A Controlled Atmosphere Characterization Environment for Bioregenerative Life Support Technologies
, Moritz Koslowsky 1, Sophia Krogmann 1, Jens Hauslage 1,2,3 and Gerhild Bornemann 1,*
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsI am reviewing this manuscript as a person interested in the topic of humans in space rather than as an expect.
The authors well present a small-scale implementation of a Bioregenerative Life Support Systems (BLSS). They demonstrate that these BLSS are needed as a path to large-scale testbeds. Also, the implication that such testing of life support systems is essential for sustained lunar and Mars expeditions in undeniable. As such, I recommend this manuscript for publication after consideration of these points.
- The Introduction gives good background and motivation for the study, but it needs to outline the paper to prime the reader for what follows.
- The testing seems quite sufficient, but the size of the cabin should be justified in terms of scaling in respect of the large-scale testbeds.
- Contamination by life-harming components, such as toxic gases and particulates, are not mentioned. Even in this cabin, the tea light candle (lines 536-7) is one source of contaminants. There will be many more in a spacecraft. I do understand that the study had limited targets for its testing, but at least a mention of the need to test for contaminants will give the paper a broader context.
- There is a significant conclusion in section 4, the Discussion and Outlook: "The absence of measurable overshoot experimentally confirms the design decision to omit integral and derivative control components" (lines 656-9). This seems to need further evidence, and also consideration of whether additional control will be needed for applications in spacecraft where the atmosphere dynamics may not be so slow as for this study.
Overall, I found this article interesting, valid, and significant for keeping humans alive and well in space. Thank you!
Author Response
Reviewer 1
1. The Introduction gives good background and motivation for the study, but it needs to outline the paper to prime the reader for what follows.
Response:
We thank the reviewer for this suggestion. We have revised the concluding part of the Introduction to provide a clearer outline of the manuscript and better prepare the reader for the subsequent sections. Specifically, the revised text now indicates that the manuscript first describes the system design, followed by its verification under representative operating conditions, as well as disturbance scenarios. (Lines 110-116)
2. The testing seems quite sufficient, but the size of the cabin should be justified in terms of scaling in respect of the large-scale testbeds.
Response:
We thank the reviewer for this comment. We have clarified that the cabin dimensioning was originally selected to enable quantification of the C.R.O.P.®-system in its intended experimental configuration. At the same time, it was designed to be payload-independent and enable characterization of other BLSS technologies that fit within the available cabin envelope, as well. Accordingly, the cabin dimensions are driven by the intended subsystem-level characterization use case rather than by scaling with respect to larger BLSS testbeds. (Lines 170-173)
3. Contamination by life-harming components, such as toxic gases and particulates, are not mentioned. Even in this cabin, the tea light candle (lines 536-7) is one source of contaminants. There will be many more in a spacecraft. I do understand that the study had limited targets for its testing, but at least a mention of the need to test for contaminants will give the paper a broader context.
Response:
We thank the reviewer for highlighting this important aspect. We agree that potentially harmful gaseous and particulate contaminants should be considered in future BLSS applications, particularly with respect to astronaut safety. The manuscript has therefore been revised to address this point in the System Concept (Lines 152-157), where we have added the description of a gas-sampling interface, as well as a liquid-phase sampling interface for payloads. These were not mentioned before and may contribute valuable insights for users of the platform that may want to use these for analyses. We have also updated Figure 1 to depict these interfaces. Finally, we have added in the Discussion and Outlook section, that future applications may require enhanced sensing capabilities, as well as dedicated test campaigns using the gas- and liquid-phase sampling interfaces.
4. There is a significant conclusion in section 4, the Discussion and Outlook: "The absence of measurable overshoot experimentally confirms the design decision to omit integral and derivative control components" (lines 656-9). This seems to need further evidence, and also consideration of whether additional control will be needed for applications in spacecraft where the atmosphere dynamics may not be so slow as for this study.
