CA1049288A - Time temperature integrating indicator - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The temperature history of a product is visually displayed as a color front on an indicator, the distance of front advancement being a function of the temperature time integral.
The indicator measures the gas generation in a first compartment by a wick in a second compartment, the wick also being in communication with the first compartment. Optionally, a gas permeable film separates the gas generating material and the wick.

Description

Detail Description The present invention pertains to an indicator system which visually displays the time-temperature integral to which a product has been exposed.
The desirability of detecting whether or not a frozen product has been allowed to thaw has long been recognized and numerous tell-tale devices are described in the literature.
One class of these relies upon material which is frozen but which melts at some preselected temperature so as to irreversibly activate an indicator, either chemically or physically. Typically of these devices are those described in the following U.S.
patents:

Nos. 1,917l048Nos. 2,753,270Nos. 2,955,942

2,216,127 2,762,711 3,047,405 2,277,278 2,788,282 3,055,759 2,340,337 2,823,131 3,065,083 2,553,369 2,850,3g3 3,194,669 2,617,734 2,852,394 3,362,834 2,662,018 2,951,405 3,437,010 All of the above devices merely signal "thaw" with no attempt to measure the period during which the product is thawed or the temperature which the product attains while thawed.
A second class of known indicators utilizes diffusion or capillary action of a liquid on a wick or similar permeable member. These devices while often cumbersome, provide some degree of gradation and are typified by the devices of the following U.S. Patents:

Nos. 2,560,537Nos. 3,243,303 2,716,065 3,414,415 302,951,764 3,479,877

3,118,774 The majority of the prior art devices however are directed primarily at the phenomenon of thawing and the attendant damage which occurs. It is now recognized that various natural and synthetic materials deteriorate with the passage of time even when taking the precaution of storing under adequate 104~Z~38 refrigeration. This is true even with such additional or alternative precautions as packaging in an inert atmosphere, sterilization or adding spoilage retardants. Thus, for example, foods, films, pharmaceuticals, biological preparations and the like, can demonstrate decomposition with the passage of time, even when sterilized or maintained at sufficiently low temperatures to preclude microbiological degradation. Such decomposition occurs for various reasons, including strictly chemical reactions, such as oxidation, and enzymatic processes. Frozen foods and ice cream show deterioration even when held in a frozen state.
A system which would monitor such decomposition or deterioration would be extremely valuable.
The deterioration kinetics involved in such processes however, are exceedingly complex. For example, while it is clear that deterioration is a function of temperature, the rate of this deterioration of such products can also vary with temperature. One rate of deterioration will exist at a first temperature while a different rate obtains at a second temperature.
The total amount of deterioration will depend upon the time at which the product is held at each temperature; i.e. the integral of time and temperature.
The quotient of (a) the rate of change at one temperature of an article's property whose deterioration is being monitored to (b) the rate of change at a lower temperature is often expressed for ten degree increments and represented by the symbol "Qlo"
for the Celsius scale and "ql0" for the Fahrenheit scale. This quotient is substantially constant within limited temperature ranges.
The practical effect of the foregoing can be seen for example from two comparable samples of frozen food which are processed and packaged at the same time. If in the course of distribution or storage one package is allowed to rise in ~04~Z88 temperature by 10 or 20C, even without thawing, its life will be reduced as compared with the other package which was maintained at a lower temperature for its entire storage life since the rate of decomposition of the contents of the first package is accelerated during the storage at the higher temperature. A
consumer about to purchase these packages, both of which are now stored at normal freezer temperature, has no way of ascertaining this difference in temperature histories.
Systems have been suggested for monitoring the temperature history of a product. Thus U.S. Patent No. 2,671,028 utilizes an enzyme such as pepsin in indicator systems while U.S. Patent No. 3,751,382 discloses an enzymatic indicator in which urease decomposes urea with the reaction products causing a change in the pH of the system. The activity of the enzyme, and thus rate of decomposition, is dependent on temperature so that the change in pH resulting from this decomposition can be monitored by conventional acid-base indicators. This type of system, which appears to be directed at the spe~ific problem of microbiological putrefaction rather than the broader problem of monitoring temperature histories, suffers from the inherent limitation of any enzymatic reaction. Thus while enzyme activity is a function of temperature, it is also sensitive to the very passage of time being measured, enzymatic activity generally decreasing with time. Enzyme activity is also sensitive to pH
change and such change is the operative factor in, for example, the system of U.S. Patent No. 3,751,382. A more sophisticated system is described in U.S. Patent No. 3,768,976 in which time temperature integration is achieved by monitoring permeation of oxygen from the atmosphere through a film, utilizing a redox dye to provide a visual read out. This device is however dependent upon the presence of atmospheric oxygen and somewhat cumbersome in configuration and dimensions.

