CA1158063A - Time-temperature indication - Google Patents

Abstract

TIME-TEMPERATURE INDICATOR

ABSTRACT
This invention relates to time-temperature indication apparatus. The shelf life in many products, including food, drugs, and photographic and X ray film is determined by the ambient temperature to which the product is exposed. Time-temperature indication by chemical means is known; however, such indicators are played over a relatively narrow temperature range or tend to have only two operable states. In any event, they do not accur-ately follow the actual shelf life of such products which tends to be a continuous function of ambient temperature and time. The disclosed invention accomplishes time-temperature indication by generating an electrical current having an amplitude-temperature relationship thus proportional to the reciprocal of the time-temperature relationship characteristic of a substance to be monitored. The current is integrated and an indication is given when the integrated current reaches a predetermined value, to signal a change in the characteristic of the substance being monitored. The disclosed indicator includes a galvanic cell having a cell component that is consumed as electrical current is drawn from the ceil. In effect, the consumable component serves as an integrator of the current drawn from the cell which is made dependent upon temperature by an electrical circuit. The indicator is actuated when a predetermined percentage of the component is consumed.

Description

TIME-TEMPERATURE INDICATION

BACKGROUND OF THE INVENTION
-This Tnventlon relates to tlme-temperature indlcatlon and~ more partlcularly, to the development of a tTme-temperature indicatlon by electrlcal means.
The shelf llfe of many products~ Includlng food, drugs, and fllm tphotographlc and ~-ray~ Ts determlned by the ambient temperature to which the product is exposed. Generally, ~o the hlgher the temperature, the lower the shelf Itfe. Thus, tlme-temperature indlcatlon by chemlcal means Is known. An Indicator matertal Is separated from a reactant materlal for ~he indicator by a dlffusion barrler. Passage of the reactant through the dlffuslon barrier depends upon the amblent temper-ature. In one such tlme-temperature indlcator, dlsclosed In U.S.
Patent No. 3,967,579, the reactant has a clearly definad msltlng polnt at a preselected tempera~ure; when the amblent temperature rlses above the preselected value, the reactant melts and begins to flow through tha d1ffuslon barrier to the Indlcator. In another such tlme-temperature Indlcator, whlch Is dtsclosed In U.S.Patent 4,057,029, the diffuslon barrler changes from an Impermeable state to a permeabla state at a clearly defIned preselected temperature. In both the descrlbed davices, the rate of passage of the reactant above the preselected temperature is essenttally independent of the amblent temperature, T.e., below the preselected temperature there Is no passage of reactant, and above the presetected temperature there Ts passage at an almost constant rate. But, the shelf lIfe of many products is a continuous, usually, an Inverse exponential, function of ambTent temperature.
As the ambient temperature rises, the shelf life gradually de-creases. Thus, the descrlbed ttme-temperature Indicators do not adequately account for how much hlgher the amblent tempera~
ture is than the preselected temperature.
Another time-temperature Indlcator Is disclosed In U.S.
Patent No. 2,782,749, and Is based on the use of temperature sensitlve materlals havlng non-specific meltTng polnts~, such as fats and waxes, whlch gradually become less vlscous as temperature increases. These temperature sensltive matèrlals are formed as a layer on a porous paper barrTer and at or above 11~8063

