Abercromby, Andrew

Bekdash, Omar

Fricker, John

Meginnis, Ian

Norcross, Jason

NASA Technical Reports Server

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Characterization of Variability Sources Associated with 
Measuring Inspired Carbon Dioxide in Spacesuits
Omar Bekdash1, Jason Norcross1, John Fricker2, Ian Meginnis3, and 
Andrew Abercromby3
1 KBRWyle, 2400 NASA Pkwy, Houston, Texas
2 Oceaneering 16665 Space Center Blvd, Houston, Texas
3 NASA Johnson Space Center 2101 NASA Parkway Houston, Texas
January 23 2017
https://ntrs.nasa.gov/search.jsp?R=20170001376 2019-08-31T19:28:25+00:00Z
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Background
• Adequate elimination of CO2 produced by respiration is an essential 
requirement for spacesuits. 
o Exposure to excessive levels of carbon dioxide (CO2) can lead to hypercapnia.
• Washout refers to the ability of a suit’s ventilation design 
to remove CO2 from the helmet environment. 
• Previous studies indicate that the accuracy and reliability 
of inspired CO2 measurements depends on many 
variables:
o Measurement equipment setup
o Analysis methods used
o Human subjects 
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Study Objectives
1. Review existing methodologies for CO2 washout measurement and 
analysis. 
2. Characterize sources of variability associated with spacesuit CO2
washout measurement equipment and methods.
3. Define a methodology that minimizes measurement error for use 
with future human testing.  
o Hardware configurations
o Analysis methods
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Existing Carbon Dioxide Washout Measurement Methods
• Industries that use respiratory protective equipment such as diving, 
firefighting, or aviation, typically require donning a mask that seals 
over the nose and mouth. 
o Provides a direct means of sampling inspired CO2 from the small dead space 
volume inside the oral-nasal cup for human test standards
• NIOSH and European standards also employ breathing simulator 
based methods to limit variability in human breathing both intra- and 
inter- wearers
o Impractical for suited testing
o Human testing is important to quantify the real variability that occurs in CO2
washout due to factors such as breathing characteristics, fitness levels, face 
anthropometry, and head positioning and movement. 
o During the developmental stages of spacesuit design before designs have 
been finalized it is not practical to construct unique manikins for each 
design iteration
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NASA CO2 Washout Methods Development 
• Subjects breathe through a small 
mouthpiece
• Inspired CO2 calculated from minimum 
inspired value
1969
Ongoing effort since Gemini EVAs demanded crewmembers 
operate at higher work rates
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NASA CO2 Washout Methods Development 
• These investigations span multiple suits, both EVA prototype and 
Launch/Entry/Abort (LEA)
Recent NASA testing has evaluated different methods of affixing sampling lines to the person 
• Several iterations on the oral-nasal 
mask design were evaluated
• Potential for the mask to alter the 
nominal air flow path inside the 
helmet 
• Dead space volume inside the 
mask may affect measurement
• None were able to provide repeatable 
measurement of respiratory data 
• Unacceptable human factors 
• Test subject comfort 
• Ability to use a Valsalva device for 
pressure equalization 
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NASA CO2 Washout Methods Development 
Variability of results using masks and a desire to improve human factors aspects of 
the sampling system led us to use a nasal cannula for in suit sampling. 
• Provides a low profile sampling line that is placed 
directly in the nasal cavity
• Reduces dead space 
• Prevents interference with nominal flow paths of 
the suit.
• Possible concerns related to breathing style ( nose 
vs nose\mouth)
• Potentially resolved through standardizing 
sampling methodology
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Calculation of Inspired CO2
Comparison against industry standards for certification of CO2
washout performance suggests that the whole inspiration cycle and
not just the local end-expired minimum be considered
Average of minimum inspired Time weighted average of inspired
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Typical Spacesuit Testing Equipment Configuration
1. Nasal Cannula worn by subject
2. Cannula tubing reducer
3. Suit pass-through
4. Needle valve or Rotameter
5. CO2 Sensor
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Simulated Breathing Gas Testing Technique
Labview Data 
Collection 
Software
Tedlar Gas 
Sample Bags
Luer Lock  
Stopcock Valve
NI-USB 6212 
DAQ
AEI CD-3A 
CO2 Analyzer 
Vacuum Pump
Sample Tubing
• A manual valve is switched between 
1% (inspired) and 4% (expired) CO2
calibration gas. 
• A vacuum pump draws gas to the 
sensor through a sample tube. 
 
