DE2112059A1 - Hygrometer with visual display - Google Patents

Description

Dr. phil. G. B. HAGEN

Patent attorney

MUNICH-SOLLN 21 120b

Franz-Hals-Strasse 21
Telephone 796213

SBE 2880 Munich, March 5, 1971

Dr. H./P./fr

Stanley Bennett Elliott 7125 Conelly Boulevard
Bedford, OMo 44146, T * St. A.

Hygrometer with visual display

Priority: March 12, 1970; U.S.A. No. 18,921

The invention relates to a hygrometer, dast changes the raltiven moisture indicates visually and in the case of birefringent crystals, their birefringence changes the relative humidity responds, are arranged between a polarizer and an analyzer, so that changes in the Birefringence can be observed. By choosing suitable birefringent crystals, their birefringence either starting or ending at different relative humidities, a number of will be visually appealing Obtain representations that make a very effective hygrometer.

There are a number of accurately working hygrometers known, the functioning of factors such as first of all, the change in Length of a hair or membrane as a result of the change in humidity, second, the ability of a hygroscopic

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Solution film, its electrical resistance with the change as a result of the changes in the moisture content of the gas tariffs above Ability of a hygroscopic polymer sensor to change its electrical resistance as a result of the change of humidity, and fourthly, the ability of a hygroscopic alumina film to change its electrical resistance to change with the change in humidity depends. There are also a number of moisture meters obtainable that the humidity of a gaseous environment as a function of the difference between show wet bulb and dry bulb thermometers. Finally, there are still types available that depend on the electrolysis of the water in the gas stream to determine its concentration and thus the moisture Show. Others work with cooling the gas to determine its dew point, which is due to the moisture can be obtained using suitable tables. All of these devices are either mechanical in nature and need special handling to be read, or some external force, or they are very complicated. Although the restrictions imposed by it are generally tolerable, there is a need for a so-called secondary hygrometer, that can be easily read, that is compact and independent of any external force as a warning device can work on complex devices. Humidity control is very important today, and it can lead to great destruction if it is not carried out.

There is a very simple hygrometer on the Fact is that a certain cobalt component

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changes color from blue to red as humidity increases. This device is used when necessary widely used. However, it is difficult to accurately determine humidity using this technique due to the color shifts. Besides, the change doesn't come with an ejection and such not suitable as a visual alarm system.

Accordingly, it is an object of the invention to provide a visual display hygrometer in which an f Number of delimited areas from blue-black to brilliant white or vice versa as a result of moisture to be traversed.

Another object of the invention is to provide a visual moisture alarm display in which a A relatively wide range from blue-black to brilliant white or vice versa changes how the humidity changes changes in a desired area. In such a device, a viewer can find suitable Warning signs appear when the humidity exceeds a certain range.

Another object of the invention is to provide visual means for determining grass flow patterns in ladders, fume cupboards, etc., by placing a moist gas on a birefringent crystal, which is sensitive to moisture is, with the crystal between pierced, a spatial distance from each other having polarizer and analyzer is arranged in a sandwich construction.

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Another object of the invention is to provide a special hygrometer which has a visual display Has. Limited areas should, for example, follow one another from blue-black to brilliant white or vice versa in case of falling humidity (as a result of a rise in temperature) they are run through in a closed System with a certain absolute humidity.

In a modification of this, if you have a moisture-sensitive Component that is birefringent at high humidity and not at low humidity is birefringent, applies to a suitable substrate and encloses this in a double field window, such as it is used for isolation purposes and puts the whole thing between two crossed polarizers temperature indicating system. Is there a suitable absolute humidity in the closed system, then the temperature of the system rises when sunlight falls, the relative humidity falls accordingly, the birefringence ceases and the system no longer noticeably transmits light,

Another object of the invention is to provide a visual indication of the moisture in which it is limited Areas successively change from blue-black to brilliant white or vice versa to display the relative Humidity of the system and thus, secondarily, the moisture content of the material to be tested such as paper, tobacco or wool.

These objects are achieved by a hygrometer with visual display which, according to the invention, is a chemical

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Contains component that doubles a light beam at a first relative humidity and at a second relative humidity does not break double.

Other features and purposes of the invention result from the figures and the description of exemplary embodiments. From the figures show:

1 shows a typical system with transmission;

a transmission system with a piece of polarizing material;

a transmission system for controlling the relative humidity of a gas stream perpendicular to the polarizer and analyzer;

a typical reflection type system;

a typical transmission type system below Applying successive marks of relative humidity as well

Pig. 6 shows another embodiment of the transmission type according to FIG. 1.

In the embodiment shown in Fig. 1 comes

a light beam from a light source 5, e.g. j

a window, a tungsten lamp, a fluorescence ™

lamp or the like and runs through a polarizer and is polarized by this. The ray of light then runs through a transparent or translucent substrate 2, such as glass or isotropic plastic such as cellulose triacetate, on which a crystal layer 3 is applied.

After the beam has passed through the layer 2 and the crystal 3, it passes through the analyzer 4, its The direction of polarization is generally perpendicular to the direction of polarization of the polarizer 1, so that Extinction occurs.

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If the crystal 3 is on the substrate 2 in its non-birefringent state, then Only a small amount of light passes through the analyzer 4 and the system appears to a viewer 6 as dark field. If the humidity changes sufficiently, the crystal 3 becomes birefringent. Runs in Light beam through the birefringent material, then it is split into two components, one

where, ordinary and an extraordinary ray, / at the directions of oscillation of the two beams are perpendicular to each other and have different propagation speeds, so that a phase difference between the two phases. One component is delayed against the other. One then speaks of elliptical polarization. The two components emanating from the birefringent material hit onto the analyzer 4 and are broken down into a plane-polarized beam. However, there is a phase difference between the two components of this ray, and so there is a necessary condition for interference. With a so-called white light source, 4 brilliant colors emerge from the analyzer when the layer crystallized into large crystals. If the crystals are very small, there is a mixture of colors, and the crystal mass appears white. But in both cases the field that was previously only lit up because of the low light transmission was blue-black, now brilliant on.

Fig. 2 shows essentially the same device as in Fig. 1. However, the device shown there is

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more suitable for mass production. Here a piece of polarizing material is folded, the direction of polarization of which is less than 45 °. Two polarization layers 1 and 4 are thus formed, the polarization directions of which are perpendicular to one another, as a result of which a dark field is generated when the observer 6 holds these folded layers between himself and the light source. There is then a substrate with a applied with respect to the relative Ä

Moisture-sensitive layer 3 inserted in between, so that the resulting assembly is sandwiched and responsive to relative humidity. The layer 3 can optionally also be applied to the two inner surfaces of the layers 1 and 4 so that no special substrate is necessary.

