Research methods in ecology

by Frederic E. Clements

62. Readings. All observations should be made facing the wind, and the observer should move one or two steps during the reading to prevent the possibility of error. The cloth of the wet bulb is moistened with water by means of a brush, or, much better, it is dipped directly into a bottle of water. Distilled water is preferable, as it contains no dissolved material to accumulate in the cloth. Tap-water and the water of streams may be used without appreciable error, if the cloth is changed somewhat more frequently. The temperature of the water is practically negligible under ordinary conditions. Readings can be made more quickly, however, when the temperature is not too far from that of the air. The psychrometer is held firmly and swung rapidly through the air when the space is not too confined. Where there is danger of breakage, it is swung back and forth through a short arc, pendulum-fashion. As the reading must be made when the mercury of the wet bulb reaches the lowest point, the instrument is stopped from time to time and the position of the column noted. The lowest point is often indicated by the tendency of the mercury to remain stationary; as a rule it can be noted with certainty when the next glance shows a rise in the column. In following the movement, and especially in noting the final reading, great care must be taken to make the latter before the mercury begins to rise. For this reason it is desirable to shade the psychrometer with the body when looking at it, and to take pains not to breathe upon the bulbs nor to bring them too near the body. At the moment when the wet bulb registers the lowest point, the dry bulb should be read and the results recorded.

63. Cog psychrometer. This instrument, commonly called the “egg-beater” psychrometer, has been devised to obviate certain disadvantages of the sling psychrometer in field work, and has entirely supplanted the latter in the writer’s own studies. It is smaller, more compact, and the danger of breaking in carriage or in use is almost nil. It has the great advantage of making it possible to take readings in a layer of air less than two inches in thickness, and in any position. Fairly accurate results can even be obtained from transpiring leaves. The instrument can readily be made by a good mechanic, at a cost for materials of $1.75, which is less than half the price for the sling form. A single drawback exists in the use of short, Centigrade thermometers, inasmuch as tables of relative humidity are usually expressed in Fahrenheit. It is a simple matter, however, to convert Centigrade degrees into Fahrenheit, mentally, or the difficulty may be avoided by the conversion table shown on page [47], or by constructing a Centigrade series of humidity tables. The fact that the wet and dry bulbs revolve in the same path has raised a doubt concerning the accuracy of the results obtained with this instrument. Repeated comparisons with the sling psychrometer have not only removed this doubt completely, but have also proved that the standardization of the thermometers has been efficient.

64. Construction and use. A convenient form of egg-beater is the Lyon (Albany, New York), in which the revolving plates can be readily removed, leaving the axis and the frame. The thermometers used are of the short Centigrade type. They are 4½ inches long and read from –5° to 50°. Eimer and Amend, 205 Third Ave., New York city, furnish them at 75 cents each. The thermometers are carefully standardized and compared, and then grouped in pairs that read together. Each pair is used to construct a particular psychrometer. Each thermometer is strongly wired to one side of the frame, pieces of felt being used to protect the tube and increase the contact. The frame is also bent at the base angles to permit free circulation of air about the thermometer bulbs. The bulb of one thermometer is covered with the proper cloth, and the psychrometer is finished. Since the frame revolves with the thermometers, it is necessary to pour the water on the wet bulb, or to employ a pipette or brush. The thermometer bulbs are placed in the layer to be studied, and the frame rotated at an even rate and with moderate rapidity. The observation is further made as in the case of the sling psychrometer. As the circle of rotation is less than three inches in diameter, and the layer less than an inch, in place of nearly three feet for the sling form, the instrument should not be moved at all for extremely localized readings, but it must be moved considerably, a foot or more, if it is desirable to obtain a more general reading.

65. Hygrometers. While there are instruments designed to indicate the humidity by means of a hygroscopic substance, not one of them seems to be of sufficient accuracy for use in ecological study. The difficulty is that the hygroscopic reaction is inconstant, rather than that the instruments are not sufficiently sensitive. A number of hygrometers have been tested, and in all the error has been found to be great, varying usually from 10–20 per cent. In the middle of the scale they sometimes read more accurately, but toward either extreme they are very inexact. It seems probable that an accurate hygrometer can be constructed only after the model of the Draper psychrograph. Its weight and bulk would make it an impossible instrument for field trips, and the expense of one would provide a dozen psychrometers. In consequence, it does not seem too sweeping to say that no hygrometer can furnish trustworthy results. Of simple instruments for humidity, the psychrometer alone can be trusted to give reliable readings. Crova’s hygrometer, used by Hesselmann, is not a hygrometer in the sense indicated. As it is much less convenient to handle and to operate than the cog psychrometer, it is not necessary to describe it.

Psychrographs

Fig. 7. Draper psychrograph.

66. The Draper psychrograph. A year’s trial of the Draper psychrograph in field and planthouse has left little question of its accuracy and its great usefulness. Essentially, it consists of a band of fine catgut strings, which are sensitive to changes in the moisture-content of the air. The variations in the length of the band are communicated to a long pointer carrying an inking pen. The latter traces the record in per cent of relative humidity on a graduated paper disk, which is practically the face of an eight-day clock. The whole is enclosed in a metal case with a glass front. A glance at the illustration will show the general structure of the instrument. Continued psychrometric tests demonstrate that the margin of error is well within the efficient difference for humidity, which is taken to be 5 per cent. In the field tests of the past summer, two psychrographs placed side by side in the same habitat did not vary 1 per cent from each other. The same instruments when in different habitats did not deviate more than 1 per cent from the psychrometric values, except when the air approached saturation. For humidities above 90 per cent, the deviation is considerable, but as these are temporary and incident upon rainfall, the error is not serious. For humidities varying from 10–85 per cent, the psychrograph is practically as accurate as the psychrometer. Per cents below 10 are rare, and no tests have been made for them.

Fig. 8. Instrument shelter, showing thermograph and psychrograph in position.