U.S. patent number 3,946,611 [Application Number 05/469,851] was granted by the patent office on 1976-03-30 for time-temperature integrating indicator.
This patent grant is currently assigned to Bio-Medical Sciences, Inc.. Invention is credited to Raymond P. Larsson.
United States Patent |
3,946,611 |
Larsson |
March 30, 1976 |
Time-temperature integrating indicator
Abstract
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.
Inventors: |
Larsson; Raymond P. (Denville,
NJ) |
Assignee: |
Bio-Medical Sciences, Inc.
(Fairfield, NJ)
|
Family
ID: |
23865279 |
Appl.
No.: |
05/469,851 |
Filed: |
May 14, 1974 |
Current U.S.
Class: |
374/106;
374/E3.004; 116/216; 426/88 |
Current CPC
Class: |
G01K
3/04 (20130101) |
Current International
Class: |
G01K
3/04 (20060101); G01K 3/00 (20060101); G01K
011/12 () |
Field of
Search: |
;73/356,358,339R
;116/114.5,114 ;426/88 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Aegerter; Richard E.
Assistant Examiner: Little; Willis
Attorney, Agent or Firm: Lagani, Jr.; Anthony
Claims
What is claimed is:
1. A temperature time integrating indicator comprising:
a. a sealed envelope having upper and lower walls, each of a gas
impermeable material, the walls being sealed about their periphery,
a wick means interposed longitudinally between said walls, said
upper wall having a transverse seal at an intermediate position
thereof, said seal sealing said upper wall to the wick means and to
the lower wall in the area laterally adjacent said wick means
thereby defining first and second compartments within the envelope,
said compartments being interconnected at the wick means area only,
said first compartment being divided into a first and second
chamber by a gas permeable film interposed between said upper and
lower walls of said envelope;
b. a gas generating material in the first chamber of the first
compartment;
c. wick means extending from the second chamber of the first
compartment into the second compartment, said wick means being the
only means of gas communication across said cross-seal; and
d. 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 permeability of said film to gas is substantially
temperature independent.
3. A temperature time integrating indicator according to claim 1
wherein the permeability of said film to gas is temperature
dependent.
4. 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.
5. A temperature time integrating indicator according to claim 1
wherein said gas generating material generates a gas wherein said
gas is an acidic gas or a basic gas.
6. A temperature time integrating indicator according to claim 5
wherein the gas generated is ammonia.
7. A temperature time integrating indicator according to claim 1,
wherein the indicator composition complexes the gas generated.
8. 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.
9. A temperature time integrating indicator according to claim 1
wherein said gas is susceptible to solvolysis with the generation
of a material wherein said material is an acidic material or a
basic material and said indicator composition includes a solvolysis
agent operable to effect solvolysis of said gas.
10. A temperature time integrating indicator according to claim 9
wherein said gas is a sublimable acid anhydride and said solvolysis
agent is water or an alcohol.
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 7
wherein the gas generating material is ammonia carbonate.
13. A temperature time integrating indicator according to claim 1
wherein the gas permeable film is polypropylene.
14. A temperature time integrating indicator according to claim 6
wherein the gas generated is acetic acid.
15. A temperature time integrating indicator according to claim 1
wherein the gas impermeable material is a laminate of heat sealable
polyethylene and trifluoromonochloropolyethylene.
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. Pat. Nos.:
Nos. 1,917,048 Nos. 2,753,270 Nos. 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 2,850,393 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. Pat.
Nos.:
Nos. 2,560,537 Nos. 3,243,303 2,716,065 3,414,415 2,951,764
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 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
10.degree. increments and represented by the symbol "Q.sub.10 " for
the Celsius scale and "q.sub.10 " 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 temperature by 10.degree.
or 20.degree.C, 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. This U.S. Pat. No. 2,671,028 utilizes an enzyme such
as pepsin in indicator systems while U.S. Pat. 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 specific 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. Pat. No. 3,751,382. A more
sophisticated system is described in U.S. Pat. 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.
A further problem is that the change in rate of quality loss per
unit of temperature change differs for different products. Thus the
change in the rate of deterioration per unit of temperature change
for certain fruits and berries is vastly different from the change
in rate for lean meats. The values for dairy products are different
from both. For example, within the range of 0.degree. to
-20.degree.C, raw fatty meat and pre-cooked fatty meat have
Q.sub.10 's of about 3, whereas raw lean meat and pre-cooked lean
meat have Q.sub.10 's between 5 and 6. Vegetables generally have a
Q.sub.10 of between 7 and 8, whereas fruits and berries have a
Q.sub.10 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. Pat. 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. Pat. 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 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 microrganism 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:
FIG. 1 is a plan view of one embodiment of the present
embodiment;
FIG. 2 is a cross-section of the embodiment of FIG. 1 taken along
line 2--2 of FIG. 1;
FIG. 3 is a plan view of a further embodiment according to the
present invention; and
FIG. 4 is a cross-section of the embodiment shown in FIG. 3 taken
along line 4--4 of FIG. 3.
