Note: Descriptions are shown in the official language in which they were submitted.
F R E E Z E / T H A W I N D I C A T O R
105Z633
BACKGROUND OF THE INVENTION
Field of_the Invention
This invention relates to indicators for evidencing
when frozen foods in freezer compartments have been subject
to temperatures above a previously selected temperature for
a sufficient time to result in deterioration of the food and
relates, in particular, to indicators for use in frost-free-
type refrigerators and freezers having heating devices which
periodically heat the walls of the freezer compartment for
a length of time sufficient to remove ice and frost from the
walls.
Prior Art
The preservation of food during storage has long
been accomplished by means of refrigeration which keeps the
food in a frozen cond.~tion so that degradation by enzymes
and bacteria is prevented. Those experienced in the science
of refrigeration have found that frozen food must be main-
tained near 0F. and should not rise above 5F. for any
extended period of storage or enzymatic or bacterial action
or both can take place with the result that changes in taste
and the development of toxic substances can occur. In many
cases, power failure or improper operation of equipment have
~' ~ 30 resulted in the temperature rising above 5F. for extended
periods or even complete thawing has taken place. Sub-
sequent resumption of power re-freezes the food and in many
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1~)5;~33
cases the thawing cycle escapes detection. It is, there~ore,
not known that the food may be in a dangerous condition for
human consumption.
A number of devices have been invented to detect
whether a thawing and subsequent freezing cycle have taken
place. Most of these devices have made use of irrev,ersible
damage caused to emulsions, gels, frangible partitions,
capsules,or of the deformation of cast geometrical shapes of
ice. These types of inventions, being irreversible, can
only be used once. Consequently, every time the refrigera-
tor is defrosted,they must be replaced. ~hey will also
suffer permanent damage if subjected to freezing during
storage or shipment. A lesser number of devices have been
lnvented which are re-usable and in some cases would operate
as described in convent~onal refrigerators. However, most
modern refrigerators are of the frost-free-type which
operate on the principle that, twice a day, they pass
through a heating cycle to remove accumulated surface frost.
During the course of this defrosting cycle the temperature
in the refrigeration compartment rises between 40F. and
45F. and the elapsed time for the equipment to change from
0F. up to over 40F. and back down again to 0F. takes
about 1 1/2 hours.
None of the pre-described devices which claim to
be usable in temperature ranges of less than 0F. up to
32F., are usable in frost-free refrigerators unless they
are replaced or re-set twice a day, as they are activated
during each frost-free heat cycle. Consequently, it would
be impossible, under normal usage of the refrigerator, to
determine whether such inventions had been set off by
105;~33
improper operation ~f the equipment or whether they had been
caused to function during the heating cycle. It is, there-
fore, the object of the present invention to provide a
means of determining whether frost-free refrigeration equip-
ment will maintain a pre-selected temperature between 0F.
and 32F. during the periods between the frost-free heating
cycles. The present device is not activated during the
heating cycle and is re-usable so that it may be re-used in
case of shut-down for repairs or if changing refrigerators.
The present invention is based on both a unique engineering
concept and a very unique harmless chemical solution which
provides for successful operation under exposure to the
heat cycle in frost-free equipment.
Super-cooling of liquids in freeze/thaw detectors
i8 a ma~or problem as the well known methods of starting
crystallization, such as stirring, shaking, seeding, etc.,
are not practical in such devices and even the addition of
substances, such as silver iodide can be ineffective. Glycols,
alcohols (eg. benzyl alcohol), o-dichlorobenzene for example,
can be kept for weeks at temperatures lower than their publ.shed
freezing points without solidifying. Tests have indicated that
aqueous solutions of glycols, sucrose, sorbitol, mannitol,
alcohol, glycerol, sodium chloride, calcium chloride, magnesium
chloride, acetic acid, and ammonium chloride formed slushes
- when frozen and were difficult, even with extensive freezing,
to form a homogeneous solid which did not have a portion of
`~ liquid liquor within its mass. A solution of ammonia in water
was found to freeze sharply and solidly, but it was found that
it, along with all the above chemicals and solutions, melted
readily when subjected to even one heat cycle of frost-free
refrigerator. It was also found, that when such solutions
:105'~33
were adjusted to ~reeze between 0F. and 26F., that the
solutions that had the lower freezing points were much
quicker to melt during the frost-free cycle. Samples of
vegetable oils and fats were selected which froze sharply
between temperatùres of 0F. and 14F. but were found to melt
more readily in the frost-free heat cycle than many of the
chemicals tested. Aqueous glycol solutions and vegetable
oils were placed in separate vials so that the vials were
half-filled. These vials were hal~ imbedded in 2 1/2 inch
cubes of foamed urethane insùlation and frozen. When these
insulated samples were exposed to the heat cyclè of a frost-
free refrigerator, all samples were readily melted.
SUMMARY OF THE INVENTION
The present invention provides an indicator which
uses the liquid which can be frozen completely solid at
12.5F. and will not melt when a pre-determined quantity is
subject to repeated exposure to heat cycles of the conventional
frost-free refrigerator.