Figure 3 shows a transmission type system for controlling the humidity (and uniformity of humidity and / or air distribution) of air exiting ducts. An air flow 7 passes through apertures which are vaguely seen in a polarizer 1, the% is illuminated by a lamp. 5 The air flow then hits a moisture-sensitive layer 3 which is applied to a substrate 2. An observer looks at the system through an analyzer 4 and determines whether the birefringence remains the same.

Fig. 4 shows a system from R _e flektionstyp in which light rays from a light source 5 pass through a polarizer 10 and is polarized thereby. They then hit a moisture-sensitive layer 3 which is applied to a substrate 2 and pass through it

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through and hit a polarization-maintaining mirror 9 · The mirror reflects the beam back through the polarizer 10, which now serves as an analyzer. As a modification of this In the system, a circular polarizer can be used instead of an ordinary linear polarizer. If the layer 3 is then not birefringent, no light is reflected by the analyzer 10 because the circular polarizer has polarized the beam in the right direction or left direction so that it does not can exit back through the circular polarizer 10. If layer 3 becomes birefringent, then the polarization form of the light is such that the light reflected by the mirror is changed and can exit through the polarizer 10.

The hygrometers described can be used like moisture meters by simply enclosing the hygrometer in the hygroscopic material whose moisture meter is to be determined, so that the salts come into a state of equilibrium with the atmosphere via the hygroscopic material. A hygrometer can be buried in a product like grain as easily and long enough in it left until equilibrium with the grains is achieved. In other cases, the material whose moisture content is to be determined, enclosed in a container together with a hygrometer, until a state of equilibrium is reached. In each In this case, of course, a diagram must be prepared for the relative value associated with the particular product Humidity above the product versus the absolute moisture content in the product at this relative Humidity. From this card you can get damybei

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known relative humidity determine the moisture content of the product.

The substrate on which the moisture-sensitive Layer is applied can be smooth material such as isotropic glass when the device is in horizontal Level should be used. However, if the / gate direction is used in the vertical plane, then the moisture-sensitive layer runs when it | is in the liquid state, on the bottom of the plate. In this case the surface should for example Treated with a sandblast or etched to let the solution remove the rough surface can wet better and so that the liquid has a better hold due to the rough surface and not just runs down. Such a unit is shown in Fig. 5, in which the layers 3> 3 '»3" etc. are applied to etched areas of the substrate 2.

For further control of drainage problems, the ^

Solutions of moisture sensitive mass in whole ™ small areas of round shape can be applied in smooth or etched points on the substrate. These Trapfschön can be printed on the surface or applied over the substrate by spraying through openings in a mask. The droplets the solution can be analogous to the points, which comprise halftone images, for the formation of marks, Relative humidity symbols, alarm systems, etc. be applied.

Other methods can be used to create effective visual representations of the subject

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relative humidity sensitive salt layers. To do this, small glass spheres of the kind used in the manufacture of reflective signs are used are applied to a substrate with a suitable cement such as epoxy. The globules are only a short distance from each other and insufficient amount of adhesive is added to the Used to fill in the spaces. In this way a large number of small cavities are created the beads. A moisture-sensitive saline solution can penetrate these cavities and form a visually effective representation when water evaporates and crystallization occurs.

Moisture-sensitive saline solutions can also be applied in narrow channels or holes that open up reside in substrates such as clear acrylic. Such channels serve to hold the salt in its dissolved state Form, the effective arrangements after evaporation of the liquid and the formation / birefringent crystals result. The channels can control the specific relative humidity of the salt filling the channels display numbers or rectangular display boards, etc.

The solutions of the moisture-sensitive components are preferably mixed with non-hygroscopic wetting agents to lower the surface tension, so that they wet the chosen substrate. As salts or other polar components for use selected as moisture-sensitive components, wetting substances are mostly of a non-ionic form.

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This is sufficient to ensure effective but controlled wetting of the substrate Wetting agents preferably mixed into the solution in order to achieve a surface tension that is close to but not below the critical surface tension of the substrate. This way one becomes smaller Contact angle ensured, but the wetting occurs not spontaneously over the entire surface, so that

the problem of drainage of the liquid downwards m

does not occur.

Although good wetting is generally desired, in some special cases it is low Wetting beneficial for creating alarm systems. To do this, a solution is given with regard to the relative Moisture-sensitive mass with a naturally high surface tension is sprayed onto a substrate in such a concentrated form that they are present at the spraying temperature and / or relative Humidity dries immediately. This relative humidity sensitive film on its ^ The carrier substrate can then be β in a typical hygrometer

of transmission or reflection type, in which an illuminated birefringent surface persists as long as the relative humidity is below a critical point, which is typical for the special crowd is. The hygroscopic film increases above the critical relative humidity Moisture from the air and the birefringent crystals go into solution. The high surface tension the solution then causes it to contract into droplets. In this way, even in the case in which the dangerously high relative humidity then sank again, simply by looking at the film

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the presence of droplets indicates that the hazard point has been exceeded is. Such a signaling device is in particular important for, for example, the storage of packaged equipment outdoors in the tropics, where large Temperature fluctuations Large fluctuations in relative humidity in sealed packages produce. Towards the end of the operational life of dehydrating Agents such as silica gel can reduce the relative humidity in packs at high altitude Daytime temperatures may be low enough, but become dangerously high at low nighttime temperatures. A device of the type described above is a signaling device that operates throughout the day shows dangerously high humidity.

The substance used can be of the type that it changes from the birefringent state to the non-birefringent state with increasing relative humidity. One mechanism by which this can happen is to involve the formation of unstable hydrates that are birefringent. The birefringent hydrate is stable above a specific relative humidity and converts to a non-birefringent material of dehydrated form or of a lower degree of hydration when the humidity drops below a critical humidity concentration. A good example of such a material is the tetra potassium salt of the 3 '3' 4 '4 • benzophenone-Tetrakarbondianhydride that are birefringent at approximately 46 i "relative humidity. The material remains birefringent until the relative humidity reaches a point where

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a flowing of the crystals in solution interrupts the birefringence abruptly. This occurs at about 65 % relative humidity.

