US2444613A - Low humidity meter - Google Patents

Description

y 5,194& I H. MccoMBm ETAL ZAMfiH LOW HUMIDITY METER Filed Aug. 31, 1943" 2 Sheets-Sheet 1 THERMOCOUPLES 32 INSULATION Mg(c| o4): 1

DEW- POINT MEASURING INVENTOR. Horace R. M Combie C leman D. Wilder BY ATTORNEY.

H. R. mcommg ETAL 2 LOW HUMIDITY METER Filed Aug. 31, 1943 2 49.3 1: mi 3 0* m ow mw ow 3 3 o m H- w 2 L m P ow M 3 A R w. M on 1&9 M S A wmo o-- T N m 0.. wn. c no R m m km mEotcm o o goo 3.30 522930 ow..- w m DEW POINT- DEGREES CENT/GRADE Patented July e, 1948 LOW HUMIDITY METER Horace R. McCombie, Pittsburg, and Coleman D. Wilder, Berkeley, Calii., assignors to the United States of America as represented by the United States Atomic Energy Commission Application August 31, 1943, Serial No. 500,648

2 Claims. l

The present invention relates to hygrometric processes and methods and more particularly to hygrometers.

It is an object of the invention to provide an improved hygrometric system and method which readily correlates the rate of flow of a gas and the water vapor content thereof. I

Another object of the invention is to provide an improved hygometric system and method utilizing the heat-of-absorption or heat-of-solution effect of a dehydrating agent when it is contacted with a gas containing water vapor.

Another object of the invention is to provide an improved hygrometer' comprising a moistureresponsive I element containing a dehydrating agent having a finite heat of absorption.

Another object of the invention is to provide an improved hygrometer comprising a moistureresponsive element in the form of a porous packing containing a dehydrating agent having a finite and relatively high positive heat of absorption.

A further object of the invention is to provide an improved continuous reading. hygrometer which is extremely sensitive, whereby the moisture content of a gas having dew-points of 30 C. and below may be readily measured.

A still further object of the invention is to provide a hygrometer of improved construction and arrangement which is capable of reading directly either very low dew-points or very small water vapor contents of test gases.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which Figure 1 is a diagrammatic illustration of a hygrometric system and a ion gitudinal sectional veiw of a hygrometric element incorporated therein, which embody the present invention; and Figure 2 illustrates graphically the calibration curve of a potentiometer incorporated in the system shown in Fig. 1.

Referring now moreparticularly to Fig. 1 of the drawings, there is shown a hygrometric system comprising a hygrometric element ill, an attached manometer l I, and an associated pumping device or'blower l2 of the constant speed 2 type. In the particular embodiment of the hygrometric element It illustrated in Fig. 1 of the drawings, the parts are drawn to approximately twice scale; and the element l comprises an upstanding cylindrical casing l3 having internally threaded rings l4 and I5 suitably secured in the respective upper and lower ends thereof. Also, the element comprises a removable upper cover l6 carrying an externally threaded ring l! adapted to engage the threads carried by the ring M, a. suitable gasket 18 being arranged between the upper cover l6 and the ring M in order to render the upper end of the casing l3 gas-tight; and a removable lower cover l9 carying an externally threaded ring 20 adapted to engage the threads carried-by the ring I5, a suitable gasket 2| being arranged between the cover l9 and the ring IS in order to render the lower end of the casing I3 gas-tight. v

Arranged within the casing l3 and spaced inwardly therefrom are upper and lower cylindrical baflles .22 and 23, the baflles 22 and 23- being disposed in alignment and in spaced apart relation. The upper baiile 22 is secured in place by a ring 24 positioned between the outer surface thereof and the inner surface of the casing l3; while the lower baflle 23 is secured in place by a tube 25. The tube 25 extends through an opening provided in the side wall of the casing l3 and is supported thereby; and one end of the tube 25 is securely anchored in a cooperating opening provided in the side wall of the lower baffle 23. The upper end of the lower bafiie 23 is closed by a plate 26 secured therein, the plate 26 being formed of any suitable insulating material such as Bakelite. A ring 21 is secured within the upper baille 22; a ring 28 is secured within the lower baffle 23; and an upstanding cylindrical reticulated screen '29 is embedded in the insulating 'oi the upper bailie 22.

