Fundamentals of Decompression

Fundamentals of Decompression 

• History 

• Tissue models 

– Haldane 

– Workman 

– Bühlmann 

• Physics of bubbles 

• Spacecraft cabin atmospheres 

U N I V E R S I T Y O F 

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© 2011 David L. Akin - All rights reserved 

http://spacecraft.ssl.umd.edu 


ENAE 697 - Space Human Factors and Life Support

First Class Assignment 

• Four topics in the first section of the course 

– Space Habitability 

– Human Factors 

– Anthropometrics 

– Psychosocial Aspects 

• Find a technical paper in two of the topic areas 

• Post the paper (in PDF) and a summary (~0.5-1 

page) to discussion board on Blackboard 

• No duplication! There’s an advantage in being first 

• Due Thursday March 3rd 


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Discussion of Term Project 

• Please go to Blackboard site and list team members 

(and innovative team names) for all teams 

• First phase: design interior layout of X-Hab in 

configuration for 2011 test series 

– Accommodations for four crew 

– Diagrams coming for outer envelope 

• Submit as slide package (no presentation) by 

March 18 (i.e., before Spring break) 

• Second phase: full interior layout of two layers 

with life support and habitat elements 


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Caissons 


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• Pressurized chambers 

for digging tunnels and 

bridge foundations 

• Late 1800’s - caisson 

workers exhibited 

severe symptoms 

– joint pain 

– arched back 

– blindness 

– death 



Brooklyn Bridge 


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• Designed by John Roebling, who 

died from tetanus contracted 

while surveying it 

• Continued by son Washington 

Roebling, who came down with 

Caisson Disease in 1872 

• Competed by wife Emily 

Warren Roebling 

• 110 instances of caisson disease 

from 600 workers 

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Decompression Sickness (DCS) 

• 1872 - Dr. Alphonse Jaminet noted similarity 

between caisson disease and air embolisms 

• Suggested procedural modifications 

– Slow compression and decompression 

– Limiting work to 4 hours, no more than 4 atm 

– Restricting to young, healthy workers 

• 1908 - J.B.S. Haldane linked to dissolved gases in 

blood and published first decompression tables 


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Supersaturation of Blood Gases 

• Early observation that “factor of two” (50% drop in 

pressure) tended to be safe 

• Definition of tissue ratio R as ratio between 

saturated pressure of gas compared to ambient 

pressure 

R = PN2 

Pambient 

• 50% drop in pressure corresponds to R=1.58 

(R values of ~1.6 considered to be “safe”) 


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=0.79 (nominal Earth value) 

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Tissue Models of Dissolved Gases 

• Issue is dissolved inert gases (not involved in 

metabolic processes, like N2 or He) 

• Diffusion rate is driven by the gradient of the 

partial pressure for the dissolved gas 

dPtissue(t) 

dt 

where k=time constant for specific tissue (min -1 ) 

P refers to partial pressure of dissolved gas 


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= k [Palveoli(t) − Ptissue(t)] 

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Tissue Saturation following Descent 


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Tissue Saturation after Ascent 


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Effect of Multiple Tissue Times 


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Haldane Tissue Models 

• Rate coefficient frequently given as time to evolve 

half of dissolved gases: 

T 1/2 = 

• Example: for 5-min tissue, k=0.1386 min -1 

• Haldane suggested five tissue “compartments”: 5, 

10, 20, 40, and 75 minutes 

• Basis of U. S. Navy tables used through 1960’s 

• Three tissue model (5 and 10 min dropped) 

• 1950’s: Six tissue model (5, 10, 20, 40, 75, 120) 


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ln (2) 

k 

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k = 

ln (2) 

T 1/2 



Workman Tissue Models 

• Dr./Capt. Robert D. Workman of Navy 

Experimental Diving Unit in 1960’s 

• Added 160, 200, 240 min tissue groups 

• Recognized that each type of tissue has a differing 

amount of overpressure it can tolerate, and this 

changes with depth 

• Defined the overpressure limits as “M values” 


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Workman M Values 

• Discovered linear relationship between partial 

pressure where DCS occurs and depth 

M=partial pressure limit (for each tissue compartment) 

M0=tissue limit at sea level (zero depth) 