There are two basic types of substances that can be used as active agents in temperature sensitive or for relative humidity sensitive devices. The first type comprises those substances which are double refractive in the hydrated state and in the dehydrated state ύ or, if the water of hydration is below a critical point, in the non-birefringent state. There are a large number of substances that show birefringence when viewed between the polarizer and analyzer. Many of these components contain water. However, most of them are impractical for use as sensors of relative humidity or temperature because the water of hydration is so tightly bound in the structure. Once a hydration compound is created from solution, the water can be slowly driven off, but usually only through the use of impractically low humidities and extremely high temperatures. Unfortunately, this usually destroys the mechanics of these solid structures and powder forms. That is, each crystal is crushed so that it cannot easily be restored to its original coherent birefringent shape by simply increasing the relative humidity to the original level at which the hydrated material was stable. Such devices with polarized light, as they have been described here, are therefore not feasible.

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The invention provides new compounds that one
make such a visual sensor possible. Such films made from such substances are characterized by
submicroscopic openings in the structure that allow water to easily enter the system.
and can step out of it. It means that
the hydrated birefringent structure is formed quickly and easily under suitable conditions and the
dehydrated or less hydrous non-birefringent form can easily be restored when the original conditions have been re-established.

These materials are the salts of relatively symmetrical components that have at least two rings with carboxyl, hydroxyl, or sulfonic groups. The "" will be Inge
bound by a bond or bonds of a type likely to allow a ring to rotate about the bond to result in bulky, non-planar structures. Ordinary planar molecules form compact, highly structured structures. It is believed, however, that the molecules of the new class of constituents form crystal structures that are entirely open to the passage of water molecules. In particular, these components include the alkali salts, pure or mixed, of molecules that have at least two rings with carboxyl, hydroxyl * or sulfur groups as
Wear substitutes. The substitutes can be on the same or on separate rings.

Make such groups as -OK, -OORb and -SO-Na
Ions to hydrates, and these individual groups
become highly polar. Are such groups asymmetrical
then result in molecules which have a high

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Have dipole moment. On the other hand, the most polar group, when balanced by other polar groups, may comprise a portion of the molecules which as a whole are essentially non-polar. Due to a symmetrical distribution around the molecule, the polar groups have ions that can hydrate. A structure is formed that can hydrate or dehydrate, and the individual molecules are not drawn into fixed intermolecular orientations by polar effects. %

In summary, it can be said that molecular configurations are not set up identically or precisely can be. However, a wide range of molecular shapes can be expected depending on the general configuration and the type and arrangement of the substituting groups, of molecules, in which the ring is essentially flat (small volume) to those in which the rings are strong from deviate from one level (very voluminous). It is believed that the water of hydration is within and outside of molecular clusters of the original j

Molecules moved firmly with considerably more difficulty than with the latter. In other words, it is a relatively low driving force (like a low water vapor pressure) appropriate to move the water to the alkali metal ions of an open salt structure, where everything else is the same, when compared to a legal structure *

The best substances, the relative at a first moisture doppelbrechead ύ & ά at a second relative humidity are not doppeIbrächend are alkali metal salts of 3, 3 ¹ "4 4 ^I benzophenone-Tetrakarboxyld anhydrides! And Alkaliaetallsalze of 1, 1, 3-

5? ^{Ri = 1} ethyl-5-carboxyl-3- (p-carboxylphenyl) indanes ♦ 109839/1234

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The molecular structure of them is given below at an approximate relative humidity at which various alkali metal salts of them birefringent will:

Alkali metal salts of 1,3-trimethyl-5-carboxyl-3- (p-carboxylphenyl) indane

Λ>

with A = an alkali metal

Relative humidity of the alkali-metal salts;

Li ₂ salt »74 $> Na ₂ salt = 62 $> K ₂ salt = 53 1 ° 45 #

Alkali metal salts of 3t3 * »4" ^t 4 Tetrakarbotyl benzophenone dianhydride

n
0

with A = an alkali metal

6 "

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Relative humidity of the alkali metal salts;

Li ₄ -SaIz m 68

Na ₄ salt »57 K ₄ salt »43 Rb ₄ -sal - 20th

In general, the alkali metals include sodium and Potassium, which are preferred for convenience and economy, as well as silicon, rubidium and

Cesium. \

Though not s ^ good as the alkali metal salts described above the indane and benzophenone substances are also the composition of the alkali metal salts of the Nap / fchochrome green, the disodium salt of which is birefringent are preferred substances at a relative humidity of about $ 84.

Other suitable substances are the alkali metal salts of 1,1 · -Binaphthyl-8-8 · -dicarboxylic acid. The Na ₂ salts begin to show birefringence was at about 53 ° i relative humidity and 21 ⁰ C. The sodium salt * coated, to initiate crystallization, and after repeated commuting the birefringence was irreversible. The Kp-SaIa did not crystallize so that there was no perception.

Other suitable compositions are alkali metal salts of the benzophenone - ^^ '- dicarboxylic acid, the sodium salt is birefringent, at about 96 fo relative humidity and their Kaliumsala whose Rubidiumsäfe birefringent at about 75 fo relative humidity at about 85 $> and

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The following list contains suitable alkali metal salts of substances that have been tested. At around the temperatures indicated, the alkali metal salts were birefringent:

substance

Alkali metal

Temperature ⁰ g

Remarks

3,3-Bie-U-hydroxy phenyl) oxindole

123

2,4 'Benzophe non-dicarboxylic acid

80

o-Benzoyl-phenyl-formic acid

K Well

Rb

149

2,4,4 ⁸ -trihydroxy bensophenone K

105

Deleted to induce birefringence after a few periods became irreversible.

First a slas film forms (not birefringent), which then crystallizes and an observation is possible power.

No drainage at 149 (no cessation of birefringence)

No drainage at 149 (no cessation of birefringence)

Films are difficult to watch because glass-like films are difficult crystallize. Considerable background birefringence is above 149 for K-SaIz available.

A thin film is required for drainage

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substance

Alkali metal

Temperature remarks

2-Hydroxy -4 »-methoxy-benzophenone-5-sulfonic acid K ₀

135 No coating necessary to induce birefringence.

2-hydroxy -4-methoxy benzophenone

2,2 »-dihydroxy 4,4 * -dimethoxy Benzophenone

50 To induce birefringence in a film, it is necessary to paint over it.

121

2 - (- hydroxy -bene

zoyl) -phenyl-

Formic acid

105

2.2 * dihydroxy
Benzophenone

121

o-hydroxy benzophenone

45

4.4 * dihydroxy
Benzophenone

127

Chlorobenzoyl-phenyl-formic acid

143 Tends to form a

considerable background birefringence when commuting.

2,4 dihydroxy benzophenone

K, no drainage (no disappearance of birefringence) at 149 C.