Response:
We thank the reviewer for this comment. We have revised the corresponding discussion to clarify that the proportional pulse-width controller was found to be sufficient for the dynamics and operating conditions investigated in the present study. We further added that future adaptations involving faster dynamics or different payload characteristics may require retuning or extension of the control architecture. (Lines 709-710)
Reviewer 2 Report
Comments and Suggestions for AuthorsI appreciate the well-organized article describing a test bed for supporting biological life support payloads and systems.
Line 18: With respect to the characterization environment, are aerosol components considered including particulates, spores and microbes?
Line 44: atmosphere revitalization is more commonly used than atmosphere regeneration
Lines 151-152: With respect to the description of Figure 1-A, please describe the maximum gas speed established by the circulation fan including manufacture and model for the fan. Also, briefly describe the CO2 scrubber (e.g., calcium hydroxide sorbent), since details are given later.
Line 164: Please clarify what is meant by welded PVC. Are the pieces joined with an adhesive?
Figure 2: Please label major components in the Figures 2-A through 2-C to improve clarity.
Line 234: Please state the acceptable tolerances on O2 and CO2 concentrations for the standard atmosphere composition.
Line 236: Define the ranges of atmospheric pressure, temperature, humidity and gas compositions that can be supported by the system.
Eqn 4: Please provide the heat of reaction for the reaction of CO2 with Calcium hydroxide.
Lines 318-319: State the required heat rate capacity for the coldplate and the actual capacity of the HE500-S coldplate.
Line 333: Please include a schematic of electrical power provision to the article.
Line 345: Please include a diagram of data flow for Command and Data Handling.
Line 399: Please clarify if the atmosphere for Experiment 2 had O2 present.
Lines 414-416: Provide rationale to support that stability criteria have been met. An increase in O2 concentration of 0.7 vol% or nearly 5% would seem high. Explain why the controller did not correct for this relatively steady increase in O2.
Lines 419-420: Provide the acceptance criteria for stability. A fairly steady decrease of 0.15 vol% CO2 or >10% appears substantial. Especially, since this chamber seeks to characterize biological payloads.
Lines 450-455: The reported drift rates seem nonnegligible. Please compare them with respect to the literature such as the Advanced Plant Habitat.
Lines 481-482: I don't think that one can conclude that the gas exchange rate (or leakage) is low for the CO2 removal experiment, since the ambient vol% CO2 is approximately the same as the measured value within the cabin. There's no driving force between the two atmospheres.
Lines 536-537: Please provide additional data of wax type, mass and approximate rate of combustion.
Line 551: It is suggested that the authors only report the number of digits that are significant given the sensor error.
Lines 584-585: What manufacturer and model of bimetallic heater?
Author Response
1.Line 18: With respect to the characterization environment, are aerosol components considered including particulates, spores and microbes?
Response:
We thank the reviewer for this question. The characterization environment includes dedicated interfaces that enable subsequent analysis of both gas- and liquid-phase samples. We have clarified this capability in the System Concept section (Lines 152-157) and updated Figure 1 to depict the corresponding interfaces. A mechanical ball valve allows cabin gas samples to be withdrawn for subsequent analysis of aerosol components, including particulates and spores. In addition, the payload to be implemented can be equipped with a liquid-phase sampling option to enable withdrawal of payload liquid for subsequent analysis. Additionally, controlled supply or exchange of feed liquids during operation are possible. These interfaces provide the capability to investigate particulate and microbiological contamination in future payload experiments. We also added a discussion on these possibilities within the section Discussion and Outlook (Lines 744-751).
2. Line 44: atmosphere revitalization is more commonly used than atmosphere regeneration
Response:
We thank the reviewer for this clarification. We have replaced the term “atmosphere regeneration” with the term “atmosphere revitalization” in the revised manuscript. (Line 46)
3. Lines 151-152: With respect to the description of Figure 1-A, please describe the maximum gas speed established by the circulation fan including manufacture and model for the fan. Also, briefly describe the CO2 scrubber (e.g., calcium hydroxide sorbent), since details are given later.
