CA1209020A - Relative humidity or water activity apparatus - Google Patents
Relative humidity or water activity apparatusInfo
- Publication number
- CA1209020A CA1209020A CA000470586A CA470586A CA1209020A CA 1209020 A CA1209020 A CA 1209020A CA 000470586 A CA000470586 A CA 000470586A CA 470586 A CA470586 A CA 470586A CA 1209020 A CA1209020 A CA 1209020A
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- indicators
- display surface
- deliquescent
- relative humidity
- water activity
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N31/00—Investigating or analysing non-biological materials by the use of the chemical methods specified in the subgroup; Apparatus specially adapted for such methods
- G01N31/22—Investigating or analysing non-biological materials by the use of the chemical methods specified in the subgroup; Apparatus specially adapted for such methods using chemical indicators
- G01N31/222—Investigating or analysing non-biological materials by the use of the chemical methods specified in the subgroup; Apparatus specially adapted for such methods using chemical indicators for investigating moisture content
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- Health & Medical Sciences (AREA)
- Analytical Chemistry (AREA)
- Molecular Biology (AREA)
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- Chemical & Material Sciences (AREA)
- Biophysics (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Non-Biological Materials By The Use Of Chemical Means (AREA)
Abstract
Relative Humidity or Water Activity Apparatus Anthony Nelson Sharpe, 230 Christian St.
Almonte, Ontario K0A lA0, Canada ABSTRACT
An apparatus for measuring relative humidity or the water activity level in a food sample, within defined limits, gives a reading within seconds. The apparatus comprises a rough or porous display surface to which is applied a pattern of graded indicator substances, each of which deliquesces at a known relative humidity. Said indicator substances are applied in such quantity that they are invisible or nearly so when dry, but become severally or wholly visible again on exposure to a moist atmosphere, according to the relative humidity of the atmosphere in relation to the range of relative humidities covered by said deliquescent indicators. If said indicators are applied in the form of the decimal fractions corresponding to their relative humidity values the apparatus provides a digital readout of relative humidity, directly and without electronics. In a modification the display surface is of glass treated with dichlorodimethylsilane after the indicators have been applied. In another the display surface is a thin porous polymer film. In further modifications the display is enclosed in a shallow dish which may be applied to the surface of a food, or in a spearlike container which may be plunged inside a food.
In a final modification the deliquescent indicators are mixed with a dye which diffuses into a removable porous layer to provide a permanent record of the humidity or water activity.
Almonte, Ontario K0A lA0, Canada ABSTRACT
An apparatus for measuring relative humidity or the water activity level in a food sample, within defined limits, gives a reading within seconds. The apparatus comprises a rough or porous display surface to which is applied a pattern of graded indicator substances, each of which deliquesces at a known relative humidity. Said indicator substances are applied in such quantity that they are invisible or nearly so when dry, but become severally or wholly visible again on exposure to a moist atmosphere, according to the relative humidity of the atmosphere in relation to the range of relative humidities covered by said deliquescent indicators. If said indicators are applied in the form of the decimal fractions corresponding to their relative humidity values the apparatus provides a digital readout of relative humidity, directly and without electronics. In a modification the display surface is of glass treated with dichlorodimethylsilane after the indicators have been applied. In another the display surface is a thin porous polymer film. In further modifications the display is enclosed in a shallow dish which may be applied to the surface of a food, or in a spearlike container which may be plunged inside a food.
In a final modification the deliquescent indicators are mixed with a dye which diffuses into a removable porous layer to provide a permanent record of the humidity or water activity.
Description
i _elative Humi-lity or Water Activity Apparatus__ _ _ A ony Nelson Sharpe, 230 Christian St.,_ Almonte, Ontario KOA lA0, Canada ABST~ACT
An apparatus for measuring relative humidity or the water activity level in a food sample, within defined limits, gives a reading within seconds. The apparatus comprises a rougll or porous display surface to which is applied a pattern of graded indicator substances, each of which deliquesces at a known relative humidity. Said indicator substances are applied in such quantity that they are invisible or nearly so when dry, but become severally or wholly visible again on exposure to a moist atmosphere, according to the relative humidity of the atmosphere in relation to the range of relative humidities covered by said deliquescent indicators. If said indicators are applied in the form of the decimal fractions corresponding to their relative humidity values the apparatus provides a digital readout of relative humidity, directly and without electronics. In a modification the display surface is of glass treated with dichlorodimethylsilane after the indicators have been applied. In another the display surface is a thin porous polymer film. In further modifications the display is enclosed in a shallow dish wnich may be al)p-Li(nl to the surEIce of a food, or in a spearlike container which may be plunged inside a food.
In a final modification the deliquescent indicators are mixed with a dye which diffuses into a removable porous layer to provide a permanent record of the hunnidity or water activity.
