Carbon Dioxide/Dewpoint Monitor 

STANLEY LUCZKOWSKI 

The portable Carbon Dioxide/Dewpoint Monitor was designed to permit measurements of carbon dioxide partial pressure (PCO2) and dewpoint and ambient gas temperature at any place within the Saturn Workshop. It required no vehicle interface other than storage. All components necessary for operation, including battery power source, were incorporated in the instrument. 

Hardware Description 

Basic Design Configuration.—The carbon dioxide monitoring system consists of two electrochemical sensors and associated amplifiers. The solid state amplifiers drive a readout meter on the front panel of the monitor.

Each carbon dioxide sensor is a small electrochemical cell consisting of a pH sensitive glass electrode, a reference electrode, an electrolyte gel, and a thin membrane. Both electrodes are enclosed within a single housing and are bridged by the electrolyte. The membrane is stretched across the sensor portion of the glass pH electrode. The membrane is permeable to carbon dioxide but is impermeable to airborne solid or liquid contaminants. The electrolyte pH changes with exposure to carbon dioxide. Electrode potential is proportional to the logarithm of the partial pressure of carbon dioxide in the air sample.

The dewpoint ambient temperature sensor contains a mirror surface which is bonded to a small thermoelectric cooling module. The module pumps heat from the mirror and lowers the temperature of the mirror surface; as the temperature reaches the dewpoint, fog appears on the mirror surface. The mirror surface reflects light to a photoelectric sensor which operates in a bridge. The bridge output is amplified and used as feedback to control the thermoelectric cooling module. The mirror temperature is stabilized by the servo loop at the dewpoint (mirror surface just fogged). The mirror temperature is then measured and displayed by the panel meter as the dewpoint temperature.

A hand-operated air sampling pump is located at the top of the Carbon Dioxide/Dewpoint Monitor. The pump draws air through an inlet into the air sampling compartment where the air contacts the dewpoint ambient temperature sensor and both of the carbon dioxide sensors. The pump also serves as the instrument handle. Figure A.II.b.-1 shows the monitor and Figure A.II.b.-2 depicts its typical use.

Performance Reguirements.—The monitor was designed to utilize existing hardware from previous programs. The CO2 sensors and amplifiers were furnished from excess inventory on the Apollo Portable Life Support System program and the dewpoint/ambient temperature sensors and associated electronics were designed and produced for the Gemini program. Consequently, the Skylab effort for this test consisted of packaging these items together with appropriate meters, battery, and switching and sample pump. A regulator circuit was designed to convert the battery voltages, 28 V, to the precision +10 V d.c. and -8 V d.c. required by the carbon dioxide system and a timer module was designed to turn the instrument off after 5 minutes of operation to prevent battery depletion. The block diagram of the system (fig. A.II.b.-3) shows the subsystem interfaces. Specific design requirements are:

Size                                                                        2745 cm³

Weight                                                                   2.66 kg

Range (PC02)                                                       1.3 to 4000 N/m³

Range (ambient temperature)                                  278 to 311 K

Range ( Dewpoint temperature)                              278 to 311 K

Accuracy (CO2)                                                    ±15%

Accuracy (temperature)                                          ±1.6%

Useful battery life                                                    6 hours

Output                                                                    Meter readout

Response time                                                        (CO2) 3 minutes

Response time (temperature)                                  10 seconds 

Summary 

The Carbon Dioxide/Dewpoint Monitor was planned for use only during the first mission to measure carbon dioxide, dewpoint, and temperature in various workshop locations. The initial problems associated with the damaged workshop folowing launch exposed the instrument to environments outside of the design considerations. Low pressure cycles and high temperature presented conditions conducive to failure of the carbon dioxide sensor membrane and possibly to failure of electronics parts in dewpoint monitor circuitry.

Development testing with these sensors for the Apollo Program demonstrated that electrolyte can be drawn through the membrane by exposure to pressures of approximately 0.17 Pa as was experienced on the Orbital Workshop during post-launch venting to purge it of potential toxic gases. This depletion of electrolyte caused improper sensor operation.

Also, with the instrument stabilized in an excessively warm stowage location, the entire mass temperature influenced the temperature and carbon dioxide readings when the instrument was used to obtain measurements in a significantly cooler portion of the Orbital Workshop. 

Acknowledgment 

Beckman Instruments, Inc. redesigned and packaged the instruments originally packaged for the Gemini and Apollo Programs. Dewpoint Sensor design was previously accomplished by Cambridge Systems, Inc. 

Bibliography 

Technical Manual, Vol. I, Subsystem Data, CO2/Dewpoint Monitor. Document No. FM-1086-301. Beckman Instruments Inc., Fullerton, California.

Final Report, Design Development and Testing Dewpoint Hygrometers for the NASA/Gemini Programs, Contract NAS 9-4793. Cambridge Systems Inc., Newton, Mass.

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