Chapter 9

The carbon-dioxide production, water-vapor elimination, and oxygen absorption of the subject during 1 or 2 hour periods are recorded in a general way by the amounts of carbon dioxide and water-vapor absorbed by the purifying vessels and the loss of weight of the oxygen cylinder; but, as a matter of fact, there may be considerable fluctuations in the amounts of carbon dioxide and water-vapor and particularly oxygen in the large volume of residual air inside the chamber. With carbon dioxide and water-vapor this is not as noticeable as with oxygen, for in the 1,300 liters of air in the chamber there are some 250 liters of oxygen, and slight changes in the composition of this air indicate considerable changes in the amount of oxygen. Great changes may also take place in the amounts of carbon dioxide and water-vapor under certain conditions. In some experiments, particularly where there are variations in muscular activity from period to period, there may be a considerable amount of carbon dioxide in the residual air and during the next period, when the muscular activity is decreased, for example, the percentage composition of the air may vary so much as to indicate a distinct fall in the amount of carbon dioxide present. Under ordinary conditions of ventilation during rest experiments the quantity of carbon dioxide present in the residual air is not far from 8 to 10 grams. There are usually present in the air not far from 6 to 9 grams of water-vapor, and hence this residual amount can undergo considerable fluctuations. When it is considered that an attempt is made to measure the total amount of carbon dioxide expired in one hour to the fraction of a gram, it is obvious that fluctuations in the composition of residual air must be taken into consideration.

It is extremely difficult to get a fair sample of air from the chamber. The air entering the chamber is free from water-vapor and carbon dioxide. In the immediate vicinity of the entering air-tube there is air which has a much lower percentage of carbon dioxide and water-vapor than the average, and on the other hand close to the nose and mouth of the subject there is air of a much higher percentage of carbon dioxide and water-vapor than the average. It has been assumed that the composition of the air leaving the chamber represents the average composition of the air in the chamber. This assumption is only in part true, but in rest experiments (and by far the largest number of experiments are rest experiments) the changes in the composition of the residual air are so slow and so small that this assumption is safe for all practical purposes.

Another difficulty presents itself in the matter of determining the amount of carbon dioxide and water-vapor; that is, to make a satisfactory analysis of air without withdrawing too great a volume from the chamber. The difficulty in analysis is almost wholly confined to the determination of water-vapor, for while there are a large number of methods for determining small amounts of carbon dioxide with great accuracy, the method for determining water-vapor to be accurate calls for the use of rather large quantities of air. From preliminary experiments with a sling psychrometer it was found that its use was precluded by the space required to successfully use this instrument, the addition of an unknown amount of water to the chamber from the wet bulb, and the difficulties of reading the instrument from without the chamber. Recourse was had to the determination of moisture by the absolute method, in that a definite amount of air is caused to pass over pumice-stone saturated with sulphuric acid. It is of interest here to record that at the moment of writing a series of experiments are in progress in which an attempt is being made to use a hair hygrometer for this purpose.

The method of determining the water-vapor and carbon dioxide in the residual air is extremely simple, in that a definite volume of air is caused to pass over sulphuric acid and soda-lime contained in U-tubes. In other words, a small amount of air is caused to pass through a small absorbing-system constructed of U-tubes rather than of porcelain vessels and silver-plated cans. Formerly a very elaborate apparatus was employed for aspirating the air from the chamber through U-tubes and then returning the aspirated air to the chamber. This involved the use of a suction-pump and called for a special installation for maintaining the pressure of water constant. More recently a much simpler device has been employed, in that we have taken advantage of the pressure in the ventilating air-system developed by the passage of air through the blower. After forcing a definite quantity of air through the reagents in the U-tubes, it is then conducted back to the system after having been measured in a gas-meter.

This procedure is best noted from fig. 30. The connected series of three U-tubes on the rack on the table is joined on one end by well-fitting rubber connections to the tube leading from the mercurial manometer and on the other end to the rubber tube A leading to the gas-meter. On lowering the mercury reservoir E, the mercury is drained out of the tube D and air passes through both arms of the tube and then through the three U-tubes. In the first of these it is deprived of moisture, and in the last two of carbon dioxide. The air then enters the meter, where it is measured and leaves the meter through the tube B, saturated with water-vapor at the room temperature. To remove this water-vapor the air is passed through a tower filled with pumice-stone drenched with sulphuric acid. It leaves the tower through the tube C and enters the ventilating air-pipe on its way to the calorimeter.