SPECIFICATION
In the manufacture of and control of the microbiological quality of food it is necessary to measure the property known as water activity or a~, which describes the availability of water molecules in aqueous solutior-s. Water activity levels profoundly affect the ability of spoilage or pathogenic microorganisms to grow in food. The a~ of pure ~ ,~
water i9 1, and tne value decreases with increasing concentration of dissolved solids. Solutions of known a~ can be prepared by dissolving accurate amounts of salts or other solutes in water, or simply by preparing saturated solutions of others.
Most food factories routinely measure the a~ of their products and in many countries such measurements are a statutory requirement.
As a result, many types of apparatus have been developed to simplify and speed up the measurernent. Most of these depend Oll the direct relationship existing between a~ and the equilibrium relative humidty or RH of the atmosphere in a closed vessel containing the food.
Hair-type hygrometers are simple and popular but require several hours to respond and need to be recalibrated frequently against solutions of known a~. Many different types of electronic instruments are available, most of them based on the changing electrical properties of a nygroscopic salt. These respond faster but are more expensive and, in time, are seriously perturbed by volatile hygroscopic compounds released from foods. In consequence, they require special protective devices and must be frequently recalibrated against known solutions.
A simple an inexpensive device based on sprinkling small quantities of appropriate deliquescent salts onto a surface that is smeared with petroleum jelly has attracted much attention recently (Northolt and Heuvelman, Journal of ~ood Protection, 1982, 45: 537-540). When inserted into a closed container containillg a s~)ecilllen oE the tes~ Eoo-l these salts either deliquesce or not, according to the relative humidity in the container. It is thus possible to determine, without need for recalibration, whether the a~ is above a certain value. Being prepared fresh each time such a device is less affected by volatile substances from the Eood. Ilowever, it is slow to respond, needing at least three hours to produce a reliable and easily readable indication. Having observed deliquescence in some of the crystals the user must then refer to a table of values for the salts being used. In addition, grinding and sieving the salt crystals to obtain a suitable si~e range, and washing and preparing the surfaces for re-use are tedious and time consuming. Moreover, because the salt solutions drip away or dry out ;~z~z~
again when the container is opened there is no permanent record, which is a distinct disadvantage when the data are required by law to be inspected.
Although salts have been most comrnonly employed in this test, sugars or other water soluble compounds could also be used in the interests of obtaining a more closely graded set of a~ indicators.
I refer to such substances generally as deliquescent indicators in the remainder of this specification.
I have realized that while the deliquescent indicator test is useful, popular, and free from the need to recalibrate, its slow response and labor intensive preparation are problems to be overcome.
In particular, it would be highly desirable if such an accurate and absolute test could be made to provide a permanent record of ~ rather than a transitory indication.
During experiments to develop such a desirable apparatus I
determined that the speed of response depends partly on the quantity of water vapor which must diffuse into the deliquescent indicator crystals in order to make it obvious that they have dissolved.
Therefore, if the quantity of deliquescent indicator is greatly reduced the response is faster. ~owever, in order to make the deliquescence of such minute quantities of indicator easily visible it is necessary to provide an inexpensive means of amplifying the small optical changes occurring when the indicator substance dissoLves. 'rhis I have acllieved by applying the deliquescent indicators to a suitlble insoluble surface reEerred to as the display surface in the remainder of this specification.
Speed of response also depellds on the distance water molecules must diffuse Erom the food before reaching the deliquescent indicator.
In the usual form of the test a food specimen is placed in a lidded vessel with the deliquescent indicator applied to the inside of the lid. Typically, a 250 mL or 500 mL container is used, and perhaps only one-fifth of this is occupied by food. Equilibration is thus slow.
I realized that by providing a container which allows much closer apposition of the deliquescent indicator and the food, or by providing a spea-rlike container for the ind:icator which can then simply be plunged inside a sufficiently voluminous specimen of the food, much of the diffusion path can be eliminated and equilibrium achieved quickly.
Typically, embodiments of my relative humidity or water activity invention give a stable response within a few seconds, whereas previous]y it took severa] hours.
Such devices are useful for rapidly screening specimens for a~ and, by virtue of their rapid drying and rehydration, can be quickly transferred from sample to sample during a cursory inspection of a large food consignment. Moreover, by applying the deliquescent indicators to the display surface in the form of a simple pattern, and particularly in the shapes of the digits representing the numerical values of a~ for each indicator, the reading of a~ can be greatly simplified since the user merely reads off directly the highest figure visible when the device has reached equilibrium.
However, even in this greatly improved formJ the apparatus still provides a transitory indication of RH or a~ rather than a permanent record. ln order to provide such a permanent record I have further developed the apparatus. In this further modification a water soluble stain or dye is incorporated into the deliquescent indicators, and a second, porous layer is removably attached to the display surface.