-2 a predetermtned teMporature, the fat or wax beglns to dlffuse through the paper barrler and eventually reaches a polnt of vlslblllty on the opposlte slde of the barrier, The rate of dlffuslon of the materlal through the barrler is dlrectly related to fhe temperature. Devices of thls type, due to the nature of fats and waxes, have a relatlvely narrow temp~rature range at which they can be used and It Is dlfflcult to control the dlffuslon rate through the paper barrler. In addltlon, the paper barrier must b~ relatlvely thlck, on IO the order of 0.75 Inchas~ 50 that the devlce Is relatlvely bulky, thus givlng rtse to handling and storage problems when attached to packages or contalners.
SUMMARY OF THE INVENTION
___ The present Inventlon accompllshes tlme-temperature Indlcatlon by electrlcal means, namely, by generating an electrical current havlng an amplltude-temperature relatlon-ship that Is proportlonal to the reclprocal of the tlme-temperature relatlonshlp of a characteristlc of a substance to be monltored. The current Is integrated to represent the generatsd charge. An indicatlon Is glven when the generated charge reaches a predetermlned value, to signal a;change In the characteristic of the substance belng monltored. By way of e*ample, the change In the characterlstlc could represent spollage of the substance being monitored; in such case the tlMe-temperature relatlonshtp represents shelf life.
A feature of the inventlon Is a time temperature indlcator comprising a galvanlc cell having a polr of output termlnals and a cell component that Is consumad as electrical current Is drawn from the cell. A current-controillng external circult Is connected between the termlnals of the cell for llmlting the current flow through the cell, at any Instant, to an amplitude whlch bears a predetermlned relatlon-ship to the temperature of the product. Pr~erably, thl;s relationship Ts exponentlal so that the current at~any Instant Is dlrectly proportlonal to the rate of deterloratlon of the product. In effect, the consumed component of the cell serves as an Integrator of the currsnt drawn from the cell, whlch Is dependsnt upon~the temperature.~ An Indlcator is actuated when a~predetermined perc~ntage~of the~component 0~3

-3-is consumed. Prs~erably, the consumed component of the cell is one of Its electrodes, the Indlcator comprlsss a window covering one slde of the consumable electrode anti a patterned mask coverlng portions of ths other slde of the consumable electroda. When the portlons of the electrode not covered by the mask are completely consumed, the electrolyte becomes v7slble through the wlndow and a pre-selected not7ce ts dlsplayed.
Thus, the Invention permlts a contlnuous time tempera-ture measurement to be made that preclsaly matches therelatlonshlp between product characteristlcs, such as shelf llfe, and temperature. It is contemplated that tlme-tempera ture indlcators embodylng the Inventlon can be mass produced at very low cost by using exlsting integrated clrcuit iechnology and related manufacturlng techniques.
In accordance wlth one broad aspect, the invention Is related to an apparatus for Indlcatlng the deterloration of a packaged perlshable product that deterlorates at a rate approxlmately exponentlally related to the temperature of the product, the apparatus b~lng adapted to be attached to the package In which sald product ls conta1ned. The apparatus includes a substantially planar, !~abel-llke, galvanic cell hav1ng a consumable electrode,~an electrolyte, and o patterned mask posltioned between said electrode and the electrolyte and protecting a predetermlned portion of sald electrode agalnst galvanlc eroslon to prov1de a predetermlned visual Indicatlon when the tlm~ In~egral of the Instantaneous~cur-rents passecl through sald cell amounts to a predetermined coulombic total, and circuit means including a semlconductor device having a temperature dependent current characterlstic connected to sa1d cell for controil1ng the through-put~current In a manner such that the Instantaneous current amplltude at any glven time ls a direct and substantial;ly expônentlal ~ ;
function of the temperature at such tlme, with sald function substantlaliy~corresponding to the t1me rate of deterlora~
tion as a function of temperature characterlstlc of the~
perishable product to which the apparatus ls to be~attached,~
whereby said through-put current consumes that portlon~of the consumable eiectrode not profected by~said patterned mask, .

