IN experimental plant physiology the problem of the control of

Transcription

1 [ 119 ] THE CONTROL OF ATMOSPHERIC HUMIDITY IN A CLOSED SYSTEM BY B. D. BOLAS From the Department of Plant Physiology and Pathology, Imperial College of Science and Technology, London (With 4 figures in the text) IN experimental plant physiology the problem of the control of atmospheric humidity is one of considerable difficulty. The apparatus described in this paper has been designed to meet this difficulty. Atmospheric humidity may be expressed in two ways, it may be stated in terms of " relative humidity," that is the percentage saturation of the atmosphere, or in terms of the "saturation deficit," this being the difference in vapour pressure between that of saturated aqueous vapour and that of the water vapour actually present in the air, both being measured at the same temperature. A brief consideration of the subject will show that these two concepts "relative humidity" and "saturation deficit" are widely different. In, for e.xample, the case of a somewhat moist atmosphere such as air which is 75 per cent, saturated at 15 C, the humidity may be expressed by the statement that the pressure of saturated aqueous vapour at 15 C. being equivalent to mm. of mercury, the pressure of aqueous vapour present in the air under consideration is equivalent to 9-56 mm. of mercury, i.e. that 75 per cent, of the possible water is present; such an atmosphere is said to have a relative humidity of 75 per cent, at 15 C. The saturation deficit, on the other hand, is not expressed as a percentage but is given by the absolute difference between the two vapour pressures, being equivalent in this case to , i.e mm. mercury. An atmosphere 75 per cent, saturated at 25 C. woukl clearly have a very different saturation deficit from that of an atmosphere 75 per cent, saturated at 15 C, because at 25 C. the pressure of saturated aqueous vapour equals nim., and the pressure of water vapour present must therefore be equal to 75 per ccnl. uf this or mm., giving a saturation deficit of 5-92 mm., so that although we have the same relative humidity in each ease, the

2 120 B. D. BOLAS saturation deficit, and hence the evaporating power, is very much greater at the higher temperature. The relations between certain saturation deficits, temperatures, aitd relative humidities are shown graphically in Fig. i. In experiments on the plant it is, therefore, rather the saturation deficit than the relati\-e humidity that should be maintained at a constant value, and attempts have been made in various ways to attain this end. One method is so to circulate the air in the system where Fig. I. Relative Humiditv - (5-~.^' ': X x = pressure of saturated aiiuemis vapuur at air tcmperaturi', y = saturation deiicit. containing the plant that this air passes through a solution of calcium chloride having the required vapour pressure. This method is open to several objections, among these being the fact that if any considerable amount of water is given up to or removed from the air a change in tht' concentration of the calcium chloride solution must result, thus changing the vapour pressure. Another objection is that if the air be circulated at all fast there is risk of minute globules of the fluid being carried over in spite of scrubbing devices.

3 The Control of Atmospheric Humidity Another and in many respects more satisfaetory' method is to saturate the circulating air with water at or above the temperature of the growing chamber, and then to pass it through a condenser which is immersed in a bath maintained at a temperature below that of the growing chamber, the temperature of the bath being such that saturation at this temperature gives the required saturation deficit when the temperature of the circulating air again rises to that of the growing chamber. The only important objection to this method of humidity control is that any change in the temperature of the growing chamber or of the condenser involves a change in the saturation deficit, and this difficulty cannot be overcome by maintaining the condenser at a temperature which differs by a fixed amount from that of the growing chamber, owing to the fact that, in order to maintain a fixed saturation deficit at different temperatures of the growing chamber, the required difference between the temperatures of the condenser and the growing chamber is not fixed but is a mathematical function of the temperature of the growing chamber. Thus, if the growing chamber be maintained at a temperature of 14-6 C. and the air passing into this be saturated at a temperature of 10" C, it will have a saturation deficit of 3-2 mm. on attaining the temperature of 14-6 C. If, however, the growing ehamber be maintained at a temperature of 24-5 C, the temperature of the condenser required to maintain the same saturation deficit is 22-9 C., the difference in t( mperature required to maintain the saturation deficit of 3-2 mm. thus being.\-(f Q.. at the lower temperature and only i-fi" C. at the higher. If, from a table of vapour pressures, one calculates the temperatures of the condi'nser and the growing chamber required for a particular saturation deficit at different growing chamber temperatures, and plots these in the form of a graph, it will be seen that for certain physiologically important ranges of growing chamber temperature and saturation deficit the graph appm.ximates to a straight line; three such graphs are shown in Fig. 2. These graphs suggest that the temperature may be treated as a linear function of the temperature of the growing chamber, and if this assumption be permissible it should be a comparatively easy matter to design a thermostatic device which would autdiuatirally maintain a constant saturation deficit in spite of lrnii>eiat\ire variations in the growing chamber. With the object of iii\e-..tigating this possibility the linear equations of closi'st lit were determined by the method of least squares for three saturation I2i

4 122 B. D. BoLAS deficits, namely 3-2 mm., 6-o mm. and io-omm.; these also are shown in Fig. 2, and it will be seen that they approach closely to the values obtained from the vapour pressure tables 1. Having found the linear equations connecting growing chamber temperature and condenser temperature, it remained to determine the error produced in the saturation deficit by treating these equa Fig. 2. Graphs of /, = o + bt^, where a = constant, b = constant. (, = condenser temp., t^ air temp Points plotted are from Landolt's Tables. Straight lines are linear equations of closest fit. tions as true, and Fig. 3 shows these errors for the same three saturation deficits. It will be seen from the curves in this figure that in none of the cases considered is the error produced by treating the condenser temperature as a linear function of the temperature of the growing ehamber greater than plus or minus 6-6 per cent, of the saturation deficit, except at a temperature above 24 C. with a satura- ' A point of some interest, although it cannot be elaborated here, is that the constants a and b in the equations appear to be approximately linear functions of the saturation deficit.