When the deliquescent indicator deliquesces on expos~re to a sufficient relative humidity the resulting stain solutiotl spreads into this recording layer and is permanently visible there a~ter the apparatus is removed from the food. Such permanently recording apparatus can, of course, only be used once.
Embodimen~s o~ the invention will now be described by reference to the accompanying diagrammatic drawings in which~
Figure 1 shows plan and sec~ion of a simple a~ apparatus in which a set of graded deliquescent indicators is applied to an insoluble display surface to be used as the lid of a vessel containing the food specimen.
Figure 2 illustrates the typical appearance of this apparatus in z~
use.
Figure 3 shows a section through an apparatus in which the display surface is a mesh or porous membrane.
Figure 4 illustrates an apparatus which may simply be pressed against the surface of a suitable solid food specimen.
Figure 5 shows plan and section of an apparatus in which the display surface is enclosed in a rigid spearlike porous sheath which can be plunged into the interior of a food specimen.
Figure 6 shows a section through an apparatus in which a porous recording layer is removably attached to the treated display surface.
Referring to Figure 1, a rigid or semi-rigid plate 1 is provided with a display surface 2 by roughening it with abrasives or by etching.
The material of the plate may be, for example, glass or acrylic plastic, but other materials are not excluded. Ilowever, they should be slightly wettable, the molecules of the polymers containing a proportion of polar or hydrophilic groups. The degree of roughness is not particularly important and is typically that obtained by grinding the plate on 200 to 600 mesh abrasive. The material may be transparent in which case the display surface is normally translucent and becomes temporarily transparent in use or, if the material is opaque it is preferably black in which case the display surface is normally lighter but becomes temporarily darker in use. The cle~n display surface is now moistened in difFerent areas with solutions of tlle deliquescent indicators 3, which are allowed to dry.
In the preferred form the deliquescent indicators are applied by print:ing means, each deliquescent indicator being applied in the form of the decimal fraction representing its particular a~ value. The concentration and quantity of each deliquescent indicator applied must be determined for each indicator. Typically solutions of 1 to 10 per cent concentration will be used so that the dried print is invisible or nearly so on the rough display surface but contains ~ufficient indicator to yield an easily visible image when exposed to a ~ ~3~
sufficiently humid atmosphere. 1~ the display surface is prepared from a transparent plate the print will usually be reversed so that the numbers appear erect when viewed from the other side of the plate.
The dimensions or the apparatus and the print are not particularly important but should be consistent with easy readability of the numbers and the ability to cover a convenient quantity of food specimen.
Referring to Figure 2, to use this apparatus when it is constructed from a transparent plate the food specimen 4 (for example, a slice of fermented sausage) is placed in the bottom of a shallow dish 5 which is then covered by the apparatus with the display surface facing the food. The dish and display surface should make good contact or be provided with sealing means such as a rubber O-ring so that the atmosphere inside the dish is completely enclosed.
On exposure to the moist atmosphere inside the dish each deliquescent indicator possessing an a~ value less than that of the food will deliquesce, while those of higher a~ values will remain dry.
When an indicator deliquesces its solution fills the minute cracks and pores of the roughened display surafce and reduces the amount of light scattered there. In this example the translucent plate becomes transparent wherever it is wetted and the numbers representing those indicators 6 which deliquesced become visible on a surface which was original~y blank white. After a suitable period, therefore, t:he user simply makes a note of the highest decimal ~r.clctioll visible and recol-ds this as the a~ value. ~uch 1n apparatLIs provides a reading of a~ wit.hin a few seconds.
If the apparatus is made of black opaque material it will be used in such a manner that the numbers (which in this case will be black against a lighter background) are visible through a transparent wall in the dish.
A slight disadvantage of this form of the apparatus is that, if the user leaves i~ for several hours before looking at it, the solutions of deliquescing indicators spread across the display surface and become blurred or even unreadable. In a modification of this basic apparatus the deliquescent indicators are app]ied to a roughened glass surface ~2&~?~
which is then exposed to a reagent such as dichlorodimethylsilane.
This reagent invisibly siliconizes and renders the surface hydrophobic except in those areas which are masked by the applied indicators. When this apparatus is usecl tlle solutions of deliquescing indicators are still able to wet the surface beneath them and render iL transparent but are prevented from spreading because they will not wet the surrounding hydrophobic surface.