, ' ' 11~8063 exposing the electrolyte to view ~hrough said electrode and providing a visual indication corresponding to the pattern of said mask.
BRIEF DESCRIPTION OF T~IE DRAWINGS
The features of specific embodiments o the best mode contemplated of carrying out the invention are illustrated in the drawings, in which:
Figure 1 is a schematic circuit diagram of a time-temperature indicator incorporating the principles o the invention;
Figure 2 is a side sectional view of a product package including a time-temperature indicator incorporating the principles of the invention;
Figures 3A, 3B, and 3C are graphs depicting the current, temperature, and time relationships upon which the invention is based; and Figure 4, appearing on the same drawing sheet as Figure 1, is a schematic circuit diagram of an alternative version of a voltage reerence source, which permits variation o the relationship between the current generated by the cir-cuit of FiguTe 1 and the ambient temperature.
DETAILED DESCRIPTION OF THE
SPECIFIC EM~ODIMENTS
In Figure 1, a so-called Leclanche cell 10, has a consumable zinc anode, a carbon cathode, an ammonium chloride electrolyte, a positiva terminal connec-ted to the carbon electrode, and a negative terminal connected to the zinc electrode. The zinc electrode has a known mass, and therefore a known number of zinc atoms available for ionization, i.e., charge cnpaclty (the charge capa-city of one gram of zinc is 0.81 ampere-hours). The positive output terminal of cell 10 is directly connected by a normally open, manuaIly operable switch 11 to the collector of an NPN transistor 12. The negative output terminal of cell 10 lS directly connected to the emitter of transistor 12. Transistor 12 is completely untemperature compensated, and its collector-to-emitter current, ~, .

and thus the current drawn from cell 10, is a function of temperature. An operational amplifier 13 has an output connected to the base o~ transistor 12.
A battery 14, which could also comprlse a Leclanche c~ll, or other type of galvanic cell, has output terminals connected to -4a-a63 the power supply inputs to operatlonal ampllfier 13. The charge capacity of battery 14 is large enough to supply power untll the zinc alectrode of cell 10 Is consumed. If battery 14 is a Leclanche cell, its components Tncludlng the zlnc electrode are sufficiently large as not to be consumed before the zinc electrode of cell 10. A reslstor 15 Is connected betwean the negatlve Input of operatlonal ampllfier 13 and the nega-tlve output terminal of battery 14. A source o~ posittve voltage 16, whlch Ts a reference voltage havlng a very constant magnitude for all temperature and load condltlons, Is connected between the positlve Input of operatlonal ampllfier 13 and the negatlve output termlnal of battery 14~
An adjustable feedback resistor i7 ts connected from the output of operational ampllfler 13 to Its negatlve Input. After swltch 11 Is closedJ current Is drawn from cell 10 through translstor 12. The constant reference voltage from source 16 Is amplified by operational ampllfler 13, which serves to present a very low, almost zero, source Impedance to translstor 12.
By virtue of the constant base-to-emltter blas across transis-tor 12, the current drawn from cell 10 Is a function of;theambient temperature and, more partlcularly, a dlrect exponential functlon thereof. When current is drawn from cell l0, its zinc electrode is consumed. As the ambtent temperature ~in-creases, the current drawn from c811 1O Increases and the time Interval unttl the inc electrode Is Cnsumed decreases. Thus, the zlnc electrode of ceil lO functlons as a coulombmeter, Integratlng the temperature dependent current drawn from cell 10. When a certatn percentage of the zlnc electrode is consumed, an Indlcatlon Is given, for example, tn the manner~
described below In connectlon wlth Ftg. 2. `~
In one embodlment, translstor l2 Is a 2N3904 type translstor; operatlonal ampllfler 13 Including source 16 1s a monollthlc llnear Integrated clrcuit (Natlonal;Semi-conductor LM10/LM10~ (L) ~LMIOC~L) Op Amp and Voltage Reference whereln the output termlnals of battery 14 are connectèd to plns 7 and

