Sensor used for electrical heating measurement

A sensor used for electrical heating measurement comprises a sensor element having a rod, plural through holes provided straightly through the sensor in the longitudinal direction and plural metal thin wires inserted into and passed through the through holes and an electrically insulating member covering the sensor element, preferably the metal thin wires thinner than through holes are used, spaces around the metal thin wires are filled with ceramics powder and the powder is sintered by heating at lower temperature than the sintering temperature thereof.

BACKGROUND OF THE INVENTION 
This invention relates to a sensor for measuring properties of many kinds 
of fluid by the socalled electrical heating measurement method. 
The term "fluid" as used in the present specification means all kinds of 
fluid including a gaseous substance, a liquid substance or a solid 
substance such as powder or of two or more of these substances as well as 
a fluid of the type whose phase changes with time. 
The inventors of this application have already proposed, in Japanese Patent 
Laid-open Application No. 62(1987)-56849, a sensor as follows. 
A sensor for measurement by electrical heating has a core rod covered with 
an electrically insulating member, a thin metal wire wound about the core, 
and an electrically insulating member covering the thin metal wire. 
In the above-mentioned traditional sensor, the thin metal wire is several 
times longer than the sensor since the thin metal wire is wound around the 
core rod. With this, the sensor has some advantageous such that the 
electric resistance increases according to the length of wire and 
therefore a large heat flux perunit length of the sensor can be obtained 
by a small electric current. Further, the sensor in itself is not easily 
cut, broken or bent. 
On the other hand, since the thin metal wire is would around the core rod 
and spirally configured, a stress-strain occurs in the thin metal wire. 
And so there is a disadvantage such that, when an annealing treatment is 
performed in order to prevent the stressstrain thereof, the electric 
resistance thereof is largely changed from the initial value to some value 
which varies significantly with time. 
the change of electric resistance of each sensor is different from other 
sensors and so it is difficult to determine the specific value common to 
each sensor. 
Therefore, it is difficult to produce a sensor having a desirable electric 
resistance both before and after annealing. thus, the sensors are not 
interchangeable since each sensor has different electric resistance and so 
a relational equation between the electric resistance and properties of 
materials must be determined with each sensor. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a sensor having 
no stress-strain in the thin metal wire and where the electric resistance 
value is constant with time after an annealing treatment. 
The object of the present invention can be attained by a sensor used for 
electrical heating measurement, comprising a sensor element having a rod, 
a plurality of holes (apertures) provided longitudinally through the 
sensor and a plurality of thin metal wires inserted into and through the 
through holes, and an electrically insulating member covering the sensor 
element. 
According to the present invention, a stress-strain in a thin metal wire 
does not occur which is different from the traditional sensor formed by 
winding a thin metal wire around a core rod. Therefore, interchangeable 
sensors having a desired and predictable electric resistance value can be 
obtained. Further, in case of production on a large scale, each sensor 
will have a stable electric resistance value and so no adjustment for 
individual sensors is necessary. 
Further, according to another aspect of the present invention, in addition 
to the above-mentioned technical means, there is provided a sensor for use 
in electrical heating measurement wherein the thin metal wires are thinner 
than the through holes and are inserted into and through the through 
holes. Spaced between the thin metal wires and the through holes is 
disposed a ceramic powder and the powder is sintered by heating at a 
temperature which is lower than the sintering temperature thereof. 
In accordance with the second technical means, since in case of volumetric 
expansion with heating, the thin metal wire is not pressed against the 
inside of the holes in the rod, and the electric resistance of the sensor 
cannot be influenced by the stress-strain in the thin metal wire. 
Furthermore, when two or more thin metal wires are inserted into holes in 
the rod in the longitudinal direction and are connected in series, the 
electric resistance of the wires increases to a corresponding extent, i.e. 
the same extent as the traditional sensor with wound wire, and a large 
heat flux per unit length of the sensor can be obtained by a small 
electric current.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a sensor according to the present invention, which is formed 
by covering a sensor element 10 with a cover 40. 
Firstly, the sensor element 10 will be described hereinafter. 
A rod 12, as shown in FIG. 2, has ten through holes 18 for thin metal 
wires, which holdes pass entirely through the rod in the longitudinal 
direction and are equally spaced with respect to each other along the 
circumference of the rod. Further, four through holes 22 are provided 
adjacent to the center portion (axis) of the rod and inside the 
disposition of the through holes 18 for the thin metal wires. A plurality 
of straight thin metal wires 20 are inserted into and pass through the 
through holes 18. 
The thin metal wires 20 are bent in a U-shape at the back end 16 and a pair 
of end portions of each thin metal wire is inserted into and pass through 
a pair of through holes 18 adjacent to each other from the back end 
surface 16 to the front end surface 14 of the rod 12. In the example in 
FIG. 2, five U-shape wires 20 are inserted into and pass through each pair 
of through holes 18 for the wires, and at the front end surface 14 an end 
portion of one wire is properly connected, e.g. by the spot welding, to an 
end portion of other wire adjacent to the above end portion. With this, 
wires 20 are connected in series and, thus, at the front end surface 14 
only one resistance differential is formed between an end portion 20A and 
an end portion 20B. 