The method of manipulation is very simple. After connecting the U-tubes the pet-cock connecting the tube C with the pipe is opened, the mercury reservoir E is lowered, and air is allowed to pass through until the meter registers 10 liters. By raising the reservoir E the air supply is shut off, and after closing the stop-cock at C the tubes are disconnected, a second set is put in place, and the operation repeated. The U-tubes are of a size having a total length of the glass portion equal to 270 millimeters and an internal diameter of 16 millimeters. They permit the passage of 3 liters of air per minute through them without a noticeable escape of water-vapor or carbon dioxide. The U-tubes filled with pumice-stone and sulphuric acid weigh 90 grams. They are always weighed on the balance with a counterpoise, but no attempt is made to weigh them closer than to 0.5 milligram.

GAS-METER.

The gas-meter is made by the Dansk Maalerfabrik in Copenhagen, and is of the type used by Bohr in many of his investigations. It has the advantage of showing the water-level, and the volume may be read directly. The dial is graduated so as to be read within 50 cubic centimeters.

The Elster meter formerly used for this purpose was much smaller than the meter of the Dansk Maalerfabrik we are now using. The volume of water was much smaller and consequently the temperature fluctuations much more rapid. While the residual analyses for which the meter is used are of value in interpolating the results for the long experiments, and consequently errors in the meter would be more or less constant, affecting all results alike, we have nevertheless carefully calibrated the meter by means of the method of admitting oxygen from a weighed cylinder.[23] The test showed that the meter measured 1.4 per cent too much, and consequently this correction must be applied to all measurements made with it.

CALCULATION OF RESULTS.

With an apparatus as elaborate as is the respiration calorimeter and its accessories, the calculation of results presents many difficulties, but the experience of the past few years has enabled us to lessen materially the intricacies of the calculations formerly thought necessary.

The total amount of water-vapor leaving the chamber is determined by noting the increase in weight of the first sulphuric-acid vessel in the absorber system. This vessel is weighed with a counterpoise and hence only the increment in weight is recorded. A slight correction may be necessary here, as frequently the absorber is considerably warmer at the end of the period than at the beginning and if weighed while warm there may be an error of 0.1 to 0.2 gram. If the absorbers are weighed at the same temperature at the beginning and end, this correction is avoided.

The amount of carbon dioxide absorbed from the ventilating air-current is found by noting the changes in weight of the potash-lime can and the last sulphuric-acid vessel. As shown by the weights of this latter vessel, it is very rare that sufficient water is carried over from the potash-lime to the sulphuric acid to cause a perceptible change in temperature, and no temperature corrections are necessary. It may occasionally happen that the amount of carbon dioxide absorbed is actually somewhat less than the amount of water-vapor abstracted from the reagent by the dry air-current as it passes through the can. The conditions will then be such that there will be a loss in weight of the potash-lime can and a large gain in weight of the sulphuric-acid vessel. Obviously, the algebraic sum of these amounts will give the true weight of the carbon dioxide absorbed.

The amount of oxygen admitted is approximately measured by noting the loss in weight of the oxygen cylinder. Since, however, in admitting the oxygen from the cylinder there is a simultaneous admission of a small amount of nitrogen, a correction is necessary. This correction can be computed either by the elaborate formulas described in the publication of Atwater and Benedict[24] or by the more abbreviated method of calculation which has been used very successfully in all short experiments in this laboratory. In either case it is necessary to know the approximate percentage of nitrogen in the oxygen.

ANALYSIS OF OXYGEN.

With the modified method of computation discussed in detail on page 88 it is seen that such exceedingly exact analyses of oxygen as were formerly made are unnecessary, and further calculation is consequently very simple if we know the percentage of nitrogen to within a fraction of 1 per cent. We have used a Haldane gas-analysis apparatus for analyzing the oxygen, although the construction of the apparatus is such that this presents some little difficulty. It is necessary, for example, to accurately measure about 16 cubic centimeters of pure nitrogen, pass it into the potassium pyrogallate pipette, and then (having taken a definite sample of oxygen) gradually absorb the oxygen in the potassium pyrogallate and measure subsequently the accumulated nitrogen. The analysis is tedious and not particularly satisfactory. Having checked the manufacturer's analysis of a number of cylinders of oxygen and invariably found them to agree with our results, we are at present using the manufacturer's guaranteed analysis. If there was a very considerable error in the gas analysis, amounting even to 1 per cent, the results during short experiments would hardly be affected.

ADVANTAGE OF A CONSTANT-TEMPERATURE ROOM AND TEMPERATURE CONTROL.

