In order to control the work of ensuring a given gas composition, to manage these means and evaluate the suitability of the atmosphere for the stay of astronauts, you need to have information about the current amount of each of those components that affect human life. This means that it is necessary to regularly analyze the chemical composition of the atmosphere. The frequency and methods of control for different components can be different, depending on the degree of their effect on the human body and the maximum possible speed of quantitative changes. Particular attention in the analysis is always paid to the measurement of partial pressure of oxygen, carbon dioxide and water vapor. This is due to the fact that these parameters are most dynamic, and therefore require almost continuous control. For its implementation, automatic gas analyzers are installed on board.
The measurement of the partial pressure of oxygen is usually made using electrochemical sensors. The principle of operation of these sensors is based on the use of the dependence of the electrochemical processes occurring in the galvanic cells of the sensors, from the amount of oxygen entering the cells.
Air is pumped through the gas cavity with a microvant. The liquid cavity is filled with a solution of potassium hydroxide and two electrodes are placed in it: a silver cathode and a lead anode. Cavities are separated by a membrane permeable for gas and impenetrable for the solution. The cell works as follows.
The humidity of the atmosphere is determined by electrolytic sensors, in each of which there is a container where the air of the residential compartment is in contact with a saturated aqueous solution of hygroscopic salt. The principle of action of the sensor is based on the fact that between the partial pressure of the water vapor of the analyzed air and the vapor of the solution, a hygro-thermal equilibrium is established. As a result, the concentration of the solution (due to evaporation or absorption of additional moisture) changes. Two electrodes are placed in the salt solution, to which alternating voltage is supplied. With a change in the concentration of the solution, its electrical resistance changes, and hence the value of the current passing through the solution and the temperature of the solution. Measuring this temperature, they receive information about the partial pressure of water vapor.
In the atmosphere of such harmful impurities as carbon monoxide, methane, ammonia, acetone, hydrogen sulfide, during the normal course of the flight, it changes very slowly, so monitoring them can not be carried out continuously, but discrete, with large time intervals (several weeks or even months). In cases where there is regular transport communication between the land and the manned device, control is carried out on the ground in the laboratory by chemical analysis of air samples delivered from the side of the apparatus in special ampoules. If there is no transport connection, then the number of impurities can be determined in the flight conditions.
To do this, usually at the disposal of astronauts there is a set of such chemical reagents, each of which, interacting with one of the controlled components, changes its color. The concentration of the component is determined by the nature of the color of the reaction product. Generally speaking, it is advisable to provide the possibility of controlling harmful impurities with on -board means for any manned flight. It should be borne in mind that the need for such control may appear suddenly, for example, after the elimination of the fire of the fire, when there is a probability of unacceptable atmospheric pollution by combustion products.