The indicator of the present invention comprises a
float which is immersed and frozen in place in a liquid and
which only rises when the liquid thaws after being subject
to temperatures in excessive of a pre-determined time.
A detailed description following, related to the
drawings, gives exemplification of apparatus according to
the invention which, however, is capable or expression in
means other than those particularly described and illustrated.
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105'~33
D ES CR I PT I ON OF TH E DRAW INGS
Fig. 1 is a cross-sectional view of one embodiment of the
invention shown in an activating position,
Fig. 2 is a perspective view of a cylindrical design of
float, also shown in Fig. 1,
Fig. 3 is one form of cap and float depressor, also
shown in Fig. 1,
Fig. 4 is another cross-sectional view of the device shown
in Fig. 1,
Fig. 5 is a pers'pective view of another embodiment of the
indicator.
20 DETAILED DESCRIPTION
Referring to Fig. 1, which shows one embodiment, 1 the
invention has a cylindrical container la with an open top end
and an exaggerated taper on the lower half. This container
is preferrably made of a transparent flexible plastic, such
as polypropylene, but may be made of any other suitable
transparent material such as glass. The numerical reference 2
(also shown in Fig. 2), designates a cylindricalfloat 2 which
fits loosely in the container la. This float 2 is of thin-
walled construction and has abouyancy component 3 consisting
of preferrably a light-weight foamed plastic disc. The walls
~05'~f~33
o~ the component float have holes 4 located just above the
buoyant disc 3 and also about half-way up the walls of th~
float cylinder. This float is composed preferrably of a light-
weight plastic material such as polyethylene and is colored
preferrably bright red, although some other color may be used.
The buoyancy of the float is so designed that the upper
edge of the float will rise to the top of the outer container
when the float is immersed in a liquid 4.5 used in the device.
This liquid (subsequently described) is used to fill the
outer container in such a manner that when the float is
depressed to the bottom of the container la(as shown in Fig. l)
the liquid is slightly above the upper lip of the cylindrical
float. A Cap 5, with an attached depressor vane 6, can be
snapped on the top of the container la in two positions
(as shown in Fig8. 1 and 4). In Fig. 1 it is shown holding
down the float and in Fig. 4 it is shown projecting upwards,
outside of the container. This cap is composed preferrably
of polyethlene but any other noncorrosive suitable material
may be used. An outer mask 7 covers the lower half of the
container so that the liqu~d and depressed float are hidden
; from view. The upper edges of this mask are arranged to extendslightly above the edges of the liquid. The mask may consist of
any covenient material, such as paint, foil or a label. A
base 8 is attached to the lower end of the outer container to
provide stability for standing the container in an upright
position.
The liquid which is an aqueous solution of urea in
the proportion by weight of not less than ohe part urea ana
~ two parts water freezes sharply at 12.5F. without any proble~s
encountered with super-cooling.
1~5;~633
A solution of urea, prepared by dissol~ing 50 grams
of urea in 100 grams of water, will freeze sharply at 12.5 F.
without any problems being encountered with super-cooling.
It was found that increasing the amount of urea to 60 grams,
or more, to 100 grams of water did not lower the freezing point
any further. These stronger solutions still freeze sharply at
12.5 F. Such solutions of urea freeze completely solid at
12.5 F. and will not melt when a 25 ml. sample is subjected
to repeated exposures to the heat cycles of frost-free refrig-
erators. Such solutions are also:
(1) odourless
(2) non-toxic
(3) of sharply defined melting point when frozen
(4) free of super cooling effects
(5) stable during storage
(6) economical and easy to manufacture
(7) of high specific heat in the solid frozen state
(8) of high heat of fusion when in the solid frozen state
(9) of low conductivity of heat
(10) of high solubility of the solute
(11) of very high negative heat of solution of the solute
(12) non-reactive with components of the device
(13) very buoyant
(1~) sufficiently high in solubility of solute at low
temperatures to maintain required temperatures as a
result of the effects of negative heat of solution
(15) not melted when subjected to the heating/defrosting
cycle of a frost-free refrigerator, but do melt if
the temperature between heating cycles is sustained
at an unsafe temperature (for example 5 F.).
16)5'~t;33
Small amounts of magnesium chloride can be added
to the pre-described urea solution so that the freezing point
can be adjusted to a preferred 6 F. and that such frozen
solution, tested with 25 ml. volumes, will not liquify when
subjected to continuous exposure to successive frost-free
heat cycles. The adjustment for freezing point is not limited
to 6 F. but can be set between 0 F. and 32 F. 6 F. is
preferred as it is slightly above the 5 F. required for safe
food storage. The magnesium chloride can be added to solutions
if the freezing point is desired to be lower than 12.5 F.
Three examples of suitable solutions are as follows:
Example 1. 60 grams of urea are dissolved in 100 ml.
of water. This solution solidifies at 12.5 F.