3 the lower surface of the insulating plate 23 to define a lower compartment 3| within the casing I3. The insulating plate 28 has a number of openings extending therethrough which are arranged in any suitable symmetrical pattern and in which a suitable number of thermocouples '32 are anchored. In the illustrated embodiment of the element I fifty-five thermocouples 32 are anchored in the openings provided in the insulating plate 23 and are arranged in series circuit relation, the intermediate portions of the thermocouples 32 being supported in the openings pro.- vided in the insulating plate 25 in such a manner that the insulating plate 26 is impervious to the seepage of gas therethrough. Preferably, the thermocouples 32 are or the iron-constantan types. In this arrangement, the

- upper ends of the thermocouples 32 arranged in the upper compartment 30 constitute cold ends; while the lower ends of the thermocouples 32 arranged in the lower compartment 3| constitute hot ends, as will be more fully explained hereinafter. As previously noted, the thermocouples 32 are arranged in series circuit relation and are terminated by two terminals 33.

The upper compartment 3|! contains a porous packing 34 formed or sharp, clean sand, this sand being screened and sized such that it will pass a 20 mesh screen but will not pass a 30 mesh screen. The packing 34 arranged in the upper compartment 30 embeds the upper ends of the thermocouples 32 and is retained in place by a reticulated plate 35 positioned in the upper end The reticulated plate 35 is retained in place by a compressed coil spring 35 arranged between the upper surface of the reticulated plate 35 and the lower surface of the upper cover IS. The lower compartment 3| contains upper and lower porous packings 31 and 38, respectively, arranged in series, the packing 31 and 33 being separated by a reticulated plate 39 loosely positioned within the ring 28. The packing 31 is formed of a dehydrating agent, this dehydrating agent being screened and sized-such that it will pass a 20 mesh screen but will not pass a 30 mesh screen. Preferably, this dehy -drating agent comprises Mg(ClO4)2 in the form sold as Anhydrone. However, it may comprise other compounds of a class including CaClz and P205, as explained more fully hereinafter. The packing 38 is formed of sharp, clean sand, this sand being screened and sized such that it will pass a 20 mesh screen but will not pass a 30 mesh screen. The packing 37 arranged in the lower compartment 3| embeds the lower ends of the thermocouples 32; and both the packings 31 and 38 are retained in place by a reticulated plate 40 positioned in the lower end of the lower baille 23. The reticulated plate 40 is retained in place by a compressed coil spring 4| arranged between the lower surfa-ce of the reticulated plate 40 and the upper surface of the lower cover I9.

Finally, the element In comprises a tube 42, one end of which is anchored in a cooperatin opening provided in the side wall of the casin l3 above the ring 24,and adjacent the upper end of the upper baille 22. In the element Ill the tube 42 constitutes a gas inlet tube communicating with the upper compartment 30; while the tube 25 constitutes a gas outlet tube communicating with the lower compartment 3|. The gas inlet tube 42 is connected to a pipe 43, to which the manometer II is also attached; while the gas outlet tube 25 is connected to a pipe 44, extendin; to the blower I 2.

Chromel-Alumel or Considering now thegeneral flow of gas through the element I0, it is noted that when the blower I2 is operating a partial vacuum is established in the gas outlet tube and gas may be drawn into the gas inlet tube 42 from the pipe 43.- This gas flows from the pipe 43 through the gas inlet tube 42 into the upper portion of the casing I3, upwardly in the cylindrical space between the upper portion of the casing l3 and the upper baffle 22, and then downwardly through the open end of the upper bailie 22 through the reticulated plate into the upper compartment 30. This gas is thoroughly diffused by the packing 34 arranged in the upper compartment 30 through andaround the upper ends of the thermocouples 32,

