-L8 ADT MB PMG.

When diving with more than one gas

mixture, Galileo displays the PDIS depth

according to the following rules:

- if the PDI stop calculated for the bottom

mix (gas 1) is deeper than the switch

depth, than this calculated value is

displayed;

- if the PDI stop calculated for gas 1

is shallower than the switch depth to

the next gas (gas 2 or gas d), then the

displayed PDI stop is a function of the

next gas.

In case of a missed gas switch, Galileo

reverts to the PDI stop for the actively

breathed mix.

4. Diving with PDIS

FNOTE:

To use the PDIS feature, you must set

PDIS to ON in the PDIS menu under

DIVE SETTINGS. The default setting

is OFF.

English

When the calculated PDI stop is deeper

than 8m/25ft, Galileo shows it on the display

and continues to do so until you reach the

displayed depth during an ascent. The

displayed value changes during the dive

as Galileo tracks the uptake of nitrogen in

the 8 compartments and updates the PDIS

depth accordingly to reﬂect the optimum at

all times.

How the PDIS is displayed differs depending

on whether you are in NO STOP or

DECOMPRESSION and on the screen

conﬁguration.

4.1 PDIS during NO STOP diving

In CLASSIC and FULL screen

conﬁgurations:

The PDIS depth is shown to the left of the

no-stop time, with the label PDIS. As soon

as you reach that depth during an ascent, a

2-minute countdown will appear in place of

the no-stop value with label PDIS TIMER.

You can have one of three situations:

you have spent 2 minutes within 3m/10ft

above the indicated depth. The

countdown timer disappears, the PDIS

label and value are replaced by the label

PDIS OK as an indication that you have

performed the PDI stop;

- you have descended by more

than 0.5m/2ft below the PDIS. The

countdown timer disappears and will

reappear again, starting at 2 minutes,

the next time you ascend to the PDIS

depth;

- you have ascended by more than

3m/10ft above the PDIS. The PDIS

label, value and countdown timer

are replaced by the label PDIS NO

to indicate the fact that you have not

performed the PDI stop.

PDIS display (FULL)

PDIS timer (CLASSIC)

PDIS OK (FULL)

PDIS NO (CLASSIC)

English

In LIGHT screen conﬁguration:

The PDIS depth is shown in the MORE

sequence after the O2% and before the

temperature. As soon as you reach that

depth during an ascent, a 2-minute

countdown will appear in place of the

no-stop value with label PDIS TIMER. You

can have one of three situations:

- you have spent 2 minutes within

3m/10ft above the indicated depth while

the timer is showing on the display.

The PDIS label and value in the MORE

sequence are replaced by the label

PDIS OK as an indication that you have

performed the PDI stop;

- you have descended by more

than 0.5m/2ft below the PDIS. The

countdown timer disappears and will

reappear again, starting at 2 minutes, the

next time you ascend to the PDIS depth;

- you have ascended by more than

3m/10ft above the PDIS. The PDIS

label and value in the MORE sequence

are replaced by the label PDIS NO

to indicate the fact that you have not

performed the PDI stop.

FNOTE:

Galileo issues no warnings relating to

a missed PDI stop.

4.2 PDIS during DECOMPRESSION

diving

CLASSICandFULLscreenconﬁgurations,

upon entering decompression the PDIS

information is moved to the lower left

corner and becomes part of the MORE

sequence. This prevents the potential

misinterpretation between the PDI stop and

the decompression stop. Additionally, in

order to avoid conﬂicts between the PDIS

range and a decompression stop, the PDIS

depth is displayed only if it is 3m/10ft deeper

than the deepest decompression stop.

As soon as you reach that depth during an

ascent, the 2-minute countdown is initiated.

A timer as described in the NO-STOP case

is not displayed here, but the switch of the

label from PDIS to PDIS OK alerts you

when the timer has reached 2 minutes.

Descending by more than 0.5m/2ft below

the PDIS or ascending by more than

3m/10ft above the PDIS follows the same

rules as described in the NO-STOP case.

4.3 PDIS during dives with MB

levels

When diving with MB levels, PDIS follows

the same rules as described above. MB

levels, however, introduce stops earlier

and deeper than the L0 base algorithm. As

such, the PDIS display may be delayed and

for certain dives it may not be displayed at

all. This for instance would be the case for a

shallow dive with air (21% oxygen) and MB

level L5.

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