Disposable sensor

Two lead wires for each, connected to opposite ends of a heat generating element are secured to a holder. These lead wires form four-point-terminals at their other ends and thereby a disposable sensor is provided. A current source and a voltmeter are connected to the terminal and changes in various properties of fluid are determined on the basis of a value of current supplied from the current source and a value of voltage measured by the voltmeter.

BACKGROUND OF THE INVENTION 
The present invention relates to a disposable sensor used to measure 
properties, conditions and the other factors of fluid. 
In many industrial and scientific fields, it is important to know 
properties of fluid such as specific gravity and coefficient of viscosity 
as well as conditions thereof such as temperature, flowing velocity and 
flow rate, and sensors of various types have already been proposed to 
measure such factors. 
The inventors disclosed, for example, in Japanese Disclosure Gazette No. 
1985-152943, a method for measuring changes in properties of liquid 
substance or semisolid substance, in which suitable metallic wire is 
supplied with electric current so that a temperature of the substance as 
well as a temperature of the metallic wire may be maintained at a 
predetermined value, simultaneously, a value of such electric current is 
measured and therefrom a heat transfer coefficient on the surface of the 
metallic wire is calculated to determine changes in the properties. 
The inventors also disclosed, in Japanese Disclosure Gazette No. 
1987-185146, a method for measuring conditions of fluid, in which 
temperatures of one or more sensor elements, a temperature of the fluid 
and differential temperatures between the respective sensor elements and 
the fluid are comparatively measured to determine conditions of the fluid. 
This Disclosure Gazette describes a stainless rod carrying platinum wire 
wound therearound and then coated with teflon, as a sensor used to perform 
this method. 
It is also known, for example, from, Japanese Disclosure Gazette No. 
1986-210959 to form a passage into which a liquid sample is introduced for 
detection of chemical ingredients thereof within this passage or for the 
like purposes and to assemble the passage and elements contained therein 
such as a detecting sensor and a pump into a disposable cartridge. 
For properties, conditions and the other factors to be accurately measured, 
the surface of the sensor brought into contact with fluid to be measured 
must be free from any contamination. 
However, in the well-known art as mentioned above, if said fluid to be 
measured comprises gelable substance, blood, detrimental substance for 
human body or the like, it will be difficult for such substance to be 
completely removed or washed away from the sensor when the sensor is not 
reused to prevent danger, or even when it is reused, even though the 
sensor is coated with teflon, and such inadequate washing will prevent the 
subsequent measurement from being accurately accomplished. 
The sensor of said cartridge type contains therein various elements such as 
the pump and correspondingly, the cartridge itself becomes considerably 
expensive. With a consequence, the sensor of this type has usually been 
washed after every measurement and reused several times. Here again, 
however, washing of the sensor is difficult and troublesome, and the 
inadequate washing will often adversely affect the subsequent measurement. 
Thus, such sensor of cartridge type has left the problems of prior art 
unsolved. 
Furthermore, a sensor for measurement of properties of fluid was generally 
expensive because of its complex mechanism, and the sensor was not 
suitable for disposable, essentially. The method of the present invention 
simplifies the structure of the sensor, and therefore, a practical 
disposable sensor can be constructed. 
SUMMARY OF THE INVENTION 
A principal object of the present invention is, in view of the drawbacks of 
the prior art as mentioned above, to provide an inexpensive disposable 
sensor used to measure properties, conditions and the other factors of 
fluid. 
Such object is achieved, in accordance with the prevent invention, by a 
disposable sensor utilizing so-called hot wire method to measure changes 
in various properties of fluid, said disposable sensor comprising two lead 
wires for each connected to opposite ends of a stretched heat generating 
element and secured to a holder so as to form a four-point-terminal, a 
current source and a voltmeter electrically connected to said terminal so 
that said changes may be determined from a value of current supplied from 
said current source and a value of voltage indicated by said voltmeter. 
A method to measure changes in various properties of fluid utilizing the 
disposable sensor constructed as mentioned just above will be briefly 
described. The sensor is placed in a container or the like filled with a 
fluid sample to be measured so that the heat generating element comes into 
contact with said fluid. Then, the heat generating element is supplied 
with electrical current through the lead wires for heat generation, 
causing heat transfer between the heat generating element and the fluid by 
conduction and convection while the current source and the voltmeter are 
electrically connected to the terminal defined by the lead wires secured 
to the holder to supply the heat generating element with electrical 
current of a predetermined level via the lead wires and to measure the 
voltage applied to the heat generating element by the voltmeter, and 
changes in said heat transfer relating to the changes in various 
properties of fluid can be measured. 
It should be understood that the sensor once brought into contact with the 
fluid and used for measurement may be disposed and replaced by a fresh 
sensor. 
Such disposable sensor of the present invention has advantageous effects as 
following: 
According to the present invention when for example, changes in properties 
of fluid is measured, the sensor provided separately of a measuring unit 
proper is disposable, and thus, it is inexpensive and an accurate 
measurement can be achieved. Moreover, not only a requirement for 
troublesome operation of sensor washing after every measurement is 
eliminated, but also a possibility that an operator might be exposed to 
toxicity of measured fluid is minimized. 
Especially, it is important in practice that the measurement is performed 
with a sensor always free from contamination and, therefore, neither a 
measurement error due to such contamination on the sensor surface nor 
contamination of the fluid due to contamination of the sensor surface 
occurs. 
Furthermore, there is no requirement for recovery of the sensor after every 
measurement, so the sensor of the present invention can be used for the 
fluid of special types, for example, as cast iron within the blast 
furnace. 
The structure of the sensor itself simply comprising platinum wire or the 
like serving as the heat generating element, the lead wires and the holder 
for said lead wires can be easily manufactured at a cost reasonably low to 
provide it as a disposable sensor. 
By constructing the sensor as a sufficiently small sensor having a 
correspondingly shorter heat generating element, it will be also possible 
to measure properties and the other factors of fluid at any location 
within this fluid.