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substance

Alkali Temperature Metal ⁰ C Remarks

Sodium 2.2V Na Dihydroxy -4,4 * -Dimethoxy 5-sulfobenzophenone

NaK, no break in birefringence at 149 C.

No disappearance of the birefringence at 149 ° C.

2.2 ·, 4.4 · -
Tetrahydroxy -
Benzophenone ^K 4 65 Phenol red K 143 p, p ¹ biphenol _K

O, O· Biphenol K ₂ 90 2, 2 ' Diphenic acid Rb ₂ 115 K_

4-biphenyl carboxylic acid K

Naphthol No drainage (no Interruption of birefringence at 149 C).

Thin layers are needed for drainage and disappearance the birefringence. A remaining birefringence stops after major drainage.

No dehydration (no break in birefringence occurs at 149 C on).

No drainage (no break in birefringence at 149 C).

No dewatering (no break in birefringence at 149 C).

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substance

Alkali metal remarks

4,4 ^f biphenyl disulfonic acid K

Et »,

Phenolphthalein K

Bisphenol Α

130

Büiydroxy -

Coumarin, (3.3 ^f -ethylene b: i3-

65 65

Κ, / Γ-hydroxy ^ Coumarin J.

nJ

Methylene disalicylic acid K

124 124

No drainage (no break in birefringence at 149 C.)

No drainage (no break in birefringence at 149 C.)

In concentrated solution, about 100 H more KOH is required than theoretically for Κ, salt to form ^ salt, which is birefringent. Even then, it is necessary to paint over the film in order to align the molecules and to develop the doubling.

Tends to gradually form birefringent crystals that are not dissociate longer when heated.

Tends to form glassy films

Benzilic acid

Rb No drainage at 149 ° C.

Strict crystal formation with high birefringence

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substance

Alkali metal temperature

Remarks

Diphenyl acetylic acid

Dichlorophene

/ 2.2 »-Methylene

b ± s- (4-chlorophenolJ7

6,6 Bi- (p-Hydroxyphenyl) -3- (1-Hydroxy 1-Methy1 Ethyl) -He ptane acid G-amma lactone

K, slow drainage with strong retention Subsurface birefringence.

No dehydration at 149 C, i.e. H. no interruption of the birefringence. Strong crystalline birefringent film.

No drainage at 149 ° C.

Hexachlorophene,
/ ~ 2,2 'methylenebis - (3,4,6-dichloro- -2-hydroxy phenyl) M _e than7

Pan Acid

2,6-divanillylidenecyclohexanone

4 ', 5,7-trihydroxy Flavenone

4.4 · oxydiphenol

2.2 · -Thiobi

(4 »6-dichlorophenol)

^ 2

'3 123

Kr 143

No dehydration at 149 C, i.e. H. no interruption of the double bre chung.

If both -GOOH or both -COOH plus- both OH are neutral, no drainage is observed at 149 G, ie the birefringence is not interrupted.
The salts tend to form glasses which crystallize at great speed.

No drainage (no interruption of the birefringence) 149 G.

No drainage (no interruption of the birefringence) at 149 C.

Strong remaining birefringence after reaching 143 C.

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substance

Alkali metal

temperature

Remarks

2.2 dithio

diphenyl

Formic acid

N / A,

Dihydroxy diphenyl sulfone

5.5 thiodisalicylic acid

110

115

No crystallization

Crystallization is slow. Strong remaining birefringence after reaching 110 C.

Remaining birefringence after reaching 115 C.

No drainage (no break in birefringence at 149 C.)

Like K ₂ -SaIz.

There is another factor to consider when choosing the molecular configuration for that Creation of a sensor that is sensitive to relative humidity or temperature and especially industrial is easily usable. As already stated, it is assumed that that the substituting groups contribute to the deformation of the rings into spatial open configurations, the degree of treeiness depends in part on the substituents. However, although the molecules are in a dissolved state are disordered in a solution with which they are applied, strong intermolecular forces come into play, when water evaporates, and the molecules are driven closer together. It follows from observation that some molecules of the kind found to be effective are capable of being totally in a tight close molecular grouping during evaporation of the water to pass. The dehydrated material shows no birefringence after thousands of dehydration-hydration periods. However, other molecules develop what is known as an underground grinder, a partial birefringence

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in the dehydrated form when observed in polarized Called light. Sometimes this foggy background develops immediately, sometimes repeatedly Periods through the phases "dehydrated solid hydrated solid - dehydrated solid".

It is believed that this fog develops because a small part of the molecules are intermolecular Arrangement is subject to generating forces, since the rings take a considerable degree of freedom of rotation the effect of the substituting groups. Coincidence can put some molecules in a firmly built structure Bring birefringent microcrystalline that is not destroyed by melting. As already stated some molecular structures are entirely free from such fogging tendencies, and these are preferred for use in display devices. Although the presence of the underground fog3 the use these substances as indicators are not prevented, the effect is much stronger when the field of Turns blue-black after great brightness or vice versa.

This tendency of some types of molecules to succumb to these order-producing forces and so from the isotropic going to the birefringent state under certain conditions can make it suitable for industrial use make valuable. In particular, it has been found that some species (e.g. the alkali salts of 1,1,3-trimethyl-5-carboxy -3 ~ (p-carboxy phenyl) indans, as shown in table is contained) are highly stable in the dehydrated state (or with minimal hydration), which characterizes is characterized by its isotropy when viewing between crossed polarizers. Reaches the surrounding moisture a critical level at which the substance

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becomes hydrated and birefringent, then the order-creating forces begin to act. In A very limited time, which varies from a few hours to a few sagas, develops new, essentially irreversibly birefringent crystal structure * That means that also with a subsequent Sending drop in relative humidity below the critical relative humidity at which birefringence originally began, the film remains permanently birefringent.

This storage property of some molecular structures is invaluable. As already stated, there are a number of industrial applications in which cyclical changes in the relative humidity occur and in which it is important that a critical humidity (such as that at which the Schimaie development begins) should not be reached. The substances just described make excellent inexpensive visual alarm indicators which are indefinitely inactive below the critical relative humidity, but which react promptly and save their message for the next reading. λ

There is another factor besides the inherent bulkiness of the cyclic organic structures that affects the openness of the final film. This has to do with the base chosen to neutralize the acid molecules. Because the atomic volume varies and structures of the various selected for combination with the organic molecules cations, the spatial arrangement of the entire molecules, from which they come, comprise a T _e il * ^zl, also affected. This in turn affects the way in which the molecules resemble others

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Molecules bind to form a macrostructure. So can different cations depending on the nature of the opposite ion of high bulk and complexity. With a skilful selection of anions and cations, substances can be prepared which are different difficult to hydrate and dehydrate. That in turn means that substances for use as sensitive Agents for visual indication are available that the substance when applied to appropriate Substrates and viewing with polarized light indicate the existence of various relative humidity levels or temperatures.

