Nicholas Penney

Ohio Aerospace Institute, Brook Park, Ohio

Janice L. Wasowski and Christopher C. Daniels

The University of Akron, Akron, Ohio

Temperature and Atomic Oxygen Effects on Helium 

Leak Rates of a Candidate Main Interface Seal

NASA/CR—2011-216830

January 2011

AIAA–2010–6986
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Nicholas Penney

Ohio Aerospace Institute, Brook Park, Ohio

Janice L. Wasowski and Christopher C. Daniels

The University of Akron, Akron, Ohio

Temperature and Atomic Oxygen Effects on Helium 

Leak Rates of a Candidate Main Interface Seal

NASA/CR—2011-216830

January 2011

AIAA–2010–6986

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contract NNC08CA35C

Prepared for the

46th Joint Propulsion Conference and Exhibit

cosponsored by the AIAA, ASME, SAE, and ASEE

Nashville, Tennessee, July 25–28, 2010
Acknowledgments

The authors wish to acknowledge the following organizations that contributed to this research: Analex Corporation for design and 

engineering support; OCM Test Laboratories for the elastomer outgas testing; Parker Hannifi n Corporation’s Composite Sealing 

Systems Division for supplying the test articles; National Aeronautics and Space Administration (NASA) Glenn Research Center’s 

(GRC) Space Environment and Experiments Branch for exposing test specimens to atomic oxygen; and NASA GRC’s Tribology 

and Mechanical Components Branch for supplying the test apparatus and funding. Additionally, the authors wish to acknowledge 

Erhard Hartman for his technical support during this study. This work was supported by the NASA under contract NNC08CA35C.

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Trade names and trademarks are used in this report for identifi cation 

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either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by NASA expert reviewer(s).
NASA/CR—2011-216830 1 

Temperature and Atomic Oxygen Effects on Helium Leak Rates 

of a Candidate Main Interface Seal 

Nicholas Penney 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Janice L. Wasowski and Christopher C. Daniels 

The University of Akron 

Akron, Ohio 44325 

Abstract 

Helium leak tests were completed to characterize the leak rate of a 54 in. diameter composite space docking seal 

design in support of the National Aeronautics and Space Administration’s (NASA’s) Low Impact Docking System 

(LIDS). The evaluated seal design was a candidate for the main interface seal on the LIDS, which would be 

compressed between two vehicles, while docked, to prevent the escape of breathable air from the vehicles and into 

the vacuum of space. Leak tests completed at nominal temperatures of –30, 20, and 50 °C on untreated and atomic 

oxygen (AO) exposed test samples were examined to determine the influence of both test temperature and AO 

exposure on the performance of the composite seal assembly. Results obtained for untreated seal samples showed 

leak rates which increased with increased test temperature. This general trend was not observed in tests of the AO 

exposed specimens. Initial examination of collected test data suggested that AO exposure resulted in higher helium 

leak rates, however, further analysis showed that the differences observed in the 20 and 50 °C tests between the 

untreated and AO exposed samples were within the experimental error of the test method. Lack of discernable trends 

in the test data prevented concrete conclusions about the effects of test temperature and AO exposure on helium leak 

rates of the candidate seal design from being drawn. To facilitate a comparison of the current test data with results 

from previous leak tests using air as the test fluid, helium leak rates were converted to air leak rates using standard 

conversion factors for viscous and molecular flow. Flow rates calculated using the viscous flow conversion factor 

were significantly higher than the experimental air leakage values, whereas values calculated using the molecular 

flow conversion factor were significantly lower than the experimentally obtained air leak rates. The difference in 

these sets of converted flow rates and their deviation from the experimentally obtained air leak rate data suggest that 

neither conversion factor can be used alone to accurately convert helium leak rates to equivalent air leak rates for the 

test seals evaluated in this study; other leak phenomena, including permeation, must also be considered. 

Nomenclature 

t0 time constant 

I. Introduction 

To successfully accomplish the Constellation Mission, the National Aeronautics and Space Administration 

(NASA) required a new docking system with advanced capabilities. The Low Impact Docking System (LIDS) was 

designed to dock with a variety of manned and autonomous vehicles with minimal forces imparted upon either 

vehicle. The system was intended to be lighter weight, and to reduce the risks associated with docking. 