Table 1. Sample line lengths, diameters, and flow rates 
tested using calibration gas methodology. Flow rates 
(mL/min) are shown in each cell of the matrix. 
  Line Length, m (ft) 
  0.61 (2) 3.0 (10) 6.1 (20) 
L
in
e 
In
te
rn
a
l 
D
ia
m
et
er
 
 m
m
 (
in
) 
1.6 
(0.063) 
1000 
250, 500, 
750, 1000 
1000 
2.4 
(0.094) 
1000 
250, 500, 
750, 1000 
1000 
3.2 
(0.13) 
1000 
250, 500, 
750, 1000 
1000 
4.8 
(0.19) 
1000 
250, 500, 
750, 1000 
1000 
 
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Sample Line Length and Inner diameter 
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0
%
 C
O
2
Time (s)
0.61 m (2 ft)
3.0 m (10 ft)
6.1 m (20 ft)
• All data collected were normalized to 0% CO2 as a baseline by subtracting the 
1% calibration gas from all data points. 
• CO2 concentrations are known and subtracted from all data
• Values of inspired CO2 greater than zero represent inherent error due to the 
testing methodology. 
Data integrity decreases as line length and inner diameter increases for a 
constant sample rate of 1000 mL/min.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0
%
 C
O
2
Time (s)
1.6 mm (.063 in)
2.4 mm (.094 in)
3.2 mm (.13 in)
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Sampling Flow Rate 
• Data integrity decreases as flow rate decreases
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0
%
 C
O
2
Time (s)
250 mL/min
500 mL/min
750 mL/min
1000 mL/min
Inspired %CO2 based on measurements of
different sample flow rates and sample line
diameters with 3.0 m long sample tube time was
assumed.
Decreased data integrity as flow rate decreases
for a constant line length (3.0 m) and diameter
(3.2 mm).
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Rotameter Location and Passthrough Fittings
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0
%
 C
O
2
Time (S)
Rotameter After Sensor
Rotameter Before SensorA rotameter has previously been positioned prior to the
sensor to provide the proper flow rate from the cannula
to the sensor external to the suit.
• Spacesuits are typically operated at approximately
29.6 to 56.5 kPa (4.3 to 8.3 psi) above ambient
• The pre-sensor rotameter had a time-weighted
inspired ppCO2 level of 0.38% CO2, compared with
the post-sensor rotameter which had a time-
weighted inspired ppCO2 level of 0.12% CO2,
assuming a 2 second inspiration.
Threaded fittings or similar pass-throughs are needed
to accommodate cannula sample line in the suit
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Accounting for impact of inspiration duration
• The shorter the duration of an inspiration, 
the larger the effect of the methodology 
error when a time-weighted average is 
calculated
• A single error value may not be applicable 
to all HITL test conditions.
• An option to account for this variability is to 
match human and methodology error 
inspiration durations when calculating 
inspired CO2 levels. 
• Human inspiration duration is available 
from the recorded data, and the same 
duration would be applied to the error 
calculation.  
0.123
0.237
0.467
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.0 0.5 1.0 1.5 2.0 2.5
C
a
lc
u
la
te
d
 I
n
s
p
ir
e
d
 C
O
2
 (
%
)
Simulated Inspiration Duration (s)
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CO2 Washout Sampling Methodology Recommendations
• Conduct subsequent testing with the 1.6 mm (0.063 inch) 
diameter, 3.0 m (10 ft) length sample line at a flow rate of 1000 
mL/min. 
o Provides a practical length for future HITL testing while minimizing  error due 
to mixing. 
• Position rotameter after the CO2 analyzer
• Specific future configurations of fittings can be tested using the 
methodology described here to identify the associated 
measurement error. 
• Consider adjusting inspired CO2 calculation to match subject 
breathing variability
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Proposed roadmap to develop evidence-based CO2
exposure requirements for suited astronauts.
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Backup
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Potential method for In-suit sampling at pressure
May be possible to avoid this problem if an open T port is used before the
sensor and the rotameter is placed after the sensor during suited testing


Characterization of Variability Sources Associated with Measuring Inspired Carbon Dioxide in Spacesuits

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