At a certain temperature, it is the water vapor pressure around the molecule that decides whether a certain molecular substance absorbs or releases water. Most hygrometers are used in one relatively narrow temperature range of around 0 ° C up to 40 ° C, i.e. in areas where the relative humidity is relatively high. Hence are Structures needed in which the water of hydration only is loosely tied. Suitable substances are shown in the table, together with the percentages of the relative Humidity at which birefringence begins when the relative humidity increases, or at falling relative humidity is terminated in the case of lower mass deposition in these sensors used. Since the alkali metals form an isomorphic series of substances, the relative humidity response of the special hygroometric series controlled by the use of different mole ratios the alkali metals as well as the cations.

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labelie 1

Birefringence, labile hydrates with the following Response to relative humidity

Relative humid 74.26 e 71.7 Anion cation A / B Li / - MoI $> cation A. speed at which 70.2 1,1,3-sriiaethyl- Mole% cation B Birefringence a 67.2 5-karboxy-3- 100 puts (p-carboxy - phenyl) indane Li / Na ti Li / Na 63.1 It Li / Na 75/25 62.5 1,1,3-trimethyl 50/50 60.6 5-carboxy -3- 25/25 57.6 (p-carboxy - 53.2 phenyl) indane N / A/- 50.7 It Na / K 49.5 II Na / K 100 47.3 It Na / K 75/25 45.1 ti K / - 50/50 69.4 It K / Rb 25/25 67.5 It K / Rb 100 66.0 It K / Rb 75/25 62.5 It Rb / - 50/50 58.6 It Li / - 25/75 55.9 3.3 \ 4.4 ^f benzo- Li / Na 100 53.2 phenon tetrakar
bon bianhydride Li / Na 100 49.8 It Li / Na 75/25 43.0 It N / A/- 50/50 39.1 ti Na / K 25/75 32.2 II Na / K 100 25.0 It Na / K 75/25 18.0 It K / - 50/50 It K / Rb 25/75 Il K / Rb 100 It K / Rb 75/25 η Rb / - 50/50 η 25/75 100

SBE 2880 - 28 -

Purely industrial purposes exist alongside the necessity after simple hygrometers the need for simple visual temperature indicators that cover a large surface area for high recognition values. These indicators can be used for temperature sensitive monitoring Equipment for displaying the temperature of a grass in a room through an opening & indureh, such as pipes and closed rooms, for displaying the temperature distribution over relatively large areas as in the creation of electric irons, heating plates, etc. Such indicators and warning displays should usually be at slightly higher temperatures, especially above about 40C work. For such uses, substances are desired in which the water is relatively tightly bound, because the water vapor pressure in open air that has been heated to high temperatures is very low. Of the rapid drops in relative humidity with increased temperature are shown in Table 2.

Table 2

io relative humidity to the air temperature 4,536 grams of moisture / 453t6 grams of dry air

Temp. ⁰ C% rel. Temp. ⁰ C i » relat. Moisture 'moisture

2.5

2.0

1.6

1.4

20th 65 65 26th 46 71 32 34 77 37 25th 82 43 18th 88 48 14th 93 54 10 99 60 8th 105

SBE 2880 - 29 -

Mainly because of the relatively wide limits that characterize a simple alarm system, can be the external relative humidity, which is the vapor pressure of water (or the back pressure leading to the Preventing hydration from dissociation) indicates than at relatively high temperatures

Λ 4-

temperatures are constantly affected. Thus, the T _e mperature in which various hydrates interrupt the birefringence can be measured with the knowledge that changes in the ambient relative humidity have (at room temperature) has a small effect on the temperature at which the birefringence disappears. Table 3 lists various substances, along with the temperatures at which the birefringence disappears.

Table 3

Substances and temperatures at which birefringence disappears

Substances temperatures ⁰ C

Potassium salt of the o-hydroxy benxophenone 44

"" 2-hydroxy -4-methoxy -

Benzophenones 50

Phenol Red 66 potassium salt

^{11 of} the 2.4 ^f -benzophenone dicarboti

acid 80

"des ©, o'-biphenol ^ p'-di-hydroxy- 90

«2- (p-Hydroxybenzoyl) - ^phenyl)

Phenyl formic acid 105

"the 2,2'-dihydroxy -4.4 ^f -

Dimethoxy -Benzophenone 121

^MM o-Benzoyl-Phenyl-Formic Acid 149

10 9 8 3 9/1234

SBE 2880 - 30 - ώΙΙ ^ υυα

Other substances can be used to advantage in building suitable responsive arrays. Under The substances in which alkali metal salts respond appropriately to various temperatures are the following:

ρ, p-biphenol (ρ, ρ * -di-hydroxybipheny1) 2.2 »diphenic acid

Phenolphthalein

Bisphenol A (4,4 ^f -di (hydroxyphenyl) propane) bis-hydroxycoumarin

Methylenedisalicylic acid

2,4,4 * -trihydroxy-benzophenone

2.2 ^f dihydroxybenzophenone

4,4 'dihydroxybenzophenone

2-Hydroxy-4-methoxy-benzophenone-5-sulfonic acid 3 »3 ^f -Bis- (4-hydroxyphenyl) -oxindole 2, 6-divanillylidenecyclohexanone

These labile substances can also be used as detectors of various kinds and for radiations. So either pure substances or those with dyes, which they for example for a selected Make radiation band highly absorbent, can be used as radiation detectors. To further increase the Sensitivity, the substances can be applied to a substrate, which is the radiation that is visible should be displayed, absorbed.