The docking system required a seal at the interface between the two vehicles to maintain the gas pressure inside 

the adjoining vehicles and minimize the leakage of breathable air from the cabin to the vacuum of space. This seal, 

referred to as the main interface seal, was 54 in. in diameter and was designed to withstand the harsh environments 

of Low Earth Orbit (LEO) and outer space. The operating environment of the docking system was expected to 

subject the main interface seal to temperatures between –75 and 125 °C. The seal was designed to function 

mechanically (i.e., be compressed) at temperatures between –50 and 75 °C. The requirements of the docking system 

stipulated that the seal maintain low leak rates at temperatures of 4 to 40 °C. 

Considering the docking system’s exposure temperature range, silicone elastomers were chosen as seal system 

candidates based upon prior successful use in space applications, a capability to survive exposure to harsh 
NASA/CR—2011-216830 2 

Figure 1.—Representative illustration of a composite seal 

assembly. 

temperatures, and an ability to be formed into seals. Select silicone elastomers could also meet other program 

requirements, including being low outgassing (Ref. 1). 

One candidate seal design consisted of a composite of a metal retainer and four elastomer seal bulbs that were 

vacuum molded into the retainer. The metal retainer provided a means to fasten the one piece assembly to the 

docking system, and limited the compression of the elastomer during docking. Additionally, the retainer minimized 

the ability of the elastomer to be removed undesirably during undocking (Ref. 2). 

Although silicone elastomers offer a wide operating temperature range, they are more permeable to gases than 

many other elastomer compounds. Additionally, the permeation rate of silicone elastomers varies with temperature 

and gas type (Ref. 3). This combination of characteristics necessitates that the leak rate behavior of silicone 

elastomers be thoroughly investigated prior to application on flight vehicles. 

Elastomers tend to adhere to contact surfaces (Ref. 4). Adhesion levels depend upon many factors, including the 

elastomer compound, temperature, amount and duration of contact, amongst others. Adhesion mitigation techniques 

include lubrication and controlling the temperature of the interface during separation. Another effective technique 

for minimizing adhesion is the pretreatment of the seal’s elastomer surfaces with atomic oxygen (AO). Studies have 

shown that moderate exposure to AO can reduce or eliminate surface adhesion (Ref. 5). A minor drawback to the 

AO pretreatment technique is a modest increase in the leak rate of the seal (Ref. 6). 

Previous investigations to characterize leak rates have been conducted on seals of similar designs using dry air as 

the test gas (Ref. 7). While useful in understanding the behavior in application, dry air is rarely utilized during 

qualification, acceptance, and verification testing processes. Tracer gases (usually helium) are used because of their 

small size and tendency to readily permeate through porous media. Using proper techniques and procedures, tracer 

gas methods can accurately predict very small leak rates. 

In this work, a helium leak detector was used to quantify the leak rate of four 54 in. diameter candidate 

composite seal assembly test specimens. A full factorial matrix of tests was conducted using test temperatures of 

–30, 20, and 50 °C on test specimens with and without AO surface pretreatment. The effects of test temperature and 

surface pretreatment on the seal leak rates, as well as a comparison of air leak rates to helium leak rates are 

discussed.

II. Description of Experiments 

A. Test Specimens 

Each test specimen consisted of four individual elastomer seals vacuum molded into a metal retainer, as shown 

in Figure 1. There were two back seals and two front seals. The two front seals had identical cross-sections, and the 

two back seals had identical cross-sections, though the front and back seals were different. The dimensions of the 

seals are proprietary and therefore are not reported. The front and back seals closest to the inner diameter are 

referred to as the primary seals, and the front and back seals closest to the outer diameter are referred to as the 

redundant seals. The seal assemblies were manufactured per a design referred to as EDU54 using Parker-Hannifin 

S0383-70 silicone elastomer compound. The metal retainer was manufactured from aluminum 7075-T651 with a 

chemical conversion coating applied per Mil-C-5541 (Ref. 8) to minimize corrosion. The test specimen’s silicone 

elastomer met mission outgas requirements of total mass loss (TML) less than 1.0% and collected volatile 

condensable materials (CVCM) less than 0.1% when tested per ASTM E595 (Ref. 9).