104~2~38 A further problem is that the change in rate of quality loss per unit of temperature change differs for di~ferent products.
Thus the change in the rate of deterioration per unit of temperature change for certain fruits and berries is vastly different fron the change in rate for lean meats. The values for dairy products are different from both. For example, within the range of 0 to -20C, raw fatty meat and pre-cooked fatty meat have Qlo ~ s of about 3, whereas raw lean meat and pre-cooked lean meat have Qlo's between 5 and 6. Vegetables generally have a Qlo of between 7 and 8, whereas fruits and berries have a Qlo of approximately 13. Consequently, a system which is dependent on a single enzymatic reaction or the permeability of a given film will be suitable as an indicator only for those materials having a similar slope for their relationship of change of rate of decomposition to temperature. Although U.S. Patent No.
3,751,382 describes a method for modifying the time at which the indicator's color change occurs, the activation energy of the enzyme system is modified only slightly and the ratio of change in reaction rate per temperature unit remains substantially the same. The same is true of the device described in U.S. Patent No. 3,768,976 which is dependent solely on gas permeability.
The present invention pertains to an indicator system which overcomes the above problems yet is extremely simple and reliable in structure and operation. Moreover, the device is extremely well suited for remote sensing: i.e., monitoring the time-temperature integrals at the interior of a package, while providing an immediate read-out of that integral on the exterior of the package.
The present system is not limited in application to monitoring long storage periods at low temperatures. The same considerations apply to short periods and to high temperature.
The present system can also be used to insure, for example, that 104~Z88 products have been adequately heat sterilized. The indicator is thus admirably suited to insure that canned goods which are autoclaved have been subjected to the appropriate time-temperature integral required to obtain a necessary degree of microorganism kill. In this case, the indicator provides visual information as to whether the necessary parameters of temperature and time have been reached or exceeded. Similarly, the present indicator can be used to insure that surgical instruments have been subjected to appropriate sterilization conditions, that pharmaceuticals have not been stored for periods in excess of that which is permissible, that dairy products have been properly pasteurized, and the like. Various other applications in which it is desirable to know the temperature history of a product are immediately apparent.
The present invention will be described in conjunction with the appended drawings in which:
FIGURE 1 is a plan view of a temperature-time-integrating indicator device constructed in accordance with the principles of the present invention, portions of the upper wall and the ampule positioning strip being broken away for purposes of clarity in depicting constructional details.
FIGURE 2 is a longitudinal vertical sectional view as taken along the line II-II in FIGURE 1.
FIGURE 3 is a transverse sectional view on enlarged scale as taken along the line III-III in FIGURE 2.
FIGURE 4 is a fragmentary plan view of the device showing an additional manner in which the sealing together of the envelope walls can be carried out.
FIGURE 5 is a side view of the ampule in which the gas generating material is confined, the ampule being enclosed in a resilient sleeve.