On the other hand, two straight lead wires 24, 24 are inserted into and 
pass through the through holes 22 for lead wires 24. The lead wires 24 are 
also bent in U-shape in the same way as the above-mentioned method of the 
thin metal wire 20 and a pair of end portions of each lead wire are 
inserted into and pass through a pair of through holes 22 adjacent to each 
other, but, from the front end surface 14 to the back end surface 16 of 
the rod 12 which is opposite to the insertion of the thin metal wire 20. 
At the front end surface 14 the end portions 20A, 20B of the thin metal 
wires 20 are connected with contact points 26, 28 of the two lead wires 
24, 24, e.g. by spot welding. 
With this, the two lead wires 24, 24 are connected with the contact points 
26 and 28 whereby the electric resistance of the thin metal wire 20 may be 
measured by the known four terminal method. For example, the thermal 
change of an atmosphere in which the sensor is disposed can be determined 
by connecting a current source and a voltmeter with the lead wires, 
adequately electrifying the thin metal wires 20, simultaneously measuring 
a voltage between the contact points 26 and 28 and calculating the 
electric resistance of the thin metal wires 20. 
As shown in FIG. 4, the thin metal wire 20 and the lead wire 24 are of 
slightly smaller diameter than that of the through holes 18 and the lead 
wire holes 22. Ceramics powder 30 are filled into the space in the through 
holes 18 and the lead wire holes 22. A glass sealing 32 is provided to 
both the front end surface 14 and the back end surface 16 of the rod 12 in 
order to prevent leakage of the ceramics powder 30. 
The filled ceramics powder 30 is sintered at a low temperature. In this 
specification, the "low temperature" means a temperature lower than the 
sintering temperature of the ceramics. 
With this, the ceramics powder 30 sintered at a low temperature can prevent 
an eccentricity of the thin metal wire 20. Further, when a volumetric 
expansion of the thin metal wire 20 occurs with heat, since the sintering 
state is easily destroyed, the thin metal wire 20 is not pressed against 
the inside of holes 18 and therefore no stress-strain occurs in the thin 
metal wire 20. 
In the sensor element 10 of the above-mentioned example, the rod 12 is a 
column having a 1.4mm diameter and 100mm length and made of a ceramics 
having a high purity (more than 99.9%) of a sintered crystallized alumina. 
The thin metal wire 20 is a platinum wire having a 0.110mm diameter and 
the lead wire 22 is a platinum wire having a 0.15mm diameter. Holes 18 
having a 0.16mm diameter are provided along the rod 12, and lead wire 
holes 22 having 0.16mm diameter are provided in the center of the rod 12. 
The rod 12 of ceramics provides high workability and high strength and the 
rod, therefore, is not changed in quality or deformed. Further, the 
coefficient of volumetric expansion thereof is more or less the same as 
that of platinum. The thin metal wire 20 is made of platinum and provides 
stability of the electric resistance thereof. 
The electric resistance of such a sensor is 10 .OMEGA., the platinum wire 
having 0.110mm diameter is 10 .OMEGA./m and therefore the desired value is 
determined according to these values. In case of a production on a large 
scale, the error of electric resistance of each sensor element 10 is about 
.+-.0.1% and the electric resistance has high stability both with time and 
with a heating-cooling procedure. 
Further, the lead wire 24 is inserted into the center of rod 10 so that the 
lead wire 24 is not directly exposed to the temperature imposed on sensor 
element 10 in order to prevent a heat outflow from the lead wire 24. With 
this the heat of the thin metal wire 20 will not radiate out through the 
lead wire 24. 
Next, a cover 40 covering the sensor element 10 will be described. As shown 
from FIG. 5, short pipes 44, 46 and a pipe 48 are provided to a back end 
portion of a pipe 42 having a slightly larger internal diameter than the 
sensor element 10. 
These pipes are fixed uniformly by calking from the circumference thereof. 
For example, stainless steel (SUS 316L), platinum, palladium and titanium 
are used as a pipe material; however, the pipe material is optionally 
determined in accordance with intended conditions. 
After inserting the sensor element 10 into the pipe 42 adjacent to the 
front end thereof, a vacuum is applied from the front end of the pipe 42A 
and a resin is filled into the cover 40 from the back end 42B thereof for 
electrically insulating it from open air. 
Ceramics powder, e.g. magnesium oxide powder and so on can be filled into 
the cover 40 in place of resin. 
As described, the lead wire 24 can be connected to a cable 50 which is 
secured to cover 40 and a spring means 52 may be provided for protecting 
the cable 50 so as to obtain a sensor having excellent durability. 
The cover 40 described above is only one example of that which may be used 
as an electrically insulating cover for the sensor element 10 and is not a 
limitation on the construction of the present sensor, with the exception 
that the cover must be an electrically insulating cover for the sensor 
element 10. 
As clarified from the above description, the sensor according to the 
present invention can be applied for use as a resistance temperature 
sensor to measure the atmosphere in which the sensor is disposed by 
electrifying the thin metal wire, simultaneously measuring the voltage 
applied to the thin metal wire to obtain the change of the electric 
resistance. 
As another application, two sensors according to the present invention are 
disposed in the fluid and one is used as a heat build-up element and 
another is used as a resistanct temperature sensor for measuring thermal 
conductivity so as to determine many of the properties of the fluid 
according to these values. Further, the present invention can be applied 
in any field as desired. 
While there has been described what is at present considered to be 
preferred embodiment of the invention, it will be understood that various 
modifications may be made therein, and it is intended to cover in the 
appended claims all such modifications as fall within the true spirit and 
scope of the invention.