DETAILED DESCRIPTION OF THE INVENTION 
The disposable sensor of the present invention utilizes the fact that 
changes in various value of properties of fluid is definitely related 
one-to-one to changes in heat transfer coefficient. 
A coefficient of heat transfer .alpha. on the surface of the heat 
generating element fixed in the fluid is expressed by an equation 
EQU .alpha.=Q/S (.theta..sub.s -.theta..sub..infin.) (1) 
where 
Q: a calorific power of the heat generating element, 
S: a surface area of the heat generating element, 
.theta..sub.s : a surface temperature of the heat generating element, and 
.theta..sub..infin. : a temperature of the fluid. 
As the inventors disclosed in Japanese Disclosure Gazette No. 1988-132149, 
the surface temperature .theta..sub.s of the heat generating element is 
related to an average temperature .theta..sub.w of the heat generating 
element and the temperature .theta..sub..infin. of the fluid as expressed 
by a following equation: 
EQU .theta..sub.s =.theta..sub..infin. +K.sub.1 (.theta..sub.w 
-.theta..sub..infin.).sup.k.sbsp.2 (2) 
where k.sub.1, k.sub.2 : constants specific to individual heat generating 
elements. 
The calorific power Q radiated from the surface of the heat generating 
element, the surface area S of the heat generating element, the average 
temperature .theta..sub.w or the heat generating element and the 
temperature .theta..sub..infin. of the fluid is directly measured 
according to the common method, and then, a coefficient of heat transfer 
.alpha. can be actually measured. 
Now a construction of the disposable sensor according to the present 
invention will be described in details. FIG. 1 shows a first embodiment of 
the disposable sensor constructed in accordance with the present 
invention. The disposable sensor 10 includes a holder 14 made of 
insulating material through which voltage lead wire sections 12a, 12c and 
current lead wire sections 12b, 12d extend. The voltage lead wire section 
12a and the current lead wire section 12b form together a single lead wire 
curved in U-shape provided at its upper portion slightly projecting from 
an upper surface 14U of the holder 14 with a contact 13a. 
Similarly, the voltage lead wire section 12c and the current lead wire 
section 12d form together a single lead wire having its upper portion 
vertically projecting from the upper surface 14U of the holder 14, and a 
junction of said section 12c, 12d defines an inverted V-shaped contact 13b 
which is inclined towards the contact 13a. A heat generating element 11 
made of platinum wire is vertically stretched and connected between these 
contacts 13a and 13b. 
As will be apparent, the disposable sensor of the present invention