A large number of materials absorb acoustic and electromagnetic radiation of various wavelengths. Usually some of the absorbed radiation is converted into kinetic energy, which is a

1 0 Ί 8 3 3/1 2 3

SB, 2880 -J ₁ -

Bringing an increase in the temperature of the material and its surroundings. Infrared heating fields and microwave heating ovens characterize applications of different radiation bands. As part of a proper application of various types of radiation, it is desirable to quickly determine the distribution of the radiation, thereby determining whether it is coming directly from an infrared field or from an ultrasonic generator. a cleaning tank or perhaps a remote radiation source and λ

focused by a radiation-transmitting lens will. Large, easily observable radiation detector arrays are very useful for such a display. For such moisture sensors with birefringence, highly labile hydrates can be deposited on a suitable substrate applied and placed between crossed polarizers. When the substances and / or substrates radiation absorb, the temperature of the materials and the overlying gas will increase with the accompanying Dissociation of hydrates and disruption of birefringence. To increase the sensitivity the system can use the polarizers that are sensitive Substance and the substrate form a sealed system around- j | grasp. The relative humidity of the encapsulated gas is preferably selected to be close to the critical humidity, at which birefringence ceases at the ambient temperature of the process. In this way, only a small amount of absorbed radiation causes the temperature to rise, a decrease in the relative humidity of the gas, an evaporation of the water of hydration, and an interruption the birefringence. Less sensitive than

109839/1234

the system described is one that is based on the occurrence of birefringence substances, who have stopped their birefringence because they have melted away.

A large number of modifications are of course possible. So the flexible shown in FIG Substrate 2 on which the crystal layer 3 is applied is to be made in roll form so that the hydrated crystal layer and the substrate Unroll and roll up closer together can become a heated, laterally movable pen all in one Recording medium. Then a tight line of non-birefringent, dehydrated crystals under the heated pen. After the passage a "curve" is observed between polarizer 1 and analyzer 4, which is drawn by the pen will. This curve is permanent if the substrate / crystal is kept below a certain level critical relative humidity. But the curve can be obliterated, and the film can be by simple Exposure to high humidity to rehydrate the previously dehydrated crystals can be used again.

For use in the devices described above, an adjunct agent is used in association with the materials described. This agent contains a number of inorganic substances with large surface and little or no actual birefringence. These materials become the solutions of the active organic substances already described is added, then a new one is added

1 0 9 8 3 ¹ Y / i 2 3 4

Sensitivity to the change in humidity is observed in the dried droplets or films formed by such suspensions. This is of great industrial importance as ^{it is} often important to detect a change in relative humidity. In fact, many mechanical moisture meter manufacturers report the time it takes their devices to adjust to a new value. When using the agents with a large surface area, only a few seconds are necessary

so that the sensitive film a new balance etc.

assumes when the relative humidity has shifted. It will continue to be a much higher driving force (low relative humidity) needed to dehydrate the film so that the birefringence disappears, when the high surface area agents are not used. Table 4 shows graphically the new sensitivity that these agents add to.

»Hurry 4

generally relative humidity at which the birefringence

disappears (as a result of drainage) when

High surface area agents are used λ

(Rb ₂ phenylindane dicarboxylate is the substance sensitive to relative humidity)

Bindumrsas-ens * "■ ^ ^a ~, ^{gr <A} S ^ens / # relative humid-

Binaungsagens area M./gr. 1 gr. Rb salt speed, with the

Birefringence disappears

no - 1098: 200 - 42.7 Filter earth relatively low 1.00 43.0 Fumed silica 100 0.30 45.2 Pyrogenic aluminum oxide 0.60 '45.2 Pyrogenic mixture of 150 Silica and alumi- 370 alumina (14 # ^A1 ? ⁰ ^) 0.45 46.0 Downcast j 9/1 2 3 k 0.16 46.0 Silica

SBE 2880 - 34 -

ia relat. Humidity,

Binding size agent / in the case of the double binding agent area 1 big break disappears . Rb salt det

Pyrogenic mixture
of silica
and aluminum oxide (1.3 # Al ₂ O ₅ )

50 1.20 51.5 80 1.00 51.5 170 0.50 51.5

These same mixing agents have the further benefit of causing the thickness of the deposited sensitive film is not critical. With some types of molecular structures was when they had deposited themselves in thick layers in a pure, unmixed form, observed that they were continued to be birefringent when hydration occurred and also when moisture was subsequently reduced well below a point at which they should give up their water. The big fat one compared to molecular dimensions, apparently stabilizes the system. The use of mixing agents with a large area reduce or eliminate this phenomenon of thickness dependency. It was theoretically deduced that the sensitive molecules distribute themselves over the large surface of the added solid particles, so that actually a two-dimensional Structure exposed to air.

It has already been stated that two basic types of Substances exist that act as active agents for temperature measurement or measurement of relative humidity are good to use. The first type and the structures required are now described. The second type that is in can be used in the same way has already been mentioned.

10 9 8 3 0/1234

It includes substances which are birefringent in the solid state and which dissolve so that the birefringent crystals their birefringence as a consequence the dissolution at a critical relative adjust humidity. It was found that materials used as suitable indicators are chemical substances that crystallize in birefringent crystals and their saturated solutions have a vapor pressure between 1.0 and 17.5 mm mercury at 19 ° C.

It was further found that the same substances, which have little or no birefringence, but one having high surface area are well suited when used with the second type of substances. They significantly improve the sensitivity of the materials to small changes in the relative value Humidity. They are advantageously mixed with the solution of the sensitive substance before application on a substrate.

As already stated, the active materials are applied in layers, droplets, channels and spaces are chosen so that the thicknesses of the layers are not substantially greater than 0.1 mm and preferably are not larger than 0.05 mm. This provides an adequate brilliance of the image to be viewed and yet minimizes the time lag due to the diffusion of water into the indicator layer or the particles in or out of them. In general the applied layer containing the birefringent substance should be at least 0.001 mm, preferably be at least 0.005 mm thick.

1 0 9 £ J "J / 1 2 3

In use, a particle absorbs the applied Materials water when the relative humidity rises and eventually dissolves. With further increase the solution droplets grow with the relative humidity in their tolumen, but retain their structural integrity and remain in place of the substrate. The grows relative humidity increases even further, then water is given off from the droplets, and at a key point starts birefringence again. This way, the droplets represent when they are in a polarized light field are an effective indicator of relative humidity. A range of substances can be selected whose birefringence as a result of dissolution with increasing relative humidity stops. A typical series can be taken from the following substances:

Table 5

$ relat. Moisture substance

30 potassium salt of levulinic acid

40 - potassium salt of phthalic acid

50 potassium salt of succinic acid

60 potassium salt of adipic acid

70 potassium salt of 1,2,4,5-benzene

Tetracarboxylic acid

80 potassium salt of fumaric acid

90 potassium salt of o-benzoyl-phenyl-

Formic acid

These are a number of potassium salts, but can also be a number of other substances used, whichever area is more relative

1 Q B: · ..., / i 2 3

2112

SBE 2880 - £ 37 - £ M

Humidity should be measured. Table 6 shows another set of suitable chemical Substances that are separated from sodium and potassium salts

.si na
gesuchy because these are relatively easily available.