Response:
We thank the reviewer for this suggestion. We have expanded the description of the atmosphere-management system in the revised manuscript. The manufacturer and model of the circulation fan are now specified (F14 PWM, ARCTIC, Germany) together with its maximum airflow rate of 126 m³ h⁻¹. In addition, the CO₂ removal system is briefly mentioned as a calcium hydroxide-based scrubber, with further details provided in the corresponding sections (Lines 160-162).
4. Line 164: Please clarify what is meant by welded PVC. Are the pieces joined with an adhesive?
Response:
We thank the reviewer for this question. The PVC panels are joined by plastic welding and not by adhesive bonding. We have therefore retained the term “welded polyvinyl chloride (PVC) cabin” in the manuscript, as it describes the manufacturing method. (Line 173)
5. Figure 2: Please label major components in the Figures 2-A through 2-C to improve clarity.
Response:
We thank the reviewer for this suggestion. Figure 2 has been revised accordingly, with main components now labeled.
6. Line 234: Please state the acceptable tolerances on O2 and CO2 concentrations for the standard atmosphere composition.
Response:
We thank the reviewer for this valuable addition. We have revised the manuscript to explicitly state the tolerance bands used for the standard atmosphere composition. The default O₂ concentration is now specified as 21 ± 0.1 %, and the CO₂ concentration as 0.04 ± 0.01 %. These setpoints are used for standard operation, where the payload shall experience no limitation with respect to its supply. As these setpoints and bands are adjustable, stress testing is however possible. (Lines 244-247)
7. Line 236: Define the ranges of atmospheric pressure, temperature, humidity and gas compositions that can be supported by the system.
Response:
We thank the reviewer for this suggestion. We have added the supported operating ranges to the revised manuscript. The system supports cabin pressures from ambient pressure up to a maximum design overpressure of +75 hPa and temperatures from approximately 5-40 °C, with the lower limit representing non-freezing operating conditions and the upper limit corresponding to the maximum ambient temperature considered in the thermal design. Relative humidity has been experimentally verified from ambient conditions up to 90 %RH. Gas-composition setpoints can be configured according to payload requirements within the technical and safety constraints of the system, with non-standard operating conditions subject to prior safety assessment. (Lines 248-258)
8. Eqn 4: Please provide the heat of reaction for the reaction of CO2 with Calcium hydroxide.
Response:
We thank the reviewer for this suggestion. We have added the standard reaction enthalpy of the carbonation reaction to the revised manuscript (reaction enthalpy of ~ ΔH°₂₉₈K = −113 kJ mol⁻¹ CO₂). The corresponding thermochemical reference (Chase, 1998, NIST-JANAF Thermochemical Tables) has also been added. (Lines 308-309)
9. Lines 318-319: State the required heat rate capacity for the coldplate and the actual capacity of the HE500-S coldplate.
Response:
We thank the reviewer for this comment. We have revised the manuscript to explicitly state both the calculated heat-removal requirement and the available cooling capacity of the implemented system. Under the conservative worst-case assumptions used in the thermal FDM model, the maximum required heat-removal rate was 71 W. The coupled system of thermal control unit and HE500-S coldplate provides 200 W of cooling capacity, corresponding to a safety factor of approximately 2.8 relative to the calculated requirement. (Lines 344-351)
10. Line 333: Please include a schematic of electrical power provision to the article.
Response:
We thank the reviewer for this suggestion. A schematic of the electrical power distribution has been added to the revised manuscript as Figure 3-A. The figure illustrates the mains supply and its rail conversion stages inside and outside the cabin, as well as the voltage levels available for payload operation.