and then flows downwardly toward the insulating plate 26 and outwardly through the reticulated screen 29 toward the mid-section of the casing I3. The gas-then flows downwardly in the cylindrical space between the lower portion of the casing I3 and the lower baffle 23, and then upwardly through the open end of the lower baffle 23 through the reticulated plate into the lower compartment 3|. This gas is thoroughly diffused by the packing 33 arranged in the lower portion or the lower compartment 3|, and then passes through the reticulated plate 39 into the upper portion of the lower compartment 3|. This gas is then thoroughly difiused by the packing 31 arranged in the upper portion of the lower compartment 3| through and around the lower ends of the thermocouples 32 and then flows upwardly toward the insulating plate 25 and outwardly through the reticulated screen 29 into the gas outlet tube 25. Finally, the gas in the gas outlet tube 25 is drawn through the pipe 44 by the blower l2 and exhausted to atmosphere. The volume of gas drawn through the element III may be readily determined by reading the manometer attached to the pipe 43 adjacent the inlet tube 42, as the blower I2 is of the constant speed type.

In the construction and arrangement of the element I0, it is noted that the packing 34 disposed in the upper compartment 30 offers an impedance to the flow of gas therethrough which is equal approximately to the sum of the impedances offered to the flow of gas through the series-arranged packings 38 and 31 disposed in the lower compartment 3|. Thisarrangement insures that the cooling effect produced by the gas as it is diffused by the packing 34 onto and around'the upper ends of the thermocouples 32 is substantially equal to the cooling effect produced by the gas as it is diffused by the packing 31 onto and around the lower ends of the thermocouples 32; whereby the temperature of the gas exerts substantially equal and opposite effects upon both the upper and lower ends of the thermocouples 32, thereby rendering the operation of the element It substantially independent of the temperature of the gas and ambient temperature conditions.

Finally, associating with the element It is a galvanometer or potentiometer 45 which is electrically connected to the terminals 33 in any suitable manner. Preferably, the potentiometer 45 is of the well-known Leeds and Northrop type. Also, it is preferred that the element II), the manometer I I, the blower I2, and the potentiometer 45 be arranged in a suitable external casing, thereby constituting a portable hygrometer.

As previously explained, the packing 31 arranged in the upper portion of the lower compartment 3| and embedding the lower or hot ends of the thermocouples 32, comprises a dehydrating agent of a group including Mg(C10-i) 2, CaClz, and

PaOs, this class oi dehydrating agents having the following specifications:

(1) Great amnity for water vapor;

(2) Very high positive heat of absorption or solution;

(3) Chemically inert to air and ordinary permanent gases;

(4) Solid granulated form such that a porous pachins can be made therefrom;

Of the dehydrating agents in this class,

Max:104):

is preferred in view or the fact that it has a very great afinity for water vapor; it has a finite and very high positive heat of absorption or hydration hydrating agent previously mentioned. When the moisture in the gas passing through the packing ti is absorbed by this dehydrating agent it forms a hydrated compound, producing a, considerable amount of heat, the heat of hydration of in rown): being approximately 25,000 caiories per mole, as previously noted. Thus, the packing Sl becomes hot due to the chemical action at the moisture forming a hydrate with the dehydrating agent, whereby the lower ends of the thermocouples 82 are heated, causing a voltage to be produced which appears across the terminals and may be measured by the potentiometer it. It is noted that the voltage appearing across the terminals 33 and measured by the potentiometer it is controlled directly by the temperature of the packing ill, which temperature, in turn, is controlled directly by the amount of heat liberated in this dehydrating agent, while the amount of heat liberated in this dehydrating agent is controlled directly in accordance with the amount of moisture absorbed thereby from the gas difiusecl therethrough. Finally, the amount of moisture absorbed by the packing :tl is directly controlled jointly and principally by the rate of flow of gasthrough the element it and the moisture content of the gas flowing therethrough. Accordingly, by maintaining substantially constant the rate of gas flowing through the element it, the amount of moisture absorbed by the packing 31 will be dependent substantially entirely upon the moisture content of the gas flowing therethrough. Hence, in the element it the potentiometer t5 meters a voltage which is proportional to the moisture content of the gas flowing therethrough.