The specimens were tested in one of two conditions. Two specimens were tested as received from the 

manufacturer, without surface pretreatment. These specimens’ sealing surfaces were cleaned prior to testing with 

lint free wipes moistened with isopropyl alcohol to 

remove any accumulated debris, and then allowed 

to dry for a minimum of 15 min. Two additional 

test specimens were pretreated with an AO fluence 

of approximately 1020 atoms/cm2

. These AO 

exposures were conducted at NASA GRC 

according to the ASTM E 2089 specification 

(Ref. 10). Prior to pretreatment, these specimens 

were cleaned as described above. After 

pretreatment, cleaning was limited to lightly 

blowing the sealing surfaces with canned 

compressed air to remove any accumulated debris. 
NASA/CR—2011-216830 3 

B. Leak Test System 

Helium leak rates of the candidate main interface seal specimens were determined using the non-actuated fullscale flow fixture, as shown in Figure 2 (Ref. 11). The flow fixture was comprised of two primary assemblies, an 

upper assembly and a lower assembly, which were nearly identical. The foundation of each assembly was a 2 in. 

thick aluminum “strongback” plate to which the other components of the assembly were attached with mechanical 

fasteners. These components, moving from the strong back to the test interface, included a phenolic thermal 

insulator plate, a heat exchanger plate, and a seal cartridge holder. Those components were identical on both the 

upper and lower assemblies. 

The test seal assembly was attached to the lower seal cartridge holder with 72 fasteners. These fasteners applied 

the preload required to fully compress the test sample’s back seals against the anodized surface (32 in. surface 

finish) of the seal cartridge holder, resulting in metal-to-metal contact between the test seal retainer and the seal 

cartridge holder. During testing, the test specimen’s front seals mated against an anodized aluminum mating 

counter-face plate (7075-T6 Al, 16 in. surface finish) that was fastened to the upper seal cartridge holder. To 

minimize leakage between the mating counter-face plate and the upper seal cartridge holder, two silicone O-rings 

were installed in grooves in the back side of the mating counter-face plate. The upper assembly was then positioned 

over the lower assembly such that the test seal aligned and mated with the mating counter-face plate. Twenty-four 

external fasteners were installed through the two halves of the flow fixture. The fasteners were then tightened to 

simulate latching loads that would be generated across the sealing interface by the LIDS on orbit. The assembled 

flow fixture isolated the test section from the ambient environment with the inner and outer isolation o-rings, as 

shown in Figure 3. The test section encompassed the test specimen, mating counter-face plate, and the outer annulus. 

Once the flow fixture was assembled, the outer annulus was evacuated through a vacuum port. This was 

accomplished with a Varian Model 979 series Helium Mass Spectrometer leak detector, with a manufacturer stated 

accuracy of 2%, and a Varian Triscroll 600 rough vacuum pump. During testing, the rough pump was isolated from 

the test section, so that none of the test gas was diverted from the leak detector. 

A helium leak standard was used to calibrate the helium leak detector at the beginning or end of each leak test, 

depending on the test. The leak standard was either a VIC Leak Detection glass orifice, with an uncertainty of 

10%, or a LACO Technologies Micro-tube capillary orifice, with an accuracy of 3.8%. Both had a leak rate in the 

range of the test specimens and were mounted to the non-actuated full-scale flow fixture at a port opposite the 

vacuum port. Helium from the leak standard circulated around the outer annulus and not through the test specimen. 

During the calibration process, the time constant, t0, which was the time required for the helium leak detector to 

indicate a steady-state leak was determined. 

Figure 2.—Photograph of non-actuated full-scale flow fixture. 
NASA/CR—2011-216830 4 

Figure 4.—Illustration of helium flow path. 

During testing, high purity helium (99.997%) was supplied between the inner and outer test specimen seals at 

0.2 psig, as shown in Figure 4. With a constant helium supply, helium permeated through both the primary and 

redundant seals, but only the helium that passed through the redundant seals to the outer annulus was measured. The 

duration of time that helium was supplied to the test specimen included the time required for the test specimen leak 

rate to reach steady-state and three time constants. 

C. Thermal Control System 

The temperature of the test specimens was controlled using a Mydax 2VLH30W Chiller/Heater unit, which 

circulated Dow Syltherm HF heat transfer fluid through passages in heat exchanger plates attached to the backside 

of the upper and lower seal cartridge holders (Fig. 2). The temperature control unit maintained the test section 

temperature to 0.5 °C. The test section temperature was monitored with 24 evenly spaced circumferentially 

mounted type-T thermocouples (Fig. 2), distributed equally on the upper and lower seal cartridge holders. Each 

thermocouple had an accuracy of 0.036 °C. 

D. Data Acquisition System 

A PC based data acquisition system was used to display and record the test data. National Instruments hardware 

and a custom written LabVIEW (National Instruments) code collected the interbulb pressure, the outer annulus 

vacuum pressure, the test section temperatures, and the helium leak rate. Unfiltered data was sampled at 25 Hz and 

recorded every 10 sec with each recorded value being an average of 25 sampled data points. 