Referring to Figure 3, a section through another modification is shown in which the display surface is a translucent mesh or porous membrane 7 fastened by suitable fastening means to a stiff black or transparent layer 8. In the preferred form the membrane is laminated by heat or other laminating means to the support film 8. In this modification an adhesive layer 9 such as polyethylene will usually be required to bring about the lamination. In the preferred form the membrane is extremely thin and contains holes of at least O.l ,um diameter so as to scatter light strongly. Membranes of polycarbonate, polyester or other slightly wettable polymers which have been made porous by the process known as track-etching following nuclear irradiation, and in which there is little lateral connection between the holes are preferred~ but other precipitated membranes or woven meshes are not excluded. The deliquescent indicators will normally be applied to the membrane after it is fastened to the support EiLm.
This apparatus will normally be usecl by inserti[lg it temporariLy into a re-usable carrier an(l discarding ;t aFter use. lt yields an image with less tendency to spread i~ overexposed to the humid atmosphere .
Referr;ng to Figure 4, a further modiEication is shown in which a display surface with de-Liquescent indicators prepared as above is incorporated into a smooth rimmecl, shallow transparent vessel lO in such manner that the deliquescing display is visible through the base of the vessel. To use such an apparatus it is inverted and pressed against the surface of a suitably flat and yielding food specimen 11 in such manner as to enclose a small volume of air. There is thus no need to remove a portion of the specimen to a separate vessel. The dimensions of the apparatus are relatively unimportant but clearance between the speciinen and display should be enough to accommodate uneven food specimens while keeping the enclosed volume small for rapid response .
Referring to Figure 5, a further modification is ShOWtl in which the display surface and deliquescing indicators are contained inside a rigid porous sheath 12. This modification permits the user to plunge the apparatus into a suitably yielding food specimen and allows measurement of a~ inside of a food specimen without need to remove 10 portions to a separate vessel. Moreover, since the apparatus is completely surrounded by the specimen the diffucion of water vapor is optimized and the fastest response obtained. However, the display is only visible when the apparatus is withdrawn from the specimen and must be read quickly before the deliquesced indicator display dries out again .
The materials and construction of the sheath are relatively unimportant provided that the sheath prevents direcl: contact between specimen and display, and are sufficently perforated to allow good diffusion of water vapor. The structure is preferably brought to a 20 point 13 so as to facilitate penetration of the specimen. It may be transparent or not, depending on whether the structure permits the display to be withdrawn from the sheath for reading.
~ eferring to Figure 6, a section through a Ellrtller moclil~ication is shown in which a water soluble stain or dye is incorporated into the deliquescent indicators and a porous recording layer 14 into which the stain solution formed during use can seep is removably attached to the display surface. In the form shown the recording Layer is of paper and the display surface 7 is of porous mesh, but any of the display surfaces described above may be used and many papers, cloths, 30 membranes or meshes may be used for both the display and recorcling layers. The means of removably attaching the two layers together will depend on the materials used, for example, by the applica~ion of sufficient heat to fuse the two layers together weakly, or by stretching over a convex surface.
If the display pattern is of the number type discussed above, the numbers may be applied to the display surface as previously described.
Alternatively, the numbers may be printed on the recording layer in negative form using a hydrophobic barrier material such as wax or rubber latex, in which case the print of deliquescent indicators may be applied simply as solid blocks or strips on the display surface. The hydrophobic barrier material thus provides both the means of removably attaching the two layers and the means of confining the transferred stain to the desired areas. This apparatus may be incorporated into any of the preceeding embodiments and is easier and less expensive to manufacture. At least one of the two layers must be porous so as to allow water vapor to reach the deliquescent indicators.
To use such an apparatus it is exposed to the humid atMosphere of the test specimen for a sufficient time to allow those deliquescent indicators which will deliquesce to do so. During this time the dye or stain also dissolves in the solution of deliquescing indicator and seeps into the recording layer. After sufficient time has elapsed the two layers are torn apart. The user reads the a~ value by noting the highest visible value stained in the recording layer. Since the recording layer is no longer in contact with deliquescent indicators, and since the visible i.mage is produced by the transferred stain rather than a deliquescent indicator, this record is permanent.
Because ad7nixing a dye or stMin with an otherwi~.e pure cleliquescent indicator may change the value of RH in which it deliquesces it will usually be necessary to remeasure the a~ value to which the deliquescent indicator responds. This calibration will be done by the manuEacturer and poses no extra labor for the user. Moreover, intermixing of tlle reagents in this way can be made use of tG provide a more closely graded set of a~ values where such a need is observed.
It will be obvious that, in its speed of response, simplicity and freedom from the need to recalibrate, my invention provides a sign;ficant advance over previous methods of measuring water activity or relative humidity. In particular, the ability of certain embodiments of the invention to provide a permanent record of a~ will be welcomed in any process where there is a requirement that records be inspected and wherever the actual value of a~ may be called retrospectively into accou~t.