A careful inspection of the elaborate method of calculation required for use with the calorimeter formerly at Wesleyan University shows that a large proportion of it can be eliminated owing to the fact that we are here able to work in a room of constant temperature. It has been pointed out that the fluctuations in the temperature of the gas-meter affect not only the volume of the gas passing through the meter, but likewise the tension of aqueous vapor. The corrections formerly made for temperature on the barometer are now unnecessary; finally (and perhaps still more important) it is no longer necessary to subdivide the volume of the system into portions of air existing under different temperatures, depending upon whether they were in the upper or lower part of the laboratory. In other words, the temperature of the whole ventilating circuit and chamber, with the single exception of the air above the acid in the first sulphuric-acid absorber, may be said to be constant. During rest experiments this assumption can be made without introducing any material error, but during work experiments it is highly probable that some consideration must be given to the possibility of the development of a considerable temperature rise in the air of the potash-lime absorbers, due to the reaction between the carbon dioxide and the solid absorbent. It is thus apparent that the constant-temperature conditions maintained in the calorimeter laboratory not only facilitate calorimetric measurements, but also simplify considerably the elaborate calculations of the respiratory exchange formerly required.

VARIATIONS IN THE APPARENT VOLUME OF AIR.

In the earlier form of apparatus the largest variation in the apparent volume of air was due to the fluctuations in the height of the large rubber diaphragms used on the tension equalizer. In the present form of apparatus there is but one rubber diaphragm, and this is small, containing not more than 3 to 4 liters as compared to about 30 liters in the earlier double rubber diaphragms. As now arranged, all fluctuations due to the varying positions of the tension-equalizer are eliminated as each experimental period is ended with the diaphragm in exactly the same position, _i. e._, filled to a definite tension.

In its passage through the purifiers the air is subjected to more or less pressure, and it is obvious that if these absorbers were coupled to the ventilating system under atmospheric pressure, and then air caused to pass through them, there would be compression in a portion of the purifier system. Thus there would be a contraction in the volume, and air thus compressed would subsequently be released into the open air when the absorbers were uncoupled. The method of testing the system outlined on page 100 equalizes this error, however, in that the system is tested under the same pressure used during an actual experiment, and hence between the surface of the sulphuric acid in the first porcelain vessel and the sulphuric acid in the second porcelain vessel there is a confined volume of air which at the beginning of an experimental period is under identically the same pressure as it is at the end. There is, then, no correction necessary for the rejection of air with the changes in the absorber system.

CHANGES IN VOLUME DUE TO THE ABSORPTION OF WATER AND CARBON DIOXIDE.

As the water-vapor is absorbed by the sulphuric acid, there is a slight increase in volume of the acid. This naturally results in the diminution of the apparent volume of air and likewise again affects the amount of oxygen admitted to produce constant apparent volume at the end of each experimental period. The amount of increase which thus takes place for each experimental period is very small. It has been found that an increase in weight of 25 grams of water-vapor results in an increase in volume of the acid of some 15 cubic centimeters. Formerly this correction was made, but it is now deemed unnecessary and unwise to introduce a refinement that is hardly justified in other parts of the apparatus. Similarly, there is theoretically at least an increase in volume of the potash-lime by reason of the absorption of the carbon dioxide. This was formerly taken into consideration, but the correction is no longer applied.

RESPIRATORY LOSS.

With experiments on man, there is a constant transformation of solid body material into gaseous products which are carried out into the air-current and absorbed. Particularly where no food is taken, this solid material becomes smaller in volume and consequently additional oxygen is required to take the place of the decrease in volume of body substance. But this so-called respiratory loss is more theoretical than practical in importance, and in the experiments made at present the correction is not considered necessary.

CALCULATION OF THE VOLUME OF AIR RESIDUAL IN THE CHAMBER.

The ventilating air-circuit may be said to consist of several portions of air. The largest portion is that in the respiration chamber itself and consists of air containing oxygen, nitrogen, carbon dioxide, and water-vapor. This air is assumed to have the same composition up to the moment when it begins to bubble through the sulphuric acid in the first acid-absorber. The air in this absorber above the acid, amounting to about 14 liters, has a different composition in that the water-vapor has been completely removed. The same 14 liters of air may then be said to contain carbon dioxide, nitrogen, and oxygen. This composition is immediately disturbed the moment the air enters the potash-lime can, when the carbon dioxide is absorbed and the volume of air in the last sulphuric-acid absorber, in the sodium-bicarbonate can, and in the piping back to the calorimeter may be said to consist only of nitrogen and oxygen. The air then between the surface of the sulphuric acid in the last porcelain absorber and the point where the ingoing air is delivered to the calorimeter consists of air free from carbon dioxide and free from water. Formerly this section also included the tension-equalizer, but very recently we have in both of the calorimeters attached the tension-equalizer directly to the respiration chamber.