Figure 3.—Illustration of assembled test section with magnification of test specimen and mating counter-face. 
NASA/CR—2011-216830 5 

III. Experimental Results and Discussion 

As shown in Figure 5, the leak detector data from a typical leak test contained five distinct regions. Regions I and 

II represented the data from the leak standard. Region I represented the time it took the leak detector to indicate a 

steady-state leak rate, t0. During the analysis of the test data, t0 was used in the determination of the test specimen’s 

leak rates. Region II represented the steady-state leak rate of the leak standard. Region III represented the system 

clean-up where the helium introduced by the leak standard into the test section, and subsequently the leak detector, 

was removed from the system. The next regions of the graph characterized the performance of the test specimen. 

Region IV represented the rise to steady-state, and Region V showed the steady-state leak rate of the test specimen. 

For each test, the helium leak rate for the test specimen was calculated from the averaged steady-state 

measurements in Region V. The results are summarized in Table 1. The reported leak rates represented the 

arithmetic average of two tests conducted at each temperature for both the untreated and AO exposed test 

specimens. These leak rates were normalized per unit length based on the circumferential length of the redundant 

seal bulb centerline. This allowed comparison between the tests conducted with helium, which measured the leak 

rate of the redundant seal bulbs, and previously reported tests conducted with air (Ref. 7), which measured the leak 

rate of the primary seal bulbs. The experimental error for the results shown in Table 1 was 33% of the reading. 

Figure 5.—Typical leak rate test data. 

TABLE 1.—HELIUM LEAK RATE RESULTS 

Temperature, °C Helium Leak Rate, 

atm-cc/s-in. 

Untreated AO exposed 

–30 2.0910–6 1.2410–5

20 5.9110–6 6.6310–6

50 1.3310–5 1.5110–5
NASA/CR—2011-216830 6 

Figure 6.—Comparison of untreated and AO exposed 

test specimens. 

A. Temperature Effects on Leak Rate 

The helium leak rates for the test specimens were 

measured at three test temperatures: –30, 20, and 

50 °C. The leak rates for the untreated test specimens 

increased as the temperature increased, as shown in 

Figure 6. This result followed the trend of previously 

reported air leak rates for the same test specimens at 

the same test conditions (Ref. 7). The temperature 

effect on the test specimens exposed to AO was not as 

straight forward. As shown in Figure 6 the leak rate 

increased as the temperature increased from 20 to 

50 °C, however, the leak rate also appeared to have 

increased as the temperature decreased from 20 to 

–30 °C. This did not correspond to the trend observed 

for the air leak rate results; the air leak rate results 

showed an increase in leak rate with temperature 

similar to the leak rates of the untreated test specimens. 

A contributing factor to the differences could be the 

associated test equipment. It has been shown that 

measurement error can occur when the nontemperature compensated leak detector was used for 

test conditions other than room temperature (Ref. 12). 

Although this was not apparent in the untreated seal 

test results, it is a factor that requires further 

investigation. 

B. AO Exposure Effects on the Leak Rate 

When the leak rates of the untreated test specimens were compared to the leak rates of AO exposed test 

specimens, the leak rates for the AO exposed specimens appeared greater than the leak rates of the untreated 

specimens, as shown in Figure 6. However, this observed increase for the 20 and 50 °C tests was within the 

experimental error of the tests and the increase was not considered significant. At –30 °C, the average leak rate of 

the AO exposed test specimens was approximately 5.9 times greater than that of the untreated specimens. 

C. Comparison of Helium Leak Rates to Air Leak Rates 

To compare the helium leak rates reported here with the previously reported air leak rates (Ref. 7), differences in 

the type of measurement (volume versus mass) and the type of gas (helium versus air) needed to be addressed. The 

conversion from a volumetric leak rate to a mass leak rate was accomplished using the density of the gas. However, 

the conversion from helium to air was not straight forward. Conversion factors typically used in industry for 

converting helium leak rates to air are 1.08 and 0.374 for viscous flow and molecular flow respectively. The viscous 

flow conversion factor is the ratio of the dynamic viscosity of the two gases, and the molecular flow conversion 

factor is the ratio of the average molecular velocity of the two gases (Ref. 13). In both the viscous and molecular 

flow cases, the conversion factors are based on flow through an orifice of finite dimensions, however, based on 

previous work, it was shown that the flow through these test specimens was dominated by permeation (Ref. 6). 