DESCRIPTION OF TH~ PR~F`ERRE~ EMBODIMENT
ThiS inventio~ relates to a device for monitoring the quality of a product. More specifically, it relates to a monitoring device which is capable of giving a visual display of the integral of time and temperature to which a product has been exposed.
With continuing reference to FIGURES 1-3, there is depicted a temperature time indicator which includes an envelope 10 comprised of elongated, generally co-extensive upper and lower walls 12 and 14 of gas impermeable material. The walls 12 and 14 while depicted as single ply components of transparent material could be plural ply and be laminated to include a metal foil layer as well as being in part opaque. The important consideration is that said walls be gas impermeable. Walls 12 and 14 are joined together to form the envelope structure by sealing them together in a continuous course extending about the periphery of each, e.g., by heat-sealing, the material of the walls of course being compatible to that purpose, and such peripheral seal being shown generally at 16 in FIGURE 2. The device also embodies a wick 18, the wick being disposed longitudinally of the envelope 1~, in a longitudinal portion thereof which constitutes an indicating section 26, and being treated with an indicator composition.
The device also includes an ampule 22 disposed in another longitudinal portion of the envelope constituting a gas generation section 26 in which is confined a gas generating material, the ampule being disposed intermediate the upper and lower walls 12 and 14 and being fixedly positioned therebetween as by connection of an overlaying gas permeable sheet 24 with one of said walls, the wick 18 having one tip end as at 19 in gas generation section 26 and its other tip end 21 remote from said gas generation section.