Example 2. 10 grams of magnesium chloride (MgC12
6H2O) and 60 grams of urea are dissolved in 100 ml.
of water. This solidifies sharply at +10 F.
Example 3. 14 grams of magnesium chloride (MgCl
- 6H2~) and 60 grams of urea are dissolved in 100 ml.
of water. This solution solidifies sharply at +6 F.
It is also preferable to add a suitable non-toxic
odourless, taint-free preservative such as alkyl trimethylam-
monium bromide, which is effective in amounts of 0.15 percent
to 0.3 percent of the total weight of the solution.
The urea solution described behaves as follows when
subjected to temperatures near the desired 0 F. to 5 F.
The pure urea solution (Example 1) shows a slight
`~05'~:;33
separation of crystals of urea, due to reduced solubility,
at 23 F. These crystals seed the solution and prevent
super-cooling when the solution reaches its freezing point
of 12.5 F. Large masses of interlocking crystals of urea
are thrown out of solution, due to reduced solubility as
the temperature is reduced to the freezing point of the re-
maining solution. This mass of urea crystals and frozen
urea solution become firmly locked into place, within and
around the float, rendering the float immobile. The spec-
ific heat and heat of fusion of the mass remains high. When
the temperature in the freezing compartment rises to over
40 F. during the heat cycle of defrosting, this frozen mass
does not melt. Although urea is very soluble, its rate of
solution is slow, particularly at lower temperatures. Urea
crystals absorb large quantitites of heat when dissolving in
water (they are endothermic). Consequently, as the tempera-
ture is rapidly rising in the freezing compartment during
defrosting, the precipitated urea crystals cause the mixture
to be "self-cooling" and prevents a rapid rise in temperature
within the frozen mass. Consequently, the float 2 is pre-
- vented from rising within the container as it can not lift
the solid mass above the surface. However, if the tempera-
tures within the freezer compartment does not return to a
sustained 12.5 F. (using Example 1 urea solution) shortly
after defrosting has occurred the frozen mass of urea will
melt and the float will rise into view, indicating a mal-
function. In similar fashion, for example, the urea-magnesium
chloride solution (Example 3) will indicate whether the
freezer temperatuxe has not been maintained at 6 F. or
lower between defrosting cycles.
It is preferred that the volume of solution used
_ g _
iOS'~33
in the present invention should be approximately 25 ml. to
30 ml. but is not intended to be restricted to such volumes.
The present invention is used in the following
manner. Referring to Fig. 1, cap 5 with attached depressor
vane 6 is snapped onto the top of the outer container 1
with the depressor vane resting on the upper edges of the
float 2, holding the float beneath the solution as shown in
Fig. 1. The device is then placed in a vertical position
in the freezer compartment and cooled until the enclosed
liquid has solidified. Solidification is obvious as the
liquid becomes opaque white in colour, hiding the float from
view even when viewed from an upper angle through the clear
upper walls of the cylinder. The liquid freezes solid in
about three hours at 0 F. At this time the float is firmly `
locked in the submerged position by an interlocking mass of
crystals and frozen solution. The cap is then removed, in-
verted and replaced with the depressor vane 6 extended up-
ward outside the container as shown in Fig. 4. The device
is then ready to indicate any undesirable rise in tempera-
ture above the pre-selected range between 0 F. and 32 F.,
other than the designed rise in temperature during the de-
frosting cycles.
In the event of malfunction of the equipment or of
power failure, when the temperature rises above the desired
level (preferably +5 F.) during the period between the de-
frosting cycles, the solution will melt and the coloured
float will rise, exposing itself to view through the trans-
parent upper portion of the device, indicating that the stored
food has been subjected to unsafe storage conditions. If
power resumes and re-frèezing occurs, the brightly coloured
-- 10 --
105'~ 3
exposed float remains in its elevated position indicating
that a thawing and re-freezing cycle has occurred. If a
partial condition of thawing occurs between the period of
deErosting cycles, the float will only rise part of the way,
as its movement upward is retarded by masses of crystals and
partly frozen liquid, both of which are trapped inside the
float, and the buoyant force on the float is insufficient
to lift the partially melted mass above the surface of the
melted portion of the liquid. Consequently, this device can
give a quantitative evaluation of the dearee of thawing that
has taken place. The mask 7 completely hides the color of
the float, if by chance~ the float touches the walls of the
container while being frozen.
Fig. 5 shows another embodiment 20 which has a
float 21 and cap 22. A matching outer square container cor-
responding to container la is not shown. The cap 22 in this
case is not required to be inverted during use. An indicator
mark 23 located on the cap 22 is rotated 90 degrees to change
the position of depressor legs 24 which are used in similar
fashion to the depressor vane, described in Fig. 3. These
depressor legs, in the position illustrated, impinge on lugs
25 holding the float submerged while the solution is being
frozen. Once frozen, the cap 22 is rotated 90 degrees so that
the legs will clear the lugs on the float, allowing the float
to rise.