Permeation does not rely on physical defects in the material to provide a leak path as required for viscous and 

molecular flow leaks. Therefore, applying viscous or molecular flow conversion factors to permeation dominated 

leak rates can produce erroneous results. 

As an example, the helium leak rates reported for the untreated test specimens were converted to equivalent air 

leak rates using both the viscous flow and molecular flow conversion factors. The resulting converted air leak rates 

were then compared to the previously reported experimental air leak rates for the same test specimens at the same 

temperature and test conditions. Using a NIST traceable leak standard, the pressure decay mass loss technique has 

been shown to have less uncertainty associated with the results when compared to the helium leak detector method 

(Ref. 12). Therefore, the experimentally determined air leak rates were considered the most accurate. A comparison 

of the converted air leak rates and the experimental air leak rates is summarized in Table 2. As shown in Table 2, 

when viscous flow was assumed, the converted air leak rates were 35 to 73% greater than the experimental air leak 

rates, however, when molecular flow was assumed, the converted air leak rates were 40 to 53% less than the 
NASA/CR—2011-216830 7 

TABLE 2.—COMPARISON OF HELIUM AND AIR LEAK RATES 

Temperature, 

°C 

Helium leak 

rate 

Converted air leak rate Experimental air 

leak rate (Ref. 7)

Experimental to 

converted leak rate ratio 

atm-cc/s-in. Viscous flow 

lbm/s-in. 

Molecular flow 

lbm/s-in. 

lbm/s-in. Viscous 

flow 

Molecular 

flow 

–30 2.0910–6 7.2410–12 2.5110–12 5.1510–12 0.71 2.05 

20 5.9110–6 1.7010–11 5.8810–12 1.2610–11 0.74 2.14 

50 1.3310–5 3.4710–11 1.2010–11 2.0010–11 0.57 1.67 

experimental air leak rates. In both cases, when the helium leak rates were converted to air leak rates, the results 

were significantly different than the experimental air leak rate values. This example highlights the importance of 

understanding the type of flow and test conditions when leak testing with helium and then converting the results to 

leak rates for other gases. 

IV. Summary 

A near-full-scale candidate composite seal assembly to be used as the main interface seal of the LIDS has been 

helium leak tested. From the experimental results, the following observations were noted: 

1. The helium leak rate for the untreated test specimens increased as the temperature increased over the 

temperature range of –30 to 50 °C, however, the helium leak rate for the AO exposed test specimens decreased 

over the temperature range of –30 to 20 °C then increased as the temperature increased over the temperature 

range of 20 to 50 °C. The inconsistent trend requires further investigation. 

2. There was an apparent increase in the helium leak rate of test specimens exposed to AO when compared to 

untreated test specimens over the temperature range of 20 to 50 °C. However, the differences were within the 

experimental error of the tests and the increase was not deemed significant. There was a significant increase in 

the helium leak rate of test specimens exposed to AO when compared to untreated test specimens at –30 °C. 

3. Using standard conversion factors for viscous or molecular flow can produce inaccurate results when applied to 

this study’s test specimens. The test specimen seal leakage was dominated by permeation, which exhibits 

different flow behavior than either viscous or molecular flow. 

References 

1. Daniels, C.C., de Groh III, H., Dunlap, P.H., Finkbeiner, J.R., Steinetz, B.M., Bastrzyk, M.B., Oswald, J.J., 

Banks, B.A., Dever, J.A., Miller, S.K., and Waters, D.L., “Characteristics of Elastomer Seals Exposed to Space 

Environments,” Proceedings of the 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, No. 

AIAA–2007–5741, AIAA, Cincinnati, OH, 8–11 July 2007. 

2. Conrad, M.C., Daniels, C.C. and Bastrzyk, M.B., “An Experimental Investigation of Silicone-to-Metal Bond 

Strength in Composite Space Docking System Seals,” Proceedings of the 45th AIAA/ASME/SAE/ASEE Joint 

Propulsion Conference & Exhibit, No. AIAA–2009–5320, AIAA, Denver, Colorado, 2–5 August 2009. 

3. Pauly, S., “Permeability and Diffusion Data,” Polymer Handbook, 4th ed., edited by J. Brandrup, E.A. 

Immergut, and E.A. Grulke, Wiley-Interscience, New York, 1999, Chap. VI. 

4. Bastrzyk, M.B. and Daniels, C.C., “The Mechanical Performance of Subscale Candidate Elastomer Docking 

Seals,” 51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, April 

2010. 