' In acc~rdance with the present invention, there is provided a gas barrier 40 at each longitudinal side of the wick 18, the gas barrier extending between walls 12 and 14 and in the instance where walls 12 and 14 are amenable to heat sealing being provided by effecting a heat sealed joinder of the walls in the pattern depicted best in FIGURE 1. The heat seal is positioned immediately adjacent the said wick longitudinal side margins. "Immediately adjacent" as used herein is intended to mean effecting the heat seal as close to the wick as practical manufacturing will permit without causing adherance of any melted wall material to the wick material. Thus any spacing 51 as may exist between the sides of the wick at the barrier is of insignificant consequence with respect to the possibility of gas transport occurring along said space without making a contact with the wick 18 at or very close to tip end 19. In this manner the possibility of random gas molecules transport through said space and into first contact with the wick at location remote from tip end l9 is inhibited.
The important requirement in the construction of the device is that the longitudinal gas barrier extend immediately adjacent the wick side margins substantially along the full length of the wick. If desired, however, the sealed joinder of the envelope walls can be extended laterally outwardly from the wick sides in the pattern 55 depicted in FIGURE 4.
Further in accordance with the present invention, the gas generating component is confined within ampule 22, and the ampule 22 is fixedly secured to the inner surface of one of the envelope upper and lower walls, in the depicted embodiment the ampule 22 being fixedly positioned by securing the same to the inner surface of lower wall 14 with the gas permeable sheet 24, the latter being heat sealed to the lower wall in the generally oval course seal pattern 57 depicted in FIGURE 1. The lV4~Z~38 ampule 22 in which the gas generatin~ material is confined desirably is an elongated component, closed at its ends and made of a frangible material, glass being preferred. Thus, when it is desired to activate the devicel the user need only apply a bending - force to the envelope in the reyion of the position of the ampule and gelierally applied intermediate the ends of the ampule to fracture the same and permit the gas to escape in the first section 28 of the envelope from whence it can flow onto the wick located in the second section 26. To provide that when ampule 22 is ruptured, resulting jagged particles of the same will not pierce or damage any of the envelope structure, the ampule can be enclosed in a resilient sleeve 60 as shown in FIGURE 5, the resilient sleeve for example being a braided fiberglass member.
It will be obvious to those skilled in the art that the gas generating material need not necessarily be sealed in an ampule. The only necessary requirement is that it be contained and isolated from the wick prior to activation. Furt'nermore, the ampule or other means for isolating the gas generating material can be completely enclosed in a pouch of the gas permeable sheet, 24. In that event the pouch must have a gas tight seal about its periphery. The pouch iLself need not be heat sealed to the walls of the gas barrier.
Upon rupture of the ampule 22 and after an initial induction period during which the partial pressure of the gas rises in chamber formed by the gas permeable sheet, 24, the gas permeates across film 24 to the wick, 18. The gas is then absorbed into wick 18. The rate of gas generation by the gas generating material is a function of temperature and the amount of gas which thus passes through the permeal film, 24, is in turn a function of temperature. If wick 18 is constructed with a substantially-constant cross-section, the distance which the gas advances along wick means 18 will thus be a direct function of 1049Z~38 the time-temperature integral to which the device has been subjected.
Deposited on wick 18 is an indicator composition which produces a color change in the presence of the gas generated by gas generating material. This indicator composition can vary widely but is selected so as to be responsive to the particular gas generated by gas generating material. Since this indicator composition produces a color change in the presence of the gas, an advancing front will be observed on wick means 18 in the indicating section, 26. The length of advancement corresponds to the time-temperature integral to which the device has been exposed and can be read through the incorporation of a graduated scale and appropriate indicia associated with the wick means.
The indicator composition may be a pH sensitive dye.
Alternately, it may be a composition which complexes with the gas generated to produce a color change.
Illustrative non-limiting examples of pH sensitive dyes useful as indicator compositions in the practice of this invention are phenolphthalein, xylenol blue, nile blue A, m-cresol purple, bromocresol green, o-cresol red, cyanidine chloride, bromocresol purple, alizarin, thymol blue, bromophenol red, methyl red, acid fuchsin, brilliant yellow, logwood extract, bromthymol blue, phenol red, phenolphthalexon, etc.
Various compounds such as copper or cobalt halides which can form complexes (e.g. with ammonia) which exhibit a color change upon complexing may be used as the indicator.
An additional compound preferably included in the wick is a quantifier material whose function is to fix the time interval over which the time-temperature indicator is operative.
Although the temperature and hence the Qlo sensitivity of the time-temperature indicator is determined by the temperature coefficients of both the vapor pressure of the gas generated and 1049ZI!38 the permeability of the rate controlling film, 24, (RCF); the timing response of the indicator, on the other hand, is determined by the amount of quantifier impregnated on the wick, as well as the thickness and effective area of the RCF.
Variations in the quantity of quantifier are best accomplished by controlling its concentration in an impregnating solution. For example, where the quantifier material is tartaric acid, a solution is prepared of 0.2N tartaric acid in ethanol and glycerol, the glycerol comprising 20% in volume of the solution, and 0.2~ of % phenol red based on the total solution. The wick is immersed in the solution and the excess material squeezed out by passing the saturated wick through a roll nip and allowing the wick to air dry.
Where the RCF is polypropylene of an area of about 525 mm and the gas generating material is (NH4)2CO3, the indicator based on a wick prepared in the above manner, (NH4)CO3 has a time scale at 0F of about 600 days for a 1/4 x 4-inch wick of 6 mil Whatman #114 filter paper. This time scale may be shortened by reducing the concentration of quantifier material in the impregnating solution.
The quantifier material can be any non-volatile material which reacts to neutralize the gas generated. Hence, the quantifier materials of choice are acidic or basic compounds which can be solvated for deposition on the wick. Illustrative examples of such quantifier are tartaric acid, potassium acid phosphate, cinnamic acid, quinine, guanidine, sodium hydroxide, sodium carbonate, etc. Certain quantifiers are preferably used with particular pH sensitive dyes as shown in the table below:
Gas Generated Quantifier pH Sensitive Dyes NH Tartaric acid Phenol red "3 Potassium acid phosphate Cresol red " Cinnamic acid Ethyl red Acetic acid Sodium hydroxide Methyl red " Quinine Methyl orange " Sodium carbonate Cresol red In lieu of a mechanical barrier, such as an ampule, the gas generating material may be isolated ~rom wick 18 prior to use by encapsulation, the details of which being well known to the art need not be elaborated here. Upon fracturing the protective coating around the individual particles of the encapsulated material/ which fracturing can be done mechanically or in the course of subjecting the particles to low temperatures, gas generation begins. The gas passes through permeable film 24 and then to the wick means 18.
An alternative to the longitudinal seals described above is a seal transverse and perpendicular to the wick 18, at or near the end of the wick l9, near the gas generating section 28. This transverse seal divides the device into its two sections 26 and 28. The function of the transverse seals or the heretofore described longitudinal seals is to prevent access to the wick, 18, of the gas generated except by capillary wicking action along the wick, 18, beginning at the end, l9, which protrudes into the gas generating section, 28. Absent, these seals gas would be free to diffuse toward the far end of the wick 21, thereby giving erroneous readings.
The gas generation section, 26, can utilize a variety of physical or chemical processes. In its simplest embodiment, the gas generation may involve simple sublimation or vaporization and thus one may utilize any substance which has a high vapor pressure, as for example, water (or ice); iodine, aliphatic and aromatic alcohols such as thymol; hydrogen peroxide; lower alkanoic and aromatic acids, such as acetic acid; acid anhydrides such as maleic anhydride; acid halides; ketones, aldehydes and the like. Alternatively the gas generating material can be a salt which decomposes with the generation of a gas, as for example ammonium carbonate, sodium bicarbonate, ammonium acetate, ammonium oxalate, ammonium formate and the like.

