Clinical thermometer

A clinical thermometer registers the maximum temperature reached and uses as the thermometric fluid a non-toxic gallium/indium alloy which adheres to the walls of the thermometer measuring tube, the adhesive force being greater than the internal cohesive force of the thermometric liquid. A Gallium/indium/tin eutectic alloy is particularly useful in such applications and others.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a clinical thermometer with a maximum 
function and a eutectic mixture suited to it. 
2. Description of the Related Art 
In order to achieve the maximum function, conventional thermometers of this 
kind are filled with mercury and show a constriction between the bulb 
exposed to the temperature to be measured and the reading or measuring 
tube, which has the effect that in the cooling down process the mercury 
thread that entered the measuring tube separates. 
Mercury is extremely poisonous and for health and environmental reasons is 
therefore increasingly met with disapproval. 
Furthermore, in making the thermometer, an additional production step is 
necessary in order to create the constriction which requires a certain 
amount of precision so that the necessary inner cross-section is achieved 
which has to be small to facilitate the separation of the mercury thread 
upon a drop in temperature after having reached the maximum temperature, 
but not so small as to interfere with the reuniting of the mercury upon 
shaking down. 
In accordance with US-PS 3,872,729 it was suggested to do without the 
problematic constriction and to coat the inside of the measuring tube to 
assure the necessary adhesion forces. The production of such a measuring 
tube is, however, very expensive and usually not very practical. 
Furthermore, it is necessary also with this thermometer to rely on toxic 
mercury. 
So far, all attempts to produce a mercury-free clinical thermometer failed 
first of all because of the necessary maximum function. Similarly 
unsuitable are also the thermometers according to DE-PS 453 184, DE-PS 454 
213 and GB-PS 246 843, which provide for the use of gallium with and 
without indium. Such thermometers have the disadvantage that their 
measuring liquid solidifies at low temperatures which may under certain 
circumstances result in the thermometer shattering. 
Similar problems also arise when using a measuring liquid according to 
SU-PS 279 108. 
The object of the present invention is therefore to specify the details of 
a thermometer with a maximum function which is simple to use, easy to 
produce and is harmless from the health and environmental standpoint. 
SUMMARY OF THE INVENTION 
According to the invention the solution to the given problems is found by 
using a eutectic alloy containing gallium in a concentration of 65-95 
wt.-%, indium in a concentration of 5-22 wt.-% and tin in a concentration 
of 0-11 wt.-%, if necessary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The use of a eutectic gallium alloy as measuring liquid is of special 
significance because of its non-toxic characteristic. Furthermore, this 
liquid has a low melting point and a high vaporization point so that the 
necessary requirements of a clinical thermometer are met. 
In this range of composition the liquid state of aggregation of the liquid 
extends from approx. -15.degree. C. to more than +1800.degree. C. under 
normal conditions. Gallium, indium and, if necessary, tin are preferential 
alloy elements because they lead to a particularly low eutectic point. 
Furthermore, this alloy is capable of conducting electricity so that it is 
also suitable for an embodiment as contact thermometer. 
In a further embodiment of the present invention according to claim 2 the 
eutectic alloy may contain up to 2 wt.-% bismuth and up to 2 wt.-% 
antimony. Antimony raises the oxidation resistance while bismuth 
positively affects the fluidity of the alloy. Furthermore, antimony and 
bismuth are, like tin, readily available and inexpensive substances, while 
gallium and indium are expensive. A content of more than 2 wt.-% pf one of 
the two additional elements Sb and Bi leads to a noticeable and 
undesirable increase in the melting point. 
In order to keep the liquid used for measuring and reading in the position 
of maximum wetting of the measuring tube, the adhesion forces inside the 
measuring tube must be greater than the cohesion forces active in the 
liquid. This is achieved, among others, by a water coat around the surface 
area of the measuring tube in contact with the liquid. Preferably this 
water coat is smaller than the permanent water coat which is normally on 
the surface of the measuring tube, so that a thin gallium oxide layer is 
formed by reaction of gallium in the liquid with the water, which deposits 
on the surface of the measuring tube, increasing the adhesion forces 
inside the measuring tube. In this respect, one can do without the 
conventional constriction which simplifies not only the production but 
also the handling of the thermometer when deliberately returning the 
measuring thread. 
One of the possible embodiments of the thermometer according to the 
invention has the measuring tube made of glass. It may show a non-circular 
preferably flat-oval to crescent-shaped cross-section. These 
characteristics serve to increase the adhesion forces between the 
measuring liquid and the measuring tube and therefore to guarantee when 
temperature decreases that the liquid thread is kept in place in the 
measuring tube which is necessary in order to achieve the desired maximum 
function. 
In a preferred embodiment, the actual measuring tube is connected to the 
bulb by an area which has an opening with a preferably circular 
cross-section in order to reduce the adhesion forces there. This results 
in a high operational reliability of the maximum function. 
Thermometers with a measuring tube having a flat or crescent-shaped 
cross-section are known. This cross-sectional form has, however, so far 
been applied in the widening and improved readability of the mercury 
thread and not in increasing the adhesion. 
In further embodiments, the outer wall of the clinical thermometer, e.g. 
the glass housing enclosing the tube, may be provided with a grip element 
which is formed by several glass mass areas melted onto the housing to 
improve the grip of the thermometer. Preferably said melted-on glass mass 
areas are arranged in the form of one or more circumferential rings around 
the housing. When using colored glass mass or glass paint a marking 
function, e.g. of the measuring range, or a reference that the clinical 
thermometer is filled with a non-toxic liquid can also be obtained. 
Surprisingly it was found that a preferred eutectic mixture which contains 
68-69 wt.-% gallium, 21-22 wt.-% indium and 9.5-10.5 wt.-% tin may be used 
for various other applications due to its special characteristics. Such a 
eutectic mixture is e.g. suitable as a lubricant especially for vacuum, 
high-vacuum and ultra-high vacuum applications. 
The eutectic mixture according to the invention should, if possible, only 
have a small degree of impurity such as lead or zinc of less than 0.001 
wt.-% , preferably less than 0.0001 wt.-%. 
Said new eutectic mixture is mainly characterized by its low melting point 
of approx. -19.5.degree. C. under normal pressure and atmospheric 
conditions. Furthermore, the vaporization point is above 1800.degree. C. 
According to the drawing, glass housing 10 encloses a bulb 11 with a 
measuring tube 13 arranged above the transition area 12. Bulb 11 is filled 
with said eutectic gallium alloy 14, which, as shown in FIG. 1, has risen 
inside measuring tube 13 to a certain height as a function of the 
temperature increase. The measuring tube area of the housing is provided 
with a scale 15. 
Measuring tube 13 has, as shown in FIG. 2, a oval to nearly crescent-shaped 
cross-section to increase the adhesion between liquid 14 and measuring 
tube 13. Transition area 12 has an opening with a preferably circular 
cross-section to reduce the adhesion forces in this area ensuring the 
desired maximum function. 
In the vicinity of the end opposing bulb 11 ten small irregular glass mass 
areas 16 are melted onto the outside of housing 10 forming one or more 
circumferential ring(s) around housing 10. Theses ring(s) does not only 
improve grip in this area of housing 10 but may also be used for colour 
coding.