An apparatus for measuring relative humidity or the water activity level in a food sample, within defined limits, gives a reading within seconds. The apparatus comprises a rougll or porous display surface to which is applied a pattern of graded indicator substances, each of which deliquesces at a known relative humidity. Said indicator substances are applied in such quantity that they are invisible or nearly so when dry, but become severally or wholly visible again on exposure to a moist atmosphere, according to the relative humidity of the atmosphere in relation to the range of relative humidities covered by said deliquescent indicators. If said indicators are applied in the form of the decimal fractions corresponding to their relative humidity values the apparatus provides a digital readout of relative humidity, directly and without electronics. In a modification the display surface is of glass treated with dichlorodimethylsilane after the indicators have been applied. In another the display surface is a thin porous polymer film. In further modifications the display is enclosed in a shallow dish wnich may be al)p-Li(nl to the surEIce of a food, or in a spearlike container which may be plunged inside a food.
In a final modification the deliquescent indicators are mixed with a dye which diffuses into a removable porous layer to provide a permanent record of the hunnidity or water activity.
SPECIFICATION
In the manufacture of and control of the microbiological quality of food it is necessary to measure the property known as water activity or a~, which describes the availability of water molecules in aqueous solutior-s. Water activity levels profoundly affect the ability of spoilage or pathogenic microorganisms to grow in food. The a~ of pure ~ ,~
water i9 1, and tne value decreases with increasing concentration of dissolved solids. Solutions of known a~ can be prepared by dissolving accurate amounts of salts or other solutes in water, or simply by preparing saturated solutions of others.
Most food factories routinely measure the a~ of their products and in many countries such measurements are a statutory requirement.
As a result, many types of apparatus have been developed to simplify and speed up the measurernent. Most of these depend Oll the direct relationship existing between a~ and the equilibrium relative humidty or RH of the atmosphere in a closed vessel containing the food.
Hair-type hygrometers are simple and popular but require several hours to respond and need to be recalibrated frequently against solutions of known a~. Many different types of electronic instruments are available, most of them based on the changing electrical properties of a nygroscopic salt. These respond faster but are more expensive and, in time, are seriously perturbed by volatile hygroscopic compounds released from foods. In consequence, they require special protective devices and must be frequently recalibrated against known solutions.
A simple an inexpensive device based on sprinkling small quantities of appropriate deliquescent salts onto a surface that is smeared with petroleum jelly has attracted much attention recently (Northolt and Heuvelman, Journal of ~ood Protection, 1982, 45: 537-540). When inserted into a closed container containillg a s~)ecilllen oE the tes~ Eoo-l these salts either deliquesce or not, according to the relative humidity in the container. It is thus possible to determine, without need for recalibration, whether the a~ is above a certain value. Being prepared fresh each time such a device is less affected by volatile substances from the Eood. Ilowever, it is slow to respond, needing at least three hours to produce a reliable and easily readable indication. Having observed deliquescence in some of the crystals the user must then refer to a table of values for the salts being used. In addition, grinding and sieving the salt crystals to obtain a suitable si~e range, and washing and preparing the surfaces for re-use are tedious and time consuming. Moreover, because the salt solutions drip away or dry out ;~z~z~
again when the container is opened there is no permanent record, which is a distinct disadvantage when the data are required by law to be inspected.
Although salts have been most comrnonly employed in this test, sugars or other water soluble compounds could also be used in the interests of obtaining a more closely graded set of a~ indicators.
I refer to such substances generally as deliquescent indicators in the remainder of this specification.
I have realized that while the deliquescent indicator test is useful, popular, and free from the need to recalibrate, its slow response and labor intensive preparation are problems to be overcome.
In particular, it would be highly desirable if such an accurate and absolute test could be made to provide a permanent record of ~ rather than a transitory indication.
During experiments to develop such a desirable apparatus I
determined that the speed of response depends partly on the quantity of water vapor which must diffuse into the deliquescent indicator crystals in order to make it obvious that they have dissolved.
Therefore, if the quantity of deliquescent indicator is greatly reduced the response is faster. ~owever, in order to make the deliquescence of such minute quantities of indicator easily visible it is necessary to provide an inexpensive means of amplifying the small optical changes occurring when the indicator substance dissoLves. 'rhis I have acllieved by applying the deliquescent indicators to a suitlble insoluble surface reEerred to as the display surface in the remainder of this specification.
Speed of response also depellds on the distance water molecules must diffuse Erom the food before reaching the deliquescent indicator.
In the usual form of the test a food specimen is placed in a lidded vessel with the deliquescent indicator applied to the inside of the lid. Typically, a 250 mL or 500 mL container is used, and perhaps only one-fifth of this is occupied by food. Equilibration is thus slow.
I realized that by providing a container which allows much closer apposition of the deliquescent indicator and the food, or by providing a spea-rlike container for the ind:icator which can then simply be plunged inside a sufficiently voluminous specimen of the food, much of the diffusion path can be eliminated and equilibrium achieved quickly.