104~Z88 In those instances in which the rate of gas gen-eration corresponds to the rates being monitored, it i8 unneces-sary to include the barrier film, and gas generating section of the envelope, 28, can have a single chamber. Even in such em-bodiments, however, it is often aesirable to interpose a highly permeable physical barrier which separates the gas generating material from the wick. The permeability of such barriers should be substantially independent of temperature since the rate determining step i8 the generation of gas. Typical of these are 6uch materials as microporous polypropylene (Celgard ) and microporous acrylic polyvinyl chloride on woven nylon cloth (Acropor ). When no film is employed, or the film is highly permeable, the rate of sublimation is in part dependent on the available surface area of the gas generating material. In such instances, it is often desîrable to impregnate the material on a carrier so that a uniform surface is provided.
Alternatively, the film, 24, can divide the gas generating section, 28, into a first and second chamber, as shown in Figure II. The film may have a more limited gas permeability and one which is temperature dependent. Typical of these temperature dependent rate controlling films (RCF) are polyethylene, polypropylene, nylon, cellulose films and the like.
It can be shown mathematically that the contribution of the gas generation and the contribution of gas transport to the Qlo of the system are cumulative so that by judicious selection of the two sy~tems it is possible to achieve an overall effect in which the change in rate of gas availability at the wick with changes in temperature parallels the Qlo of the product being minotored.
Moreover, when a film of limited permeability is utilized, the effect of surface area of the gas generating material is elimin-ated since gas transport across the film is the rate controlling step.
The gas generation process and optionally also the Trade Marks -104~2~8 permeability throuyh the film are thus selected so that the change in rate of gas availability at the wick per unit change in temperature approximates the Qlo of the product being monitored.
The activation energy values of the operative components are useful in this selection since the relationship between Qlo and the activation energy is as follows:
(Equation 1) Q1o = elEa/Tl T2 R
where Ea = the activation energy Tl = a first temperature in degrees (absolute) T2 = a second temperature ten degrees lower than Tl and R = the gas constant Within, for example, the range of -10 to -20C, an important region for frozen foods, the following values are obtained:

Ea Qlo qlO Ea Qlo qlO
Kcal/mole Kcal/mole 0.1 1.001.00 20.0 4.54 2.31 5.0 1.461.23 22.0 5.28 2.52 8.0 1.831.40 25.0 6.63 2.86 2010.0 2.131.52 27.0 7.71 3.11 12.0 2.481.66 30.0 9.61 3.52 15.0 3.111.88 33.0 12.0 4.00 34.0 13.0 4.16 It is thus possible to select gas generating materials and films in which the rates of gas generation and permeability parallel the decomposition rates of various materials, even in the course of temperature fluctuation over a period of time.
The wick means can be selected from a wide variety of known materials. These may be simple cellulosic products such as paper or fiber, various synthetic polymeric materials, such as polypropylene, polyesters, or polyamides, glass fiber paper, alumina, silica gel and the like. The nature of the wick means is relatively unimportant, provided it possesses a sufficient affinity for the gas and indicator composition and is substantially inert to both.