Further considering the construction and arrangement of the hygroinetric system, it is noted that in addition to the elements previously described, the system comprises a pumping device or blower it, a drier M, a humidifier it, and dewpoint measuring apparatus (it; The blower fit is adapted to take in air at atmospheric pressure and to deliver it at any desired gauge pressure to a connected pipe 5%, the pipe dd being suitably connected by adjustable valves ti and t2, respectlv'ely', to the drier ti and the humidifier $9. The drier ti and the humidifier it are respectively connected by adjustable valves 58 and It to a common pipe 85, which in turn is connected to the dew-point measuring apparatus 49. Also, the pipe 5! is connected by an adjustable valve 86 to the pipe 68. Finally. the pipe fit is also connected to a sampling conduit it by way of an adjustable valve 5d.

The drier s1 is of any suitable form, although it preferably comprises a series of H2804 bubblers and Mg ClO4lz drying columns, whereby the air from the pipe 50 may be delivered through the drier 51 in extremely dry condition to the pipe as. The humidifier so is of any suitable form, although it preferably comprises a series or H20 bubblers, whereby the air from the pipe ti may be delivered through the humidifier thin ex tremely wet condition to the pipe 55. Thus it will be understood that by suitably adjusting the proportion of air flowing from the pipe tit through the drier iii to the pipe be with respect to the proportion of the air iiowing from the pipe it through the humidifier it to the pipe tit, the air in the pipe 55 may be adjusted to any desired vapor content over a very wide range. The proportion or air flowing through the drier il may be readily determined by manipulating the adiustable valves ti and t8; while the proportion of air flowing through the humidifier tit may be readily determined by manipulating the adjustable valves t2 and as. A portion of the air in the pipe 55 may be admitted through the adjustable valve lit into the pipe at and conseouently into the element it, the adjustable valve 56 being manipulated in order to establish the desired flow of air from the pipe 56 through the element i t. Finally, the remaining portion of the air in the pipe tit traverses the dew-point measuring apparatus lit and is exhausted to the atmos-' phere.

By utilizing the arrangement described above, the element it may be readily calibrated with respect to the dew-point measuring apparatus tilt. in this operation the valve be associated with the sampling conduit ti is closed, and the adjustable valve tit is manipulated with respect to the constant speed of operation of the blower i2, whereby the required standard flow of air from the pipe 55 traverses the element it. Preferably the dew-point measuring apparatus t9 is of conventional construction and arrangement and comprises a silvered tube subjected to a bath oi air from the pipe t5, cooling equipment for the silvered tube, and a thermometer associated with the silverecl tube, whereby the dew-point of the air from the pipe 55 traversing the deW-pointapparatus t9 may be readily measured. Under the conditions mentioned, the moisture content of the air in the pipe 55 may be established by appropriately controlling the drier ll and the humidifier so in the manner previously explained. At this time the dew-point 0f the air in the pipe 55 is established by the dew-point measuring apparatus t9 and the potentiometer it is read. After obtaining these readings, the moisture content of the air in the pipe 55 is varied and the operations described above are repeated, whereby a series of dew-point measurements are produced 2 for the particular standard of air flow adopted;

Referring now to Fig. 2, the calibration curve illustrated is plotted with respect to dew-points in degrees C., obtained from the dew-point measuring apparatus 49, and readings in millivolts obtained from the potentiometer 45. It is further noted that the dew-points in degrees C. may be readily translated into terms of water content of the gas in milligrams per liter from any standard reference book. After a calibration curve of the character of that shown in Fig. 2 is obtained, correlating millivolt readings of the potentiometer 45 with respect to dew-points in degrees C. and water content in milligrams per liter, appropriate scales may be made and directly attached to the normal millivoltage scale of the potentiometer 45, whereby both dew-points and water contents of test samples of air may be read directly on the potentiometer 45 in an obvious manner.

'After the potentiometer 45 has been caliibrated with respect to the element ID in the hygrometric system in the manner previously explained, the adjustable valve 53 may be closed. whereby the hygrometer proper may be operated without reference to the pipe 55 and the associated apparatus. In fact. the hygrometer proper may constitute a portable instrument, in which case the adjustable valve 58 is closed and detached from the pipe 55. In this case, the test sample of air is obtained by attachment to the sampling conduit 51 and appropriate manipulaa tion of the adjustable valve 58. Accordingly, in normal use the adjustable valve 58 is manipulated, whereby the desired standard flow of air from the sampling conduit 51 through the element i is obtained as previously explained, this condition being indicated by the manometer l I.