5. Garafolo, N.G., Bastrzyk, M.B., and Daniels, C.C., “The Effects of Atomic Oxygen on the Sealing and 

Mechanical Performance of an Elastomer Seal,” Proceedings of the 48th AIAA Aerospace Sciences Meeting, 

No. AIAA–2010–1440, AIAA, Orlando, Florida, 4–7 January 2010. 

6. Smith, I.M., Daniels, C.C., Dunlap, P.H., and Steinetz, B.M., “Performance of Sub-scale Docking Seals under 

Simulated Temperature Conditions,” Proceedings of the 44rd AIAA/ASME/SAE/ASEE Joint Propulsion 

Conference & Exhibit, No. AIAA–2008–4713, AIAA, Hartford, CT, 21–23 July 2008. 

7. Wasowski, J.L., Penney, N., Garafolo, N.G., and Daniels, C.C., “Leak Rates of a Candidate Main Interface Seal 

at Selected Temperatures,” Proceedings of the 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & 

Exhibit, No. AIAA–2009–5320, AIAA, Denver, Colorado, 2–5 August 2009. 

8. USN. MIL-C-5541E: Chemical Conversion Coatings on Aluminum and Aluminum Alloys. Department of 

Defense, 1990. 
NASA/CR—2011-216830 8 

9. ASTM International. ASTM E595-07 (Reapproved 2003): Standard test method for total mass loss and 

collected volatile on densable materials from outgassing in a vacuum environment. 2003. 

10. ASTM International. ASTM E-2089-00 (Reapproved 2006): Standard Practices for Ground Laboratory Atomic 

Oxygen Interaction Evaluation of Materials for Space Applications. ASTM, 2006. 

11. Dunlap, P.H., Daniels, C.C., Steinetz, B.M., Erker, A.H., Robbie, M.G., Wasowski, J.L., Drlik, G.J., Tong, 

M.T., and Penney, N., “Full-Scale System for Quantifying Leakage of Docking System Seals for Space 

Applications,” NASA/TM—2007-215024, AIAA–2007–5742, October 2007. 

12. Daniels, C.C. and Garafolo, N.G., “Comprehensive Mass Point Leak Rate Technique. Part II: Application of 

Methodology and Variable Influences,” The Fourth Japan-US Symposium on Emerging NDE Capabilities for a 

Safer World, American Society for Nondestructive Testing, Maui, HI, June 7–11, 2010. 

13. VanAtta, C.M., Vacuum Science and Engineering, McGraw-Hill, New York, 1965, pp. 44–62. 
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Temperature and Atomic Oxygen Effects on Helium Leak Rates of a Candidate Main 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Helium leak tests were completed to characterize the leak rate of a 54 in. diameter composite space docking seal design in support of the 

National Aeronautics and Space Administration’s (NASA's) Low Impact Docking System (LIDS). The evaluated seal design was a 

candidate for the main interface seal on the LIDS, which would be compressed between two vehicles, while docked, to prevent the escape of 

breathable air from the vehicles and into the vacuum of space. Leak tests completed at nominal temperatures of -30, 20, and 50 °C on 

untreated and atomic oxygen (AO) exposed test samples were examined to determine the influence of both test temperature and AO 

exposure on the performance of the composite seal assembly. Results obtained for untreated seal samples showed leak rates which increased 

with increased test temperature. This general trend was not observed in tests of the AO exposed specimens. Initial examination of collected 

test data suggested that AO exposure resulted in higher helium leak rates, however, further analysis showed that the differences observed in 

the 20 and 50 °C tests between the untreated and AO exposed samples were within the experimental error of the test method. Lack of 

discernable trends in the test data prevented concrete conclusions about the effects of test temperature and AO exposure on helium leak rates 

of the candidate seal design from being drawn. To facilitate a comparison of the current test data with results from previous leak tests using 

air as the test fluid, helium leak rates were converted to air leak rates using standard conversion factors for viscous and molecular flow. Flow 

rates calculated using the viscous flow conversion factor were significantly higher than the experimental air leakage values, whereas values 

calculated using the molecular flow conversion factor were significantly lower than the experimentally obtained air leak rates. The 

difference in these sets of converted flow rates and their deviation from the experimentally obtained air leak rate data suggest that neither 

conversion factor can be used alone to accurately convert helium leak rates to equivalent air leak rates for the test seals evaluated in this 

study; other leak phenomena, including permeation, must also be considered.

15. SUBJECT TERMS

Spacecraft design; Testing; Performance; Mechanical engineering; Helium leak detection; Helium to air leak rate conversion 

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