104~Z88 The indicating composition which is deposited on the wick means and which results in a color change in the presence of gas can be a single component or a mixture of components operating together. The particular indicating composition must be selected for the particular ~as generated. When, for example, the gas generated is ammonia, the indicator composition can simply include an aqueous medium and a pH sensitive dye, such as methyl red or thymol blue, and an acidic substance of low volatility such as trichloroacetic, benzoic, oxalic or the like acid. Prior to absorption of any ammonia, the dye will display its first color which color will change as ammonia is absorbed.
Analogous systems are employed with acidic gases.
The indicating composition can alternatively use a redox system to produce the requisite color change. For example, the wick may be impregnated with a potassium permanganate solution.
In such an instance, the gas or vapor generated is one which is susceptible to oxidation, as for example, thymol or another oxidizable alcohol. As the thymol is absorbed on the wick and advances along its length, it is oxidized by the permanganate which in turn loses its characteristic red color.
It is also possible to utilize an indicator composition which, while not responding to the gas directly, converts it to a material which can be monitored. Thus, for example, in the case of maleic anhydride, the wick may be impregnated with an aqueous base of with an alcohol serving as a solvolysis agent.
As the anhydride is absorbed in the wick, it is hydrolyzed by the water or alcohol with the generation of maleic acid. This acid can then be monitored by incorporation in the composition of a pH sensitive dye.
The indicator composition can also complex the gas, as with potassium iodine and starch for iodine gas.
The following examples will serve to typify other systems and configurations but should not be construed as a limitation on the scope of the present invention, the invention being defined only by the appended claims.
Example l A time-temperature integrating indicator is prepared in a configuration similar to that shown in Figures l and 2.
The upper wall is a laminate of 2 mil polyethylene and l mil trifluorochloropolyethylene while the bottom wall is l mil aluminum foil laminated to l mil polyethylene. The gas permeable film is 2 mil polyethylene having an available area of l sq. inch.
The gas generating material is ammonium carbonate. The wick is Whatman No. l filter paper having a width of 0.5 inch. The indicator composition is .05 molar aqueous trichloroacetic acid, 20~ by volume glycerol and 0.1% methyl red.
Upon activation and equilibration, the ammonia generated by the ammonium carbonate migrated through the polyethylene film and produces a color change in the wick. At -18C, the front advances at a rate of .017 mm/hr. If the sensor is held at -1C, the front advances at a rate of 0.15 mm/hr. The change in the 0 rate with 10C increments corresponds to a Qlo of 3.7.
Example 2 An indicator is prepared as above utilizing, however, iodine as the gas generating material. The indicator composition consists of lO~ potassium iodine and 0.1~ starch. At -1C, the front advances at 0.033 mm/hr. while at 22C, the front advances at 0.15 mm/hr., corresponding to a Qlo of from 2.5 to 3Ø
Example 3 An indicator is prepared in a configuration similar to that shown in Figures 3 and 4, omitting however, the gas permeable film, 24. Paraformaldehyde is employed as the gas generating material. The indicator composition consists of l.l molar hydroxylamine hydrochloride, 0.8 molar sodium acetate and 0.1~ bromphenol blue and thymol blue. At -18C, the front advances at a rate of 0.065 mm/hr. while at 10C, the front advances at 0.12 mm/hr., corresponding to a Qlo of 1.5.
Example 4 An indicator is prepared in a configuration similar to that shown in Figures 3 and 4, omitting however, the gas permeable filmr 24. Thymol is utilized as the gas generating material. The wick is glass fiber paper which is impregnated with 0.01 molar potassium permanganate. A brownish yellow front advances along the initially red strip at a rate of 0.06 mm/hr.
at 21C and 0.0002 mm/hr. at -1C, corresponding to a Qlo of about S.
Example 5 An indicator is prepared as in Example 3. Maleic anhydride is employed as the gas generating material to give a Qlo of approximately 4. The indicator composition comprises O.lM
octadecanol, which hydrolyzes the anhydride, and a wide range pH
indicator such as lacmoid.
Example 6 An indicator is prepared as in Example 1, utilizing glacial acetic acid as the gas generating material. This is sealed below a 2 mil film of polyethylene as the gas permeable film, 24. The indicator composition comprises 0.1 molar sodium hydroxide, together with 0.1% thymol blue. The initially blue strip demonstrates a sharp yellow front advancing at a rate of 0.02 mm/hr. at -18C and 0.25 mm/hr. at 4.5C, corresponding to a Qlo of 3.1.

Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A temperature time integrating indicator comprising:
a sealed envelope having upper and lower walls, each of a gas impermeable material, the walls being sealed both about their periphery; a sealing means dividing said envelope into a first indicator section and a second gas generating section;
a gas generating material in the second section;
wick means extending from the first section to the second section, said wick means being the only means of gas communication between said sections;
an indicator composition deposited on said wick, said indicator composition producing a color change in the presence of the gas generated by said gas generating material.

2. A temperature time integrating indicator according to claim 1, wherein the second section of the envelope is divided into first and second compartments by a gas permeable film interposed between the upper and lower walls, said gas generating material is in the first compartment of the second section, and said wick means extends from the second compartment of the second section to the first section.

3. A temperature time integrating indicator according to claim 2, wherein the permeability of said film to gas is substantially temperature independent.

4. A temperature time integrating indicator according to claim 2 wherein the permeability of said film to gas is temperature dependent.

5. A temperature time integrating indicator according to claim 1 including a frangible shield means operable to isolate said gas generating material from said wick prior to use.

6. A temperature time integrating indicator according to claim 1 wherein said gas generating material generates an acidic gas.

7. A temperature time integrating indicator according to claim 1 wherein said gas generating material generates a basic gas.

8. A temperature time integrating indicator according to claim 7 wherein the gas generated is ammonia.

9. A temperature time integrating indicator according to claim 1, wherein the indicator composition complexes the gas generated.

10. A temperature time integrating indicator according to claim 1 wherein said gas is susceptible to chemical reduction and said indicator composition includes a redox system operable to reduce said gas.

11. A temperature time integrating indicator according to claim 1 wherein said gas is susceptible to chemical oxidation and said indicator composition includes a redox system operable to oxidize said gas.

12. A temperature time integrating indicator according to claim 1 wherein said gas is susceptible to solvolysis with the generation of an acidic material and said indicator composition includes a solvolysis agent operable to effect solvolysis of said gas.

13. A temperature time integrating indicator according to claim 1 wherein said gas is susceptible to solvolysis with the generation of a basic material and said indicator composition includes a solvolysis agent operable to effect solvolysis of said gas.

14. A temperature time integrating indicator according to claim 12 wherein said gas is a sublimable acid anhydride and said solvolysis agent is water or an alcohol.

15. The temperature time integrating device according to claim 1 wherein the envelope is divided into a first and second section by means of a transverse seal.

16. The temperature time integrating device according to claim 1 wherein the envelope is divided into a first and second section by means of a longitudinal seal about the wick.

17. The temperature time integrating device according to claim 1 wherein a quantifier composition is deposited on the wick.

18. The temperature time indicator according to claim 17 wherein the gas generating material generates a basic gas and the quantifier is tartaric acid, potassium acid phosphate, or cinnamic acid.

19. The temperature time indicator of claim 17 wherein the gas generating material generates an acidic gas and the quantifier is sodium hydroxide, quinine, or sodium carbonate.

20. The temperature time indicator of claim 18 wherein the basic gas is ammonia.

21. The temperature time indicator of claim 19 wherein the acidic gas is acetic acid.