Typically, embodiments of my relative humidity or water activity invention give a stable response within a few seconds, whereas previous]y it took severa] hours.
Such devices are useful for rapidly screening specimens for a~ and, by virtue of their rapid drying and rehydration, can be quickly transferred from sample to sample during a cursory inspection of a large food consignment. Moreover, by applying the deliquescent indicators to the display surface in the form of a simple pattern, and particularly in the shapes of the digits representing the numerical values of a~ for each indicator, the reading of a~ can be greatly simplified since the user merely reads off directly the highest figure visible when the device has reached equilibrium.
However, even in this greatly improved formJ the apparatus still provides a transitory indication of RH or a~ rather than a permanent record. ln order to provide such a permanent record I have further developed the apparatus. In this further modification a water soluble stain or dye is incorporated into the deliquescent indicators, and a second, porous layer is removably attached to the display surface.
When the deliquescent indicator deliquesces on expos~re to a sufficient relative humidity the resulting stain solutiotl spreads into this recording layer and is permanently visible there a~ter the apparatus is removed from the food. Such permanently recording apparatus can, of course, only be used once.
Embodimen~s o~ the invention will now be described by reference to the accompanying diagrammatic drawings in which~
Figure 1 shows plan and sec~ion of a simple a~ apparatus in which a set of graded deliquescent indicators is applied to an insoluble display surface to be used as the lid of a vessel containing the food specimen.
Figure 2 illustrates the typical appearance of this apparatus in z~
use.
Figure 3 shows a section through an apparatus in which the display surface is a mesh or porous membrane.
Figure 4 illustrates an apparatus which may simply be pressed against the surface of a suitable solid food specimen.
Figure 5 shows plan and section of an apparatus in which the display surface is enclosed in a rigid spearlike porous sheath which can be plunged into the interior of a food specimen.
Figure 6 shows a section through an apparatus in which a porous recording layer is removably attached to the treated display surface.
Referring to Figure 1, a rigid or semi-rigid plate 1 is provided with a display surface 2 by roughening it with abrasives or by etching.
The material of the plate may be, for example, glass or acrylic plastic, but other materials are not excluded. Ilowever, they should be slightly wettable, the molecules of the polymers containing a proportion of polar or hydrophilic groups. The degree of roughness is not particularly important and is typically that obtained by grinding the plate on 200 to 600 mesh abrasive. The material may be transparent in which case the display surface is normally translucent and becomes temporarily transparent in use or, if the material is opaque it is preferably black in which case the display surface is normally lighter but becomes temporarily darker in use. The cle~n display surface is now moistened in difFerent areas with solutions of tlle deliquescent indicators 3, which are allowed to dry.
In the preferred form the deliquescent indicators are applied by print:ing means, each deliquescent indicator being applied in the form of the decimal fraction representing its particular a~ value. The concentration and quantity of each deliquescent indicator applied must be determined for each indicator. Typically solutions of 1 to 10 per cent concentration will be used so that the dried print is invisible or nearly so on the rough display surface but contains ~ufficient indicator to yield an easily visible image when exposed to a ~ ~3~
sufficiently humid atmosphere. 1~ the display surface is prepared from a transparent plate the print will usually be reversed so that the numbers appear erect when viewed from the other side of the plate.
The dimensions or the apparatus and the print are not particularly important but should be consistent with easy readability of the numbers and the ability to cover a convenient quantity of food specimen.
Referring to Figure 2, to use this apparatus when it is constructed from a transparent plate the food specimen 4 (for example, a slice of fermented sausage) is placed in the bottom of a shallow dish 5 which is then covered by the apparatus with the display surface facing the food. The dish and display surface should make good contact or be provided with sealing means such as a rubber O-ring so that the atmosphere inside the dish is completely enclosed.
On exposure to the moist atmosphere inside the dish each deliquescent indicator possessing an a~ value less than that of the food will deliquesce, while those of higher a~ values will remain dry.
When an indicator deliquesces its solution fills the minute cracks and pores of the roughened display surafce and reduces the amount of light scattered there. In this example the translucent plate becomes transparent wherever it is wetted and the numbers representing those indicators 6 which deliquesced become visible on a surface which was original~y blank white. After a suitable period, therefore, t:he user simply makes a note of the highest decimal ~r.clctioll visible and recol-ds this as the a~ value. ~uch 1n apparatLIs provides a reading of a~ wit.hin a few seconds.
If the apparatus is made of black opaque material it will be used in such a manner that the numbers (which in this case will be black against a lighter background) are visible through a transparent wall in the dish.