In conjunction with the calibration of the embodiment of the element in illustrated it is noted that the standard rate of air flow therethrough was maintained at about 0.07 cubic foot per minute, a vacuum of about 2 cm. of water being produced at the gas inlet tube 42 of the element it as indicated by the manometer Ii. Also, between readings the element Ill was permitted to cool until the potential metered by the potentiometer 45 was zero. These cooling periods were from 3 to 5 minutes for dew-points above C. and from 2 to 3 minutes for dew-points below 25 C. In checking the calibration curve of the element 10 in conjunction with the potentiometer 45 and the dew-point measuring apparatus 49 it was found that the readings were reproduced over a considerable period and that slight changes in the condition of the dehydrating agent Mg(ClO4)2 did not aiiect the readings.

In a life test conducted upon the element 10 air was passed through the cell for hours, the air having a dew-point of 30 C. It was found that the reading at the end of this period was exactly the same as at the beginning and an examination of the dehydrating agent forming the packing 31 showed only a slight caking at the surface which apparently had no effect.

The total life of the packing 31 is very long but is entirely dependent upon the moisture content of the air diffused therethrough, and the element I0 is most satisfactory for measuring the moisture content of air containing less than 1.5 mg. of water per liter. .In fact, the hygrometer should not be employed to measure the moisture content of air when it is known that such content is relatively high and appreciably greater than 1.5 mg. per liter, as the life of the packing 31 would be unduly short.

While the calibration and use of the hygrome- 8 ter. has been described in conjunction with the measurement of the moisture content of air,-it will be understood that this hygrometer is very useful in measuring the moisture content of other permanent and relatively inert gases, such as nitrogen, hydrogen, etc. However, the hygrometer should not be employed to measure the moisture content of a highly chemically ,active gas having the characteristic of combining chemically with the dehydrating agent of which the packing 31 is formed, as such would be productive of large heats of chemical combination. However, in this regard some selectivity is permissible by appropriately correlating the known chemical reaction of the gas, the moisture content of which is to be measured, upon the dehydrating agent selected from the class mentioned of which the packing 31 is formed. In using the hygrometer it is suggested that the element iii be calibrated with the potentiometer 45 in the manner previously explained in conjunction with the particular gas, the moisture content of which is to be measured, in order to obtain most satisfactory results.

From the foregoing it is apparent that an improved hygrometric system is provided which comprises an improved hygrometer including an improved hygrometric element, which is operative in accordane with an improved method utilizing the heat-of-absorption or heat-of-solution eiiect of the dehydrating agent.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A hygrometer comprising a casing, an element supported Within said casing, said element having a porous structure and containing a dehydrating agent having a finite heat of absorption, means defining a passage through said casing including said element, means for establishing a substantially uniform and predetermined rate of flow of gas containing moisture through said passage, whereby the gas is diffused through said element into intimate contact with said contained dehydrating agent in order to control the temperature of said agent in accordance with the moisture content of said gas at said prcdetermined rate of flow, and an indicator controlled in accordance with the temperature of said agent and calibrated in terms-of moisture content of the gas contacting said agent at said predetermined rate of flow.

2. A hygrometer comprising a casing, an element supported within said casing, said element having a porous structure and containing a dehydrating agent having a finite heat of absorption, a porous packing supported within said casing adjacent said element, means defining a passage through said casing including said element and said packing, means for establishing a substantially uniform and predetermined rate of flow of gas containing moisture through said passage, whereby the gas is thoroughly diffused first through said packing and then through said element into intimate contact with said contained dehydrating agent in order to control the temperature of said agent in accordance with the moisture content of said gas at said predetermined rate of flow, and an indicator controlled in accordance with the temperature of said agent and calibrated in terms of moisture content of the gas contacting said age at said predetermined Name. Date Mase July 5,1927 Bond Oct. 6, 1931 Gllllland et a1 Feb. 26, 1935 Mample Aug. 19, 1941 Wittmann Aug. 31, 1943 FOREIGN PATENTS Country Date Great Britain Nov. 21, 1921 Great Britain Aug. 23, 1935

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