Table 6

35 Dipotassium salt of tetrahydrophthalic acid

40 potassium salt of crotonic acid

46 Dipotassium salt of maleic acid

50 sodium salt of 4-chlorophthalic acid

60 Azelaic acid disodium salt

65 Tetrasodium salt of 1,2,4,5-benzene-tetracarboxylic acid

73 potassium salt of glyoxylic acid

85 sodium salt of anthranilic acid

90 disodium salt of adipic acid

Different chemical substances can be mixed in different proportions, so that mixed crystals arise that span other areas to be measured than the pure components. Inorganic Components like sodium nitrate can of course also be used for the display of relative humidity like that organic components are used.

Various additives can be added to the carrier substances too using salt solutions are added. For example, dyes may be desired to make the crystal attractive

1098 3 3/1234

To give appearance. Antifoam agents can come in handy to prevent foaming on a Reduce minimum during the application of surface tension to salt solutions on the substrate. Various substances can also be useful to thicken the salt solution Preserves viscosity and can easily be "printed" onto suitable substrates. For the latter case In particular, a material such as an inorganic silicate that has no birefringence is selected. The field remains blue-black until the sensitive substance crystallizes. In some cases the thickening material can perform two functions, namely both thickening the solution and one represent large surface area to increase the sensitivity to the change in relative Humidity.

Another way of applying the invention, as shown in FIG. 6, consists in the possibility of the To disperse droplets of the solution of the relative humidity sensitive substances in non-aqueous carriers such as paints and varnishes. These carriers can then be placed on suitable substrates be applied, where after evaporation of the non-aqueous solution the droplets are encapsulated in the solid become bearers remain. Carriers are chosen that have the correct moisture transmission characteristics then the encapsulated substances can release their water and birefringent crystals image accordingly low relative humidity. When the relative humidity rises, the crystals then pull Moisture in the droplet cavity to dissolve,

1 098J9 / 1234

SBE ₂₈₈₀ .. ".

so that the birefringence disappears. Such applied substrates can then be placed between two polarizers and form a hygrometer of the type already described.

The preparation of the highly preferred alkali metal salts of l, l, 3-trimethyl-5-carboxy -3- (p-carboxy phenyl) indane and 3 »3 ^f , 4,4'-benzophenone tetracarbon dianhydride are described.

To form the indanoic acid, a suitable hydrocarbon can be partially oxidized by air oxidation, as stated in Towle and Baldwin's article "Make Most Aromatic Acids Using Mid-Century Oxidation Process ", published in Hydrocarbon Processing, 43 (11)» 149 (1964) or by chromium anhydride oxidation as described in an article by Ipatieff et al, J. Am. Chem. Soc, 70, 2123 (1948). The Ipatieff et al article describes a general procedure for the production of a suitable hydrocarbon by reacting an olefin such as trimethylethylene with p-cymene in the presence of sulfuric acid or hydrogen fluoride catalysts. In the TJ.S. Patent Specification 3 102 135 | is the production of l- (carboxy phenyl) indane carboxylic acids described. These two articles and this specification are incorporated herein by reference.

The preparation of 3 »3 '» 4,4'-Benzophenone Tetracarbondianhydride is described in U.S. U.S. Patent 3,078,279, and U.S. Patent No. 3,078,279. U.S. Patent 3,297,727 describes processes for producing relatively pure benzophenone dianhydride. On these patents is also referred to here.

1098 ^ / 1234

In general, carboxy-substituting Diaryl ketones of diaryl methsnene such as paraditolyl methane by oxidation thereof with air of elevated temperature and pressure, for example 90 ° C and

ρ
20.25 kg per 6.4516 cm are produced to produce benzophenone 4 »4'-dicarboxylic acid, as described in US Pat. No. 3,075,007 to Mc Cracken et al. A variation of the diaryl ketone-carboxylic acids can be prepared by oxidation with nitric acid having an initial concentration of about 5 to about 70 $> a temperature of 110 to 350 ° C for about one minute to 48 hours to obtain a 1,1-diarylalkane generally is represented by the following structural formula:

R.
H-II-R ₂

^R l

where R and R _{1 are} aryl groups which are alkyl radicals as core substituents. having 1 to 8 carbon atoms and Rp is an alkyl radical having 1 to 8 carbon atoms. The molar ratio of nitric acid at 100% nitric acid to the diarylalkanes is between 8.0 and 17 »0.

The preparation of the alkali metal salts of the aforementioned benzophenone dianhydrides and the benzophenone 4,4'-dicarboxylic acid now follows.

Manufacture of the lithium and potassium salts of the carboxy-substituted diary! Ketones

example 1

27.0 grams of benzophenone-4,4'-dicarboxylic acid were stirred in 2UÜ grams of distilled water for

1 U a b u '/ i 234

21 1? (Ί ξ

SBE 2880 - 41 -

smooth even pulp. 8.4 grams of lithium hydroxide monohydrate dissolved in 50 grams of distilled water are added to this. While stirring, the mass is heated to 65 ° C. and kept at this temperature until the suspended material has reacted and a clear solution of the dilithium salt forms, which normally takes about 15 minutes. The solution is then filtered through a filter with No. 40 filter paper for filtering out common debris, etc., and cooled. In this form, the solution can be used as precipitation agent for glass, cellulose triacetate, polymethyl methacrylate or similar substances to produce a film which, after drying, responds to the relative humidity. If a more sensitive film is desired, 17 »0 grams of a suitable inorganic material with a large surface area can be added to the filtered solution; fumed silica, for example, was added and distributed. A high performance mixer such as a Waring mixer is well suited for dispersing silica. The thickened saline solution can then be used as a precipitation agent, as can the clear filtered solution. G

Example 2

35.8 grams of benzophenone 3,3 '' 4,4 ♦ -Tetrakarbonsäure (or 32.2 grams benzophenone 3.3 ^f »4,4'-Tetrakarbondianhydrid) are stirred in 400 grams of distilled water to produce a uniform slurry. To this are added / 50 grams of distilled water dissolved 22.4 grams of potassium hydroxide. While stirring, the suspension is heated to 65 ° C. and kept at this temperature until the acid (or the anhydride) has reacted and a solution of the tetrapotassium salt is obtained. This takes about 10 to 15 minutes. The solution is then passed through a filter paper

1 ü 9 8 i 0/1 2 3 4

# 40 filtered to filter out all traces of insoluble impurities.

The production of alkali metal salts of the indane substance already described is carried out by the following Example described:

Preparation of the sodium salt of phenylindane dicarboxylic acid

Example III

32.4 grams of 1,3-trimethyl-5-carboxy -3- (p-carboxy phenyl) indane is stirred in 300 grams of distilled water to produce a uniform suspension. To this are added 8 grams of sodium hydroxide dissolved in 30 grams of distilled water. While While stirring, the suspension is heated to 65 ° C. and kept at this temperature for about 15 minutes the acid reacts and forms the disodium salt. The solution is filtered through a No. 40 filter paper, so that you get a clear solution that is also suitable Substrates can be erected to form a film that is sensitive to relative humidity

Other alkali metal salts including the rubidium and cesium salts can be prepared instead of the lithium, potassium or sodium salt, which are described in Examples I, II and III. With these Alkali metal salts give essentially the same results.