A slight disadvantage of this form of the apparatus is that, if the user leaves i~ for several hours before looking at it, the solutions of deliquescing indicators spread across the display surface and become blurred or even unreadable. In a modification of this basic apparatus the deliquescent indicators are app]ied to a roughened glass surface ~2&~?~
which is then exposed to a reagent such as dichlorodimethylsilane.
This reagent invisibly siliconizes and renders the surface hydrophobic except in those areas which are masked by the applied indicators. When this apparatus is usecl tlle solutions of deliquescing indicators are still able to wet the surface beneath them and render iL transparent but are prevented from spreading because they will not wet the surrounding hydrophobic surface.
Referring to Figure 3, a section through another modification is shown in which the display surface is a translucent mesh or porous membrane 7 fastened by suitable fastening means to a stiff black or transparent layer 8. In the preferred form the membrane is laminated by heat or other laminating means to the support film 8. In this modification an adhesive layer 9 such as polyethylene will usually be required to bring about the lamination. In the preferred form the membrane is extremely thin and contains holes of at least O.l ,um diameter so as to scatter light strongly. Membranes of polycarbonate, polyester or other slightly wettable polymers which have been made porous by the process known as track-etching following nuclear irradiation, and in which there is little lateral connection between the holes are preferred~ but other precipitated membranes or woven meshes are not excluded. The deliquescent indicators will normally be applied to the membrane after it is fastened to the support EiLm.
This apparatus will normally be usecl by inserti[lg it temporariLy into a re-usable carrier an(l discarding ;t aFter use. lt yields an image with less tendency to spread i~ overexposed to the humid atmosphere .
Referr;ng to Figure 4, a further modiEication is shown in which a display surface with de-Liquescent indicators prepared as above is incorporated into a smooth rimmecl, shallow transparent vessel lO in such manner that the deliquescing display is visible through the base of the vessel. To use such an apparatus it is inverted and pressed against the surface of a suitably flat and yielding food specimen 11 in such manner as to enclose a small volume of air. There is thus no need to remove a portion of the specimen to a separate vessel. The dimensions of the apparatus are relatively unimportant but clearance between the speciinen and display should be enough to accommodate uneven food specimens while keeping the enclosed volume small for rapid response .
Referring to Figure 5, a further modification is ShOWtl in which the display surface and deliquescing indicators are contained inside a rigid porous sheath 12. This modification permits the user to plunge the apparatus into a suitably yielding food specimen and allows measurement of a~ inside of a food specimen without need to remove 10 portions to a separate vessel. Moreover, since the apparatus is completely surrounded by the specimen the diffucion of water vapor is optimized and the fastest response obtained. However, the display is only visible when the apparatus is withdrawn from the specimen and must be read quickly before the deliquesced indicator display dries out again .
The materials and construction of the sheath are relatively unimportant provided that the sheath prevents direcl: contact between specimen and display, and are sufficently perforated to allow good diffusion of water vapor. The structure is preferably brought to a 20 point 13 so as to facilitate penetration of the specimen. It may be transparent or not, depending on whether the structure permits the display to be withdrawn from the sheath for reading.
~ eferring to Figure 6, a section through a Ellrtller moclil~ication is shown in which a water soluble stain or dye is incorporated into the deliquescent indicators and a porous recording layer 14 into which the stain solution formed during use can seep is removably attached to the display surface. In the form shown the recording Layer is of paper and the display surface 7 is of porous mesh, but any of the display surfaces described above may be used and many papers, cloths, 30 membranes or meshes may be used for both the display and recorcling layers. The means of removably attaching the two layers together will depend on the materials used, for example, by the applica~ion of sufficient heat to fuse the two layers together weakly, or by stretching over a convex surface.
If the display pattern is of the number type discussed above, the numbers may be applied to the display surface as previously described.
Alternatively, the numbers may be printed on the recording layer in negative form using a hydrophobic barrier material such as wax or rubber latex, in which case the print of deliquescent indicators may be applied simply as solid blocks or strips on the display surface. The hydrophobic barrier material thus provides both the means of removably attaching the two layers and the means of confining the transferred stain to the desired areas. This apparatus may be incorporated into any of the preceeding embodiments and is easier and less expensive to manufacture. At least one of the two layers must be porous so as to allow water vapor to reach the deliquescent indicators.
To use such an apparatus it is exposed to the humid atMosphere of the test specimen for a sufficient time to allow those deliquescent indicators which will deliquesce to do so. During this time the dye or stain also dissolves in the solution of deliquescing indicator and seeps into the recording layer. After sufficient time has elapsed the two layers are torn apart. The user reads the a~ value by noting the highest visible value stained in the recording layer. Since the recording layer is no longer in contact with deliquescent indicators, and since the visible i.mage is produced by the transferred stain rather than a deliquescent indicator, this record is permanent.