Referring now in greater detail to FIGS. 1 and 2, the
time-temperature integrating indicator includes a sealed envelope
shown generally at 11 which is constructed by sealing upper wall 12
to the lower wall 13 about their periphery 14. Upper wall 12 and
lower wall 13 are of like or different materials which are
substantially impermeable to gas, as described in greater detail
below. In the embodiment shown in FIG. 2, upper wall 12 is composed
of a transparent gas impermeable material such as a laminate of
heat sealable polyethylene and trifluoromonochloropolyethylene.
Other materials which are gas impermeable, such as coextruded
polyvinylidene chloride and polyethylene or a two layer film of
polyester and polyethylene can similarly be employed. Bottom wall
13, in the embodiment of FIGS. 1 and 2, can be a foil, such as
aluminum foil, a polymer or a laminate.
In addition to the seal about the periphery 14 of envelope 11,
upper wall 12 and lower wall 13 are sealed transversely at some
intermediate position by cross-seal 15. Cross-seal 15 divides the
envelope into first and second compartments, shown generally at 16
and 17, respectively. The position and configuration of the
cross-seal is relatively unimportant and can be varied widely, as
disclosed below. The upper and lower walls can be flexible or
rigid.
Extending longitudinally from first compartment 16 to second
compartment 17 of envelope 11 is wick means 18 which serves as the
only means of gas communication across cross-seal 15. Thus
cross-seal 15 must effectively prevent gas transport from first
compartment 16 to second compartment 17 other than through wick
means 18 within cross-seal 15.
Disposed within first compartment 16 of envelope 11 is a gas
generating material 19. A wide variety of different chemical
systems can be employed as the gas generating material and indeed
the wide latitude in selection of the gas generating material
contributes greatly to the versatility of the present device.
Optionally, first compartment 16 can be divided into a first
chamber 20 and a second chamber 21 by a gas permeable film 22. Film
22 may serve merely as a mechanical separator between gas
generating material 19 and wick means 18, in which case the
permeability of film 22 to gas should be substantially temperature
independent. Alternatively, and preferably, the permeability of
film 22 to gas is temperature dependent and this dependency thus
contributes to the response of the device in time-temperature
integration.
Gas generating material 19 can be isolated from wick means 18 prior
to use through incorporation of a frangible shield, as for example
by forming a loop of bottom wall 13 which is weakly bonded to
itself as at 23. A small amount of physical pressure behind seal
23, as for example at 24, results in the rupture of seal 23, and
the communication of gas generating material 19 with first chamber
20 of first compartment 16.
Upon rupture of seal 23, and after an initial induction period
during which the partial pressure of the gas rises in chamber 20,
the gas permeates across film 22 into second chamber 21 of first
compartment 16. The gas is then absorbed into wick 18, passing
through cross-seal 15 from first compartment 16 to second
compartment 17. The rate of gas generation by material 19 is a
function of temperature and the amount of gas which thus passes
through cross-seal 15 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 from cross-seal
15 will thus be a direct function of 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 19. 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 second compartment 17.
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.
Referring now to FIGS. 3 and 4, there is shown envelope 41 of an
alternative configuration. Upper wall 41 is sealed to lower wall 43
around its periphery 44 and the envelope is divided by cross-seal
45 into the first compartment 46 and second compartment 47. In this
instance, the cross-seal is disposed towards one end of the
envelope so that the indicating elements in compartment 47 are
remote from the sensing elements and a long channel is provided in
compartment 46. Wick means 48 extends from first compartment 46 to
second compartment 47 through cross-seal 45. Gas generating
material 49 is carried within first chamber 50 of first compartment
46 and separated from second chamber 51 by gas permeable film
52.
In lieu of a mechanical barrier, gas generating material 49 is
isolated from wick 48 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
mecanically or in the course of subjecting the particles to low
temperatures, gas generation begins. The gas passes through
permeable film 52 into second chamber 51 of first compartment 46
and then through cross-seal 45 by means of wick means 48.
As noted above, and in contrast to the device shown in FIGS. 1 and
2, wick means 48 are remote from the sensing portion of the device.
Gas, upon passage through permeable film 52, readily migrates
through the second chamber and is absorbed in that portion of wick
means 18 which extends past cross-seal 45 into first compartment
46. As a result of this arrangement, it is possible to place the
sensing portion of the indicator deep within the interior of the
product being monitored and at the same time place the indicating
portion of the device on the exterior surface for ease of reading.
This is in contrast to many of the known devices which can only be
affixed to the outside of the product and thus can monitor only the
surface temperature of the product. Prior art devices which have
attempted to monitor the inside of a product generally rely on
mechanical means such as a spring or a liquid carrying wick. The
former is subject to mechanical failure while the latter requires a
large volume of liquid.
In addition, the embodiment of FIGS. 3 and 4 utilizes a two
component upper wall. This upper wall 42 has a first portion 53 of
foil which is sealed to the second portion 54 of transparent, gas
impermeable polymer. This is desirable particularly for
applications which require measurements over a long period of time,
since the foil has a much lower gas permeability than most
polymers. The prevention of loss of gas by transport through either
the upper or lower wall is most important within the first
compartment of the envelope and this arrangement thus minimizes
this problem while providing a transparent window for inspection of
wick 48.