Claims: 10983 9/1234

Claims (30)

to Claims Hygrometer with visual display, characterized in that a polarizer (1) and an analyzer (4jP ⁿ % ine birefringent substance (3) are provided between polarizer (1) and analyzer (4) that the chemical substance (3) has a polarizer (1) and analyzer (4) dividing light beam (5) at a first relative humidity into two beams due to birefringence and that it is not birefringent at a second relative humidity. 2. Hygrometer according to claim 1, d a you r c h characterized in that the hygrometer is of the transmission type. 3 · hygrometer according to claim 1, characterized characterized in that the hygrometer λ is of the reflection type. 4. Hygrometer according to one of claims 1 to 3 »characterized in that the substance contains a component which is birefringent when hydrated and at a lower level Water content is not birefringent. 1 0 y 8; ¹ J / 12 3 4 SBE 2880 p 5 · Hygrometer according to one of Claims 1 to 3> characterized in that the substance (3) has a component with at least two Benzene rings in its structure and a polar group, the alkali metal salts of which provide hydration, contains. 6. Hygrometer according to one of claims 1 to 3 »characterized in that the substance (3) a mixture of components having at least two benzene rings in their structure and Contain polar groups, the mixed alkali metal salts of which ensure the hydration of the ions. 7. Hygrometer according to claim 5 »characterized in that the polar group is a hydroxyl, carboxyl or sulfonic radical. 8. Hygrometer according to claim 5 »characterized in that the substance an alkali metal salt of 1,3-trimethyl-5-carboxy 3- (p-carboxy-phenyl) indane. 9. Hygrometer according to claim 5, characterized characterized in that the substance is an alkali metal salt of 3 * 3 * ^^ '- benzophenone tetracarbon dianhydride having. 10. Hygrometer according to claim 5 »characterized that the substance is an alkali metal salt of 1,1 "-Binaphthy1-8-8" -dicarboxylic acid having. 109839/1234 SBE 2880 - r * - 11. Hygrometer according to claim 5, characterized in that the substance is an alkali metal salt des naphthochrome green. 12. Hygrometer according to one of claims 1 to 3, characterized in that the substance (3) is a coating on a transparent substrate (2). 13 · hygrometer according to claim 12, characterized in that λ characterized in that the substance has a coating on a translucent substrate (2) is. 14. Hygrometer according to one of the preceding claims, characterized in that the substrate (2) is essentially isotropic. 15 · Hygrometer according to claim 6, characterized that the substance (3) is an inorganic component with a large surface area contains. 16. Hygrometer according to claim 1, characterized in that the substance (2) is birefringent in the solid state after an initial relative humidity and is not birefringent in solution at a second relative humidity. 17 · Hygrometer according to claim 16, characterized that the saturated solution of the substance (3) has a vapor pressure between 1 and 17 »5 mm Has mercury at 19 ° 0. 109839/1234 2880 18. Hygrometer according to claim 16, characterized that the substance (3) is a potassium salt of o-benzoylphenyl formic acid contains. 19 · Hygrometer according to claim 16, characterized in that the substance (3) is a Has series of components whose birefringence at different relative humidities at which each of these components goes into solution, ceases. 20. Hygrometer according to claim 19 »characterized that the components are the potassium salts of levulinic acid, phthalic acid, succinic acid, adipic acid, 1,2,4,5-benzene-tetracarboxylic acid, contains fumaric acid or o-benzoylphenyl formic acid individually or in combination. 21. Hygrometer according to claim 19 »characterized in that the substances Dipotassium salt of tetrahydrophthalic acid, the sodium salt of 4-chlorophthalic acid or the tetrasodium salt of 1,2,4,5-benzene-tetracarboxylic acid individually or in Combination included. 22. Hygrometer according to claim 1, characterized in that the substance (3) is suitable to undergo at least 100 changes between birefringence and non-birefringence. 23 · Hygrometer according to claim 12, characterized in that the thickness of the coating (3) is at least 0.01 mm. 1098 3 9/1234 SBE 2880 X 24. Thermometer with visual display, thereby characterized in that a polarizer (1) and an analyzer (4) and a chemical substance (3) between these there are provided that the chemical substance (3) is birefringent at a predetermined one Temperature at a first relative humidity and not birefringent at a second temperature at a second, relative humidity. 25 · Procedure for the detection of radiation, thereby characterized in that a thermometer according to claim 24 is used as the radiation detector t. 26. Hygrometer according to claim 1, characterized that the substrate (3) has at least two chemical components, of which the first chemical component is birefringent is at a first relative humidity and not birefringent at a second relative humidity and the second chemical component is birefringent at a third relative humidity and not birefringent at a fourth relative humidity. 27. Alkali metal salt for use in a hygrometer according to one of claims 1 to 6 or 26 or in one Thermometer according to claim 24, characterized that it is a 3,3 », 4,4'-benzophenone-tetracarbondianhydride. 28. Alkali metal salt according to claim 27, characterized in that the alkali metal either lithium, sodium, potassium, rubidium or cesium is selected. 109839/1234 SBE 2880 29 · Alkali metal salt for use in a hygrometer according to any one of claims 1 to 6 or 26 or in a thermometer according to claim 24, characterized characterized that it is a salt of l, l, 3-3? rimethyl-5-carboxy -3- (p-carboxy.-phenyl) Indan acts. 30. Alkali metal salt according to claim 29, characterized characterized in that the alkali metal is either lithium, sodium, potassium, rubidium or cesium is chosen. 31 · Hygrometer according to one of Claims 1 to 3, characterized in that the substance (3) suitable for a »change from not subject to birefringent state in the birefringent state and thereby irreversibly double becomes refractive when the birefringent state lasts for more than about 72 hours. 109839/1234