5 7/. / / / /. The Control of Atmospheric Humidity 123 tion deficit of 10 mm. Over a shorter range of temperature than those plotted the error would obviously be less. From these results it woukl appear possible to design an automatic control of saturation deficit, capable of giving constant values within about 10 per cent, total variation over a limited range of temperature; the apparatus described below has accordingly been evolved. s 4 3 2?v V on Dtficii Jo a-2 ^rror cl ( 2-4 p -6 ^ 1 ^ / >?/ ;/ 1 f J I A' U \ \ \ \ \"' A ^ \ \\\ Air Temperuture "C. F'g- 3- f \ \ k\ / ' A V-- A- APPARATUS FOR THE CONTROL OF S.\TURATION DF.FICIT The apparatus is shown by Fig. 4. It consists in essentials of two mercury vessels, one B in the growing chamber, this vessel being so shaped as to take as nearly as practicable the mean temperature of the growing chamber; the other mercury vessel A is in a water bath containing the condenser, and this bath is kept well stirred by an electric stirrer (not shown). The condenser is of speeial design and is fitted with intemal glass baffles to increase its efficiency. It is also fitted with a small glass reservoir 7\, into which condensed water drains and from which it may be ilrawu nff by means uf a tube which is normally kept closed by a rubber cap.

6 124 B. D. BoLAS a o u i c % e- li i iii,',im 1 iiln'.l'il I 'I

7 The Control of Atmospheric Humidity 125 Each of the mercury vessels communicates with a small vertical capillary tube, E and H in Fig. 4, these two tubes being placed side by side and securely fastened. A hollow glass float C, havmg a platinum wire passing through it into the mercury in E, rises and falls on the mercury meniscus in this tube, which is in communication with the mercury vessel in the condenser bath. The platinum wire is so bent as to rise and fall in the capillary tube H with the rise and fall of the float C in the eapillary tube E. Contact with the wire is made either by a rise of mercury in H or by a fall of mercury in E. When contact is made a eurrent passes through two wires, one sealed into each mercury system, which are connected with a battery and a sensitive relay. An electro-magnetic device (not shown) is used to control a supply of water at 0 C. to the condenser bath, the device being so constructed that when contact is made between the two mereury columns E and H the supply of cold water is shut off. A minute gas flame is kept burning under the condenser bath so that when cooling ceases the temperature rises. As soon as contact is broken by a rise of temperature, in the condenser bath or a fall in the growing chamber, the cold water is again allowed to flow. The condenser bath is fitted with a constant-level overflow whieh is not shown. A small serew adjustment for making slight changes in the level of the mercury in tube H is provided and is shown by M in Fig. 4. If the mercury vessels A and B were of equal capacity and the capillary tubes E and H of equal bore, it follows that the difference in temjjerature of condenser and growing ehamber once set by adjustment of the mercury levels in E and H would be constant whatever the temperature of the growing ehamber; if, however, the vessels A and B are of different sizes then any desired linear relations between the two temperatures can be obtained strictly this is only true if the coefficient of expansion of mercury in glass is constant over the required range, but the error introduced by this assumption is very small. For a saturation deficit of 3-2 it was found that the required ratio of the capacity of the vessel in the condenser bath to that of the vessel in the growing chamber was as 35-6 is to 45-0, and in the experimental apparatus the vessels were made of 35-6 c.c. and 45-0 c.c. respeetively. In practice it was found necessary to increase the rigidity of tlie platinum wire which rises and falls with the float C by surrounding it by a thin sheath of glass in the form of a very fine capillary tube.

8 126 B. D. BoLAS It was also found advantageous slightly to constrict the top of the capillary tube H in order to prevent the platinum contact wire getting out of centre and so making contact with a portion of the mercury meniscus at a lower level, thus upsetting the adjustment; the contact wire itself was found to be more satisfactory when so bent that a minute horizontal portion of wire came in contact with the meniscus, rather than when the actual point made contact. A preliminary trial gave the following results: Growing Condenser Observed Required eliamber bath ditference difference K./. \^t KJ, ^ lo-o The apparatus was set at the above temperatures and the temperature of the growing chamber was then raised to 25-0 C: ^ C'5 ^-4 The apparatus was again set at; The temperature of the growing chamber was then reduced', and two readings were obtained; ^ These results, although not very accurate, indicate the value of the method; and it seems probable that the accuracy might be increased by using larger vessels. It is hoped later to publish a fuller account of the actual changes in the saturation deficit observed when using a similar type of apparatus. 1 For the purposes i)f this preliminary experiment the growing chamber was filled with water and fitted with a stirrer; the temperature was changed by the addition of hot or cold water and a few minutes allowed for the attainment of equihbrium.

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