4, respectlvely, the Junction of reststors 15 and 17 ts~con-nected to pln 8j and the base of transistor 12 ts connected to ptn l); source 16 Is 200 mill1volts; battery 14 !5 1.4~volts, reslstor 15 Is 100,000 ohms, and resistor 17 vartes between 80~3 ~, 180,000 and 270,000 ohms.
In Fig. 2, translstor 12, operatlonal amplifler 13, source 16, and reslstors 15 and 17 are Incorporated Into and integrated as indlcated by the dashed-llne box 20 In Flgure 1.
Sheets of transparent fllm 21 and 22, of a suitable dlelectrlc materlal such as ~ ~r, of the order of I mil thick have contlguous peripheral edges that are Jolned together by a heat seal 23 to form a completely closed envelope for the time-temperature Indicator.
In additlon to Integrated clrcuit chlp 20, thls envelope contains a carbon electrode 24, a zinc electrode 25, an electrolyte 27, a separator 28, an Indlcator 29, and switch 11.
Sheet 22 is secured by adheslve or the llke to a package 30 for a product 31, such as food, photographlc or x-ray film, or drugs. In one embodlment, electrode 24 comprlses a graphite-loadsd, vtnyl sheet in the order of 3 mils thTck, electrode 25 comprises a sheet of pure zinc I mil thick, electrolyte 27 comprisss a black pasty mlxture of manganese dloxide, zlnc chlorids, ammonlum chlorlde, and water in the order of 40 mils thlck, separator 28 comprlses a layer of porous paper in the order of I mil thlck, and Indicator 29 is a patterned mask or resist of materlal impervlous to electrolyte 27, such as waterproof ink, ln the order of 0.5 mlls thick. Thus, the assembly takes the physlcal form of a substantlally planar label-llke member that can be attached to a perlshable product or its container. Electrode 24 Is secured to the Inside of shset 22, electrode 25 is se-cured to the Inside of sheet 21, and electrolyte 27 fllls the space between the electrodes. Indlcator 29 Is coated on the surface of electrode 25 opposite shset 21. Indicator 29 Is a patterned mask in the sense that It forms an ou~line or negative image of letters such as '~SPOILED'~ CONTAMlNATED", "REJECT", or other symbols communlcatlng that ths shelf llfe of product 31 has been exceeded. As current Is;drawn by translstor 12 (FiG. I), the portions of electrode 25 not~
covered by Indlcator mask 29 are consumed, leavlng the masked portions thersof intact. The portion of sheet 21 contiguous to electrode 25 serves as a window. Since electrode 25 Ts between this wlndow and the electrolyte, it blocks the elec-~P~qJe )~ark l 1~8~63 --7--trolyte from view untll Its unmasked portlons are completely consumed. ~hen the unmasked portions of electrode 25 are consumed, the electrolyte is expossd to vlew through the electrode 25 to provlde a visual Indicatlon, for example a set of l~tters, correspondlng to the pattern of these unm~skad portlons. The background or contrast for th~se letters or othar vlsual indlcation Is the zTnc rematning at the masked portions of electrode 25. It wlll be understood, of course, that the area of -the unmasked portion of electrode 25 Is designed to encompass a quantlty of metal correspondlng to that quantlty whlch will be completely eroded by a through-put current 7ntegral corresponding to a pradeterml~e~ degree of aging or deterioratlon of the perishable product. Thus, thsre is provided a predatermlned vlsual dlsplay to indlcate that the time Integral of the product s aging rate has reached a predetermlned value.
It Is Important that electrode 25 have unlform thlckness so all the letters outlined by the patterned layer become vlslble at about the same tlme. Integrated ctrcuit chip 20 lies adJacent to electrode 25 and Is electrically 1solated therefrom by an insulator 32, Separator 28 envelopes electrolyte 27, elactrode 25, Indlcator 29, and Integrated clrcuit chip 20 for the purpose of preventlng Internal short circuits wlthln the cell. Swltch ll comprlses a U-shaped strip 33 of perman-ently deformable, electrically lsolattve material~lnside the closed envelope. Flat electrical conductors 34 and 35 are bonded on the Inslde, I.e., the faclng surfaces of the legs of strip 33. Switch lI ls closed by pressing the right side of sheets 21 and 22, as viewed tn Ftg. 2, togethsr to permanently deform strip 33, leavi~ng conductors 34 and 35 In contact with each other. Alternatively, swttch 11 could be located outside the closed envelope and connected to the clrcultry by lead in wlres passlng through heat seal 23. Bat~
tery 14 could a!so be located Inside or outstde thé closed envelope, in the latter case connected to the circuttry by lead in wlres passing through the heat seal. Wh~ile the electrlcal connectlons between integrated circult chlp 20, electrode 24, elsctrode 25, battsry 14, and swltch ll~are not shown in Fig. 2 " t wlll be understood tnat a palr of : ~