In the Middletown apparatus, these portions of air of varying composition were likewise subject to considerable variations in temperature, in that the temperature of the laboratory often differed materially from that of the calorimeter chamber itself, especially as regards the apparatus in the upper part of the laboratory room. It is important, however, to know the total volume of the air inclosed in the whole system. This is obtained by direct measurement. The cubic contents of the calorimeter has been carefully measured and computed; the volumes of air in the pipes, valve systems, absorbing vessels, and tension-equalizer have been computed from dimensions, and it has been found that the total volume in the apparatus is, deducting the volume of the permanent fixtures in the calorimeter, 1,347 liters. The corresponding volume for the bed calorimeter is 875. These values are altered by the subject and extra articles taken into the chamber.

From a series of careful measurements and special tests the following apparent volumes for different parts of the system have been calculated:

Liters. Volume of the chair calorimeter chamber (without fixtures) 1360.0 Permanent fixtures (5); chair and supports (8) 13.0 ------ Apparent volume of air inside chamber 1347.0 Air in pipes, blower, and valves to surface of acid in first acid vessel 4.5 ------ Apparent volume of air containing water-vapor 1351.5 Air above surface of acid in first sulphuric-acid vessel and potash-lime can 16.0 ------ Apparent volume of air containing carbon dioxide 1367.5 Air in potash-lime can, second sulphuric-acid vessel and connections, sodium-bicarbonate cans, and pipes to calorimeter chamber 23.5 ------ Apparent volume of air containing carbon dioxide, water, oxygen, and nitrogen 1391.0

These volumes represent conditions existing inside the chamber without the subject, _i. e._, conditions under which an alcohol check-test would be conducted. In an experiment with man it would be necessary to deduct the volume of the man, books, urine bottles, and all supplemental apparatus and accessories. Under these circumstances the apparent volume of the air in the chamber may at times be diminished by nearly 90 to 100 liters. At the beginning of each experiment the apparent volume of air is calculated.

RESIDUAL ANALYSES.

CALCULATION FROM RESIDUAL ANALYSES.

The increment in weight of the absorbers for water and carbon dioxide and the loss in weight of the oxygen cylinder give only an approximate idea of the amounts of carbon dioxide and water-vapor produced and oxygen absorbed during the period, and it is necessary to make correction for change in the composition of the air as shown by the residual analyses and for fluctuations in the actual volume. In order to compute from the analyses the total carbon-dioxide content of the residual air, it is necessary to know the relation of the air used for the sample to the total volume, and thus we must know accurately the volume of air passing through the gas-meter.

In the earlier apparatus 10-liter samples were used, and the volume of the respiration chamber was so large that it was necessary to multiply the values found in the residual sample by a very large factor, 500. Hence, the utmost caution was taken to procure an accurate measurement of the sample, the exact amounts of carbon dioxide absorbed, and water-vapor absorbed. To this end a large number of corrections were made, which are not necessary with the present type of apparatus with a volume of residual air of but about 1,300 liters, and accordingly the manipulation and calculations have been very greatly simplified.

While formerly pains were taken to obtain the exact temperature of the air leaving the gas-meter, with this apparatus it is unnecessary. When the earlier type of apparatus was in use there were marked changes in the temperature of the calorimeter laboratory and in the water in the meter which were naturally prejudicial to the accurate measurement of the volume of samples, but with the present control of temperature in this laboratory it has been found by repeated tests that the temperature of the water in the meter does not vary a sufficient amount to justify this painstaking measurement and calculation. Obviously, this observation also pertains to the corrections for the tension of aqueous vapor. It has been found possible to assume an average laboratory temperature and reduce the volume as read on the meter by means of a constant factor.

The quantity of air passing through the meter is so adjusted that exactly 10 liters as measured on the dial pass through it for one analysis. The air as measured in the meter is, however, under markedly different conditions from the air inside the respiration chamber. While there is the same temperature, there is a material difference in the water-vapor present, and hence the moisture content as expressed in terms of tension of aqueous vapor must be considered. This obviously tends to diminish the true volume of air in the meter.

Formerly we made accurate correction for the tension of aqueous vapor based upon the barometer and the temperature of the meter at the end of the period, but it has now been found that the reduction of the meter readings to conditions inside of the chamber can be made with a sufficient degree of accuracy by multiplying the volume of air passing through the meter by a fraction, _(h-t)/h_, in which _h_ represents the barometer and _t_ the tension of aqueous vapor at the temperature of the laboratory, 20 deg. C. Since the tension of aqueous vapor at the laboratory temperature is not far from 15 mm., a simple calculation will show that there may be considerable variations in the value of _h_ without affecting the fraction materially, and we have accordingly assumed a value of _h_ as normally 760 mm., and the correction thus obtained is (760 - 15)/760 = 0.98, and all readings on the meter should be multiplied by this fraction.