The embodiment of FIGS. 3 and 4 also includes an area 55 of
weakened wall cross-section. This permits the removal of the
indicating portion of the device by simply tearing the envelope
along the weakened wall portion. This separation immediately
terminates gas passage to wick means 48 and results in a permanent,
irreversible record of the time-temperature history of the
indicator up to the moment of separation. This permanent record can
thus be retained for various administrative purposes, or could for
example, be returned to the manufacturer or distributor to
substantiate a product complaint.
The gas generation component 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.
In those instances in which the rate of gas generation corresponds
to the rates being monitored, it is unnecessary to include the
barrier film and the first compartment of the envelope can have a
single chamber. Even in such embodiments however, it is often
desirable 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 is the generation of
gas. Typical of these are such 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 desirable to impregnate
the material on a carrier so that a uniform surface is
provided.
Alternatively the film which divides the first compartment into the
first and second chamber has a more limited gas permeability and
one which is temperature dependent. Typical of these 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 Q.sub.10 of
the system are cumulative so that by judicious selection of the two
systems 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 Q.sub.10 of the product being monitored.
Moreover, when a film of limited permeability is utilized, the
effect of surface area of the gas generating material is eliminated
since gas transport across the film is the rate controlling
step.
The gas generation process and optionally also the permeability
through the film are thus selected so that the change in rate of
gas availability at the wick per unit change in temperature
approximates the Q.sub.10 of the product being monitored. The
activation energy values of the operative components are useful in
this selection since the relationship between Q.sub.10 and the
activation energy is as follows:
where
E.sub.a = the activation energy
T.sub.1 = a first temperature in degrees (absolute)
T.sub.2 = a second temperature 10.degree. lower than T.sub.1
and
R = the gas constant
Within, for example, the range of -10.degree. to -20.degree.C, an
important region for frozen foods, the following values are
obtained: E.sub.a Q.sub.10 q.sub.10 E.sub.a Q.sub.10 q.sub.10
Kcal/mole Kcal/mole ______________________________________ 0.0 1.00
1.00 20.0 4.54 2.31 5.0 1.46 1.23 22.0 5.28 2.52 8.0 1.83 1.40 25.0
6.63 2.86 10.0 2.13 1.52 27.0 7.71 3.11 12.0 2.48 1.66 30.0 9.61
3.52 15.0 3.11 1.88 33.0 12.1 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.
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 gas 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 or 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 1
A time-temperature integrating indicator is prepared in a
configuration similar to that shown in FIGS. 1 and 2. The upper
wall is a laminate of 2 mil polyethylene and 1 mil
trifluorochloropolyethylene while the bottom wall is 1 mil aluminum
foil laminated to 1 mil polyethylene. The gas permeable film is 2
mil polyethylene having an available area of 1 sq. inch. The gas
generating material is ammonium carbonate. The wick is Whatman No.
1 filter paper having a width of 0.5 inch. The indicator
composition is 0.05 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 -18.degree.C, the front
advances at a rate of 0.017 mm/hr. If the sensor is held at
-1.degree.C, the front advances at a rate of 0.15 mm/hr. The change
in the rate with 10.degree.C increments corresponds to a Q.sub.10
of 3.7.
EXAMPLE 2
An indicator is prepared as above utilizing however iodine as the
gas generating material. The indicator composition consists of 10%
potassium iodine and 0.1% starch. At -1.degree.C, the front
advances at 0.033 mm/hr while at 22.degree.C, the front advances at
0.15 mm/hr, corresponding to a Q.sub.10 of from 2.5 to 3.0.
EXAMPLE 3
An indicator is prepared in a configuration similar to that shown
in FIGS. 3 and 4 omitting however the film. Paraformaldehyde is
employed as the gas generating material. The indicator composition
consists of 1.1 molar hydroxylamine hydrochloride, 0.8 molar sodium
acetate and 0.1% bromphenol blue and thymol blue. At -18.degree.C,
the front advances at a rate of 0.065 mm/hr while at 10.degree.C,
the front advances at 0.12 mm/hr, corresponding to a Q.sub.10 of
1.5.
EXAMPLE 4
An indicator is prepared in a configuration similar to that shown
in FIGS. 3 and 4, omitting however the film. 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 21.degree.C and 0.0002 mm/hr at -1.degree.C,
corresponding to a Q.sub.10 of about 5.
EXAMPLE 5
An indicator is prepared as in Example 3. Maleic anhydride is
employed as the gas generating material to give a Q.sub.10 of
approximately 4. The indicator composition comprises 0.1M
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. 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 -18.degree.C and 0.25 mm/hr at 4.5.degree.C,
corresponding to a Q.sub.10 of 3.1.
EXAMPLE 7
An indicator is prepared as deposited in Example 1. Prior to
sealing the upper and lower walls, an opaque polyethylene mask is
interposed between the wick and upper wall. The mask has one
opening over that portion of the wick in compartment 16 which
opening bears the indicia "Active if red". A series of openings are
regularly spaced over that portion of the wick in compartment 17
with appropriate indicia associated with each opening. This
provides an initial signal to show activation and converts the
indicator read-out to a digital system.
* * * * *