11. Line 345: Please include a diagram of data flow for Command and Data Handling.
Response:
We thank the reviewer for this suggestion. An overview has been added to the revised manuscript as Figure 3-B. It showcases the overall command and data handling structure as executed via the single-board computer that processes sensor data received from the microcontroller that governs sensors data acquisition inside and outside the cabin.
12. Line 399: Please clarify if the atmosphere for Experiment 2 had O2 present.
Response:
We thank the reviewer for this comment. We have clarified the experimental description in the revised manuscript. In Experiment 2, only the CO₂ concentration was increased to 1.15 vol.%, while the remaining atmosphere composition was otherwise left unchanged. O₂ was therefore present at approximately ambient concentration, apart from the minor dilution resulting from CO₂ injection. (Lines 437-444)
13. Lines 414-416: Provide rationale to support that stability criteria have been met. An increase in O2 concentration of 0.7 vol% or nearly 5% would seem high. Explain why the controller did not correct for this relatively steady increase in O2.
Response:
We thank the reviewer for this question. We have provided a contextualization for this experiment. We have intentionally disabled the atmosphere control system after establishing the intended initial conditions to access passive drift rates of the cabin atmosphere. Following our observations, we considered the drift rates minimal, with respect to the duration of the experiment (140 h), given that in nominal operation the atmosphere control is active. (Lines 460-465)
14. Lines 419-420: Provide the acceptance criteria for stability. A fairly steady decrease of 0.15 vol% CO2 or >10% appears substantial. Especially, since this chamber seeks to characterize biological payloads.
Response:
We thank the reviewer for this comment. We have revised the corresponding section to clarify the interpretation of the observed concentration changes in response to the previous question. We think this is a valuable addition and clarifies the experiment conditions. As these changes were do not apply when atmosphere control is active, we think this clarification helps the audience to gain insight into the experiment. (Lines 460-465)
15. Lines 450-455: The reported drift rates seem nonnegligible. Please compare them with respect to the literature such as the Advanced Plant Habitat.
Response:
We thank the reviewer for this comment. We have revised the corresponding section to better contextualize the measured passive drift rates. What we have measured are comparatively low drift rates, given that no active control was applied. Only limited corrective actuation is required to compensate for this passive drift during nominal payload characterization, representing a constant background noise for payload characterization. As the amplitude of this background noise however is quite small, we think payload characterization is possible when using the standard atmosphere control scheme. (Lines 496-500)
16. Lines 481-482: I don't think that one can conclude that the gas exchange rate (or leakage) is low for the CO2 removal experiment, since the ambient vol% CO2 is approximately the same as the measured value within the cabin. There's no driving force between the two atmospheres.
Response:
We thank the reviewer for this observation and agree with the assessment. We have therefore removed the corresponding statement from the revised manuscript. (Lines 531-532)
17. Lines 536-537: Please provide additional data of wax type, mass and approximate rate of combustion.
Response:
We thank the reviewer for this suggestion. We have added details with respect to the tea light used. It was a mixed blend of plant-based wax and paraffin with a mass of 12 g and an approximate combustion rate of 3g/h, given the maximal burn duration of 4 hours. (Lines 585-586)
18. Line 551: It is suggested that the authors only report the number of digits that are significant given the sensor error.
Response:
We thank the reviewer for this observation and agree with the assessment. We have adapted the digits of the mean O₂ concentration from 20.859 vol.% to 20.86 vol.% and updated Figure 8 accordingly. (Lines 600-601)
19. Lines 584-585: What manufacturer and model of bimetallic heater?
Response:
We thank the Reviewer for this question. The bimetallic heater used was a THEO 100 made by Hydor, Italy. We have added model and manufacturer in Line 634.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThank you for addressing my comments on your original manuscript. Without hesitation I now recommend your revised manuscript for publication. The paper will be an important contribution to the Journal Instruments.
Reviewer 2 Report
Comments and Suggestions for AuthorsThank you for addressing my review feedback. This paper makes a valuable contribution to the human spaceflight community.