Because ad7nixing a dye or stMin with an otherwi~.e pure cleliquescent indicator may change the value of RH in which it deliquesces it will usually be necessary to remeasure the a~ value to which the deliquescent indicator responds. This calibration will be done by the manuEacturer and poses no extra labor for the user. Moreover, intermixing of tlle reagents in this way can be made use of tG provide a more closely graded set of a~ values where such a need is observed.
It will be obvious that, in its speed of response, simplicity and freedom from the need to recalibrate, my invention provides a sign;ficant advance over previous methods of measuring water activity or relative humidity. In particular, the ability of certain embodiments of the invention to provide a permanent record of a~ will be welcomed in any process where there is a requirement that records be inspected and wherever the actual value of a~ may be called retrospectively into accou~t.
Claims (11)
1. An apparatus for indicating relative humidity or water activity comprising one or a plurality of deliquescent indicators consisting of deliquescent salts, sugars or other compounds for which the equilibrium relative humidities of their saturated solutions are known and which are graded according to these relative humidity values, said graded deliquescent indicators being individually and separately applied as solutions to an insoluble rough or porous display surface so that their residues are invisible or nearly so when dry but which by deliquescence become severally or wholly visible again on being exposed to a moist atmosphere, according to the relative humidity of the atmosphere in relation to the range of relative humidities covered by said deliquescent indicators.
2. An apparatus as in 1 in which said graded deliquescent indicators are applied to the display surface in a pattern in which each deliquescent indicator is applied in the shape of the decimal fraction corresponding to the relative humidity or water activity value to which that indicator responds.
3. An apparatus as in 2 in which the display surafce is of roughened glass which after receiving the applied graded deliquescent indicators is exposed to dichlorodimethylsilane, said roughened surface thereby being rendered hydrophobic except in those areas which are masked by the deliquescent indicators.
4. An apparatus as in 2 in which the display surface is of roughened acrylic or other polymer, the molecules of which contain a proportion of polar or hydrophilic groups.
5. An apparatus as in 2 in which the display surface is a mesh or porous membrane of a polymer, the molecules of which contain a proportion of hydrophilic groups.
6. An apparatus as in 2 in which the display surface is a porous membrane of a polymer, the molecules of which contain a proportion of hydrophilic groups and which has been made porous by the process known as track-etching following nuclear irradiation.
7. An apparatus as in 2 in which the display surface is attached to the base of a shallow transparent vessel which may be pressed against a specimen to determine its water activity.
8. An apparatus as in 2 in which the display surface is contained in a rigid pointed porous sheath which may be plunged inside a specimen to determine its water activity.
9. An apparatus as in 2 in which said graded deliquescent indicators contain a water soluble stain or dye.
10. An apparatus as in 9 in which said display surface is a mesh or porous membrane, said display surface being removably attached to an absorbent recording layer that absorbs stain solution from those indicators which deliquesce when the apparatus is exposed to moist air.
11. An apparatus as in 9 in which said display surface and said absorbent recording layer are removably bonded together with wax or other hydrophobic adhesive, said adhesive being applied in negative form to the recording layer and said deliquescent indicators being applied in strip or simple block patterns to the display surface.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000470586A CA1209020A (en) | 1984-12-20 | 1984-12-20 | Relative humidity or water activity apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000470586A CA1209020A (en) | 1984-12-20 | 1984-12-20 | Relative humidity or water activity apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1209020A true CA1209020A (en) | 1986-08-05 |
Family
ID=4129414
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000470586A Expired CA1209020A (en) | 1984-12-20 | 1984-12-20 | Relative humidity or water activity apparatus |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1209020A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0249732A1 (en) * | 1986-05-21 | 1987-12-23 | Drägerwerk Aktiengesellschaft | Colorimetric detection device |
| WO2009108036A1 (en) * | 2008-02-26 | 2009-09-03 | Dimas Jimenez Mendoza | Device and method for measuring the water activity of foods |
| CN114428029A (en) * | 2022-01-14 | 2022-05-03 | 南京师范大学 | Method and device for measuring liquid moisture absorption capacity |
-
1984
- 1984-12-20 CA CA000470586A patent/CA1209020A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0249732A1 (en) * | 1986-05-21 | 1987-12-23 | Drägerwerk Aktiengesellschaft | Colorimetric detection device |
| WO2009108036A1 (en) * | 2008-02-26 | 2009-09-03 | Dimas Jimenez Mendoza | Device and method for measuring the water activity of foods |
| US8632732B2 (en) | 2008-02-26 | 2014-01-21 | Dimas Jimenez Mendoza | Device and method for measuring the water activity of foods |
| CN114428029A (en) * | 2022-01-14 | 2022-05-03 | 南京师范大学 | Method and device for measuring liquid moisture absorption capacity |
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