Interconnectlng wires extend rospectively from conductor 34 to the collector of translstor 12, withln Integrated clrcuit 20, and from conductor 35 to carbon electrode 24. Further, It will be understood that the assembly, shown In cross~
section in Fig. 2, Is to be otherwlse wired to Implement circult Interconnectlons conslstent wlth Fig. 1. Electrode 25, electrode 24, and electrolyte 27 comprlse cell 10 ~Fig. 1).
Electrode 25 has a known mass of unmasked zlnc so that It contalns a known number of zlnc atoms available for lonizatlon.
As a result~ electrode 25 is consumed after a correspondTng known electrical charge has been drawn from cell 10 through translstor 12 (Fig. 1).
The graph of Flg, 3A depicts a typical shelf life, t.e., time interval in days, as a functton of amblent tempera-'S ture In degrGes centlgrade. For equal Increments of Increas-lng temperature, the shelf life Is reduced by a constant factor, e.g.~ for each 10C increase In amblent temperature the shelf llfe is halved. In general, the relatlonshlp between time and temperature of a product can be expressed by the equation -kT
where T IS the tlme interval, i.e., shelf llfe, T Ts ambient temperature, A is coefficient, and k~is the constant represent-Ing the constant factor of reductlon, I.e., the decay in shelf llfe.
The graph of Fig. 38 deplcts the current In milliamperes drawn from cell 10 as a function of amblent temperature in degrees centlgrade. Thls graph Is the reciprocal of the graph of Fig. 3A. In other words, the time tempsrature Indicator Is deslgned so for equal incremen~s of Increasing~
temperature, current drawn from cell 10 Increases by the same factor as the shelf Itfe decreases. Thus, If for each 10C Increase In ambient temperature, the shelf~lTfe Is halved, the current drawn ~rom cell 10 doubles. ~In general, the relatlonship between the current drawn from cell~10 ond ~emperature can be express~d by th~ equatlon I = BekT
where I Is the current, 8 is a coeffIcient, and T and k are as deflned abovs. Comparison of equations (I) and (2) set `

1 15~06~

forth above tliustrates that the relatlonshlp between current and temperature is proportional to the reclprocal of the relatlonship between tlme and temperature, the constant of proportionality belng dstermined by the coefflclents A and 8.
In summary, thus, transistor 12 controls the current through cell 10 so that thls current varles as ar, axponentlal functlon, i = 8enT~ of the ambtent tamperature and In dlrect proportion to the deterloration rate of the product 31.
The graph of Fig. 3C depicts the tlme Interval In days requlred to consume the zlnc alectrode of cell 10 as a functlon of the current drawn from cell 10 In mllllamperes. It can be seen that as the current increases with amblent temperature, the tlme interval decreases to slmulate the tlme-temperature relationshlp of the product characterTstlc. The valu~ of coefflcient 8 Is glven by tho equatlon B Q e -kTR t3) where TR Is room temperaturT, Q Is the charge capacity of the ~Inc electrode~ T Ts the shelf llfe of the product at room temperaturej and k Is, as defined above, the decay In 5half life, In the clrcuit of Fig. 1, coefflclent B is set by varying resîstor 17. In the example shown graphi-cally in flgs. 3A, 3B) and 3C, it is assumed that k i5 0.0693 and Q Is 0.024 ampere hours.
When source 16 prov7des a constant reference voltage of 200 millivolfs and translstor 12 ts a 2N3904, k Is ap proxlmately 0.0693, i.e., for each 10C increment of tempera-ture increase the current doubles. Other values of ~ can be provlded by substltutlng for source 16 In Flg. l, the source of reference voltage sho~n In Fig. 4, whlch is a conventlonal band gap reference clrcuit. A transistor 40 has a collector connected to the posltive termlnal of battery l4~by a~reslstor 46 and an emitter directly connected to the negative~termlnai of battery 14. A transistor 41 has a collector connected by a reslstor 42 to the collector of translstor 40~and an emitter connected by a resTstor 43 to the negat~ive terminal of battery 14. The collector of transistor 4i Is~also di~rectly connectèd to the base of translstor 40. A transistor 44 has a collector connected by a resistor 45 to the collector of franslstor 40 and an emltter directly connected to the negatlve .

`
. .

1 158~63 --10-- ..
terminal of battery 14. The bases of transistor 41 and 4 are directly connected together. A voltage divider is formed by resistors 47 and 48 connected In serles between the collector and emit~er of translstor 40. An output termlnal

5 49 at the Junctlon of reslstors 47 and 48 Is connected to the posltlve Input of oporatlonal ampli~ler 13. Th0 relatlonshlp between the voltage between the collector and emltter o~
translstor 40 and the temperature depends upon the resistance ratlo of resistors 42 and 43. Thls voltage Is nominally 1.2 volts. Thls voltage Is reduced to the reference voltage of nominally 200 mllilvolts by the described vol-tage dlvider.
For example, If the resistance ratlo Is a glven value, thc reference voltage Is constant, i.e., Ind~pendent of temperature;
If the reslstance ratlo is less than the given value, the reference voltage varies Inversely wlth amblent temperature;
and If the resistance ratto is greater than the glven value, the reference voltage varies dlrectly wlth ambient temperature.
Thus, by provldlng a resistance ratio that Is larger than the glven value, k can be Increased, I.e.j the exponentlal rlse of current drawn from cell iO can be Increased, and by provTdlng a reslstance ratio sma!ler than the glven valuej k can be decreased to match the tlm0 temperature characteristics of the partlcular product In a reclprocal manner.
The described embodiments of the Inventlon are con-sldered to be preferred and Illustratlve of the Inventlve concept; the scope of the Inventlon is not to be restricted to such embodlments. ~arlous and numerous other arrangements may be dbvlsed by one skilled In the art without departing from the spirit and scope of this Invention. For example, other consumable components of the battery, such as the cathode or the electrolyte, may be used to measure charge, although the anode of tne zlnc-carbon battery Is preferred because of the preclsion charge measurement that is attainable.
Further, other types of galvanic cells, elther primary or secondary, dry or wet, could be employed to measure charge.
Instead of a transistor, a thermlstor or other temperature responstve device could be used as the temperature dependent current drain. The inventlon also contemplates the use of other types of coulombmeters In place of a galvanlc cell, although the --I, use of a galvanlc cell In fhe manner dcscrlbcd advantageously serves both to m~asurs and Indlcat~ charge by vlrtu~ of the unmasklng of the electrolyto when thc zlnc alcctrode is cvnsumed. Usa of a consumabJc component, partlcularly an el~ctrod~ of a prlmary cell as th~ charge measurlng devlce, Is also advantacicous because of its simpllcity and eas~ of mass productlon.