ΔM=change of limit with depth (constant) 

d=depth of dive 


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M = M0 + ∆Md 

• Can use to calculate decompression stop depth 

dmin = Pt − M0 

∆M 

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PADUA (Univ of Penn.) Tissue Model 

Tissue T 1/2 (minutes) M 0 (bar) 

1 5 3.040 

2 10 2.554 

3 20 2.067 

4 40 1.611 

5 80 1.581 

6 120 1.550 

7 160 1.520 

8 240 1.490 

9 320 1.490 

10 480 1.459 


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Bühlmann Tissue Models 

• Laboratory of Hyperbaric Physiology at University 

Hospital, Zurich, Switzerland 

• Developed techniques for mixed-gas diving, 

including switching gas mixtures during 

decompression 

• Showed role of ambient pressure on 

decompression (diving at altitude) 

• Independently developed M-values, based on 

absolute pressure rather than SL depth 

• “Zurich” 12 and 16-tissue models widely used 


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Bühlmann M-Value Models 

• Modifies Workman model by not assuming sea 

level pressure at water’s surface 

M = Pamb 

+ a 

b 

Pamb=pressure of breathing gas 

b=ratio of change in ambient pressure to change in tissue 

pressure limit (dimensionless) 

a=limiting tissue limit at zero absolute pressure 

• ZH-L16 model values for a and b 

a =2T 

− 1 

3 

1/2 


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< bar > b =1.005 − T − 1 

2 

1/2 

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Physics of Bubbles 

• Pressure inside a bubble is balanced by exterior 

pressure and surface tension 

Pinternal = Pambient + Psurface = Pambient + 2γ 

where γ=surface tension in J/m 2 or N/m (=0.073 for water 

at 273°K) 

• Dissolve gas partial pressure Pg=Pamb in 

equilibrium 

• Gas pressure in bubble Pint>Pamb due to γ 

• All bubbles will eventually diffuse and collapse 


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ENAE 697 - Space Human Factors and Life Support 

r

Critical Bubble Size 

• Minimum bubble size is defined by point at which 

interior pressure Pint = gas pressure Pg 

2γ 

• rrmin - bubble will grow 

• r=rmin - unstable equilibrium 


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rmin = 

21 

Pg − pambient 



Bubble Formation and Growth 

• In equilibrium, external pressure balanced by internal 

gas pressure and surface tension 

• Surface tension forces inversely proportional to radius 


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“Clinical” Discussion of DCS 

• Tissue models are predictive, not definitive 

• Every individual is different 

– Overweight people more susceptible to DCS 

– Tables and models are predictive limits - there will be 

“outliers” who develop DCS while adhering to tables 

• Doppler velocimetry reveals prevalence of bubbles 

in bloodstream without presence of DCS 

symptoms - “asymptomatic DCS” 


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Implications of DCS in Space Flight 

• Drop from sea level pressure to ~4 psi, 100% O2 

pressure 

– Equivalent to ascent from fully saturated 120 ft dive 

– Launch in early space flight 

– Extravehicular activity from shuttle or ISS 

R = PN2 

Pamb 

• To have “safe” (R=1.4) EVA from shuttle requires 

suit pressure of 8.2 psi 


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= 14.7(0.78) 

4 

=2.87 



Current Denitrogenation Approaches 

• Depress to 10.2 psi for 12-24 hours prior to EVA 

– Full cabin depress in shuttle 

– “Campout” in air lock module of ISS 

• Exercise while breathing 100% O2 

• In-suit decompression on 100% O2 (3.5-4 hours) 


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Historical Data on Cabin Atmospheres 

from Scheuring et. al., “ Risk Assessm ent of Physiological Effects of Atm ospheric Com position and Pressure 

in Constellation Vehicles” 1 6th Annual Humans in Space, Beijing, China, May 2007 


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Spacecraft Atmosphere Design Space 




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Effect of Pressure and %O2 on Flammability 

from Hirsch, William s, and Beeson, “ Pressure Effects on Oxygen Concentration Flam m ability Thresholds of 

Materials for Aerospace Applications” J. Testing and Evaluation, Oct. 2006 


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Atmosphere Design Space with Constraints 




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Constellation Spacecraft Atmospheres 




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Fundamentals of Decompression

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