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Claims (13)

-12-

1. An apparatus for indicating the deterioration of a packaged perishable product that deteriorates at a rate approximately exponentially related to the temperature of the product, the apparatus being adapted to be attached to the package in which said product is contained and comprising:
(a) a substantially planar, label-like, galvanic cell having a consumable electrode, an electrolyte, and a patterned mask positioned between said electrode and the electrolyte and protecting a predetermined portion of said electrode against galvanic erosion to provide a predetermined visual indication when the time integral of the instantaneous currents passed through said cell amounts to a predetermined coulombic total, and (b) circuit means including a semiconductor device having a temperature dependent current characteristic connected to said cell for controlling the through-put current in a manner such that the instantaneous current amplitude at any given time is a direct and substantially exponential function of the temperature at such time, with said function substantially corresponding to the time rate of deterioration as a function of temperature characteristic of the perishable product to which the apparatus is to be attached, whereby said through-put current consumes that portion of the consumable electrode not protected by said patterned mask, exposing the electrolyte to view through said electrode and providing a visual indication corresponding to the pattern of said mask.

2. The apparatus as claimed in Claim 1 in which said circuit 2 means comprises:
(a) a transistor having its collector and emitter electrodes connected in a series circuit with said galvanic cell having a current drain characteristic such that the collector current is a substantially exponential function of temperature over the temperature range to which the perishable product is likely to be exposed, and (b) bias-circuit means connected between the emitter 10 and base electrodes for applying a forward control voltage which varies as a preselected function of temperature for adjusting said current drain characteristic to substantially correspond to the deterioration-rate-as-a-function-of-temperature characteristic of said packaged product where the deterioration rate at each temperature is the reciprocal of the shelf life of the product at that same temperature.

3. The apparatus as claimed in Claim 1 in which said circuit means comprises:
(a) a transistor having its collector and emitter electrodes connected in a series circuit with said galvanic ceil and having a current drain characteristic such that the collector current is a substantially exponential function of temperature over the temperature range to which the perishable product is likely to be exposed, and (b) bias-circuit means for applying a substantially constant forward bias voltage to the base-emitter circuit of said transistor of a magnitude such that said current characteristic is adjusted to substantially correspond to the deterioration rate characteristic, as a function of temperature, of said packaged product.

4. The apparatus as claimed in Claim 2 wherein said bias-circuit means applies a forward bias voltage to the base electrode of said transistor, with said bias voltage varying as a function of temperature sufficiently for causing the collector-current versus-temperature characteristic to substantially correspond to the deterioration rate-as-a-function-of-temperature characteristic of the perishable product.

5. The apparatus as claimed in Claim 2 wherein said bias-circuit means applies a forward bias voltage to the base of said transistor, with said bias voltage varying as a predetermined function of temperature selected for causing the collector current to vary as a function of temperature in substantially the same manner that the deterioration rate of the perishable product varies as a function of temperature.

6. The apparatus as claimed in any one of Claims 2 through 4 in which said galvanic cell provides the electromotive force for energizing the emitter-to-collector circuit of said transistor.

7. The apparatus as claimed in Claim 2 in which selected portions of the consumable electrode are separated from the electrolyte by a layer of electrolyte-resistive material disposed on the consumable electrode in a predetermined pattern such that upon complete erosion of the unmasked portions of the consumable electrode the electrolyte becomes visible through said unmasked portions to provide a visual indication corresponding to said predetermined pattern.

8. The apparatus as claimed in claims 1 or 2 wherein the several elements of the apparatus are constructed and arranged to form a planar assembly having a thickness of about 0.05 inches or less and with the length and width both being at least an order of magnitude greater than the thickness.

9. The apparatus of claim 2 wherein the galvanic cell functions simultan-eously as a current integrator, an indicating pattern display mechanism, and a power source for energizing the collector-to-emitter circuit of said transistor.

10. The apparatus of claim 5 in which said bias voltage varies as an increa-sing function of temperature.

11. The apparatus of claim 5 in which said bias voltage varies as a decrea-sing function of temperature.

12. The apparatus of either claim 1 or claim 2 in which one electrode of the galvanic cell is zinc, the other electrode is carbon, and the electrolyte includes manganese dioxide, zinc chloride and ammonium chloride.

13. The apparatus as claimed in claim 5 in which said galvanic cell provides the electromotive force for energizing the emitter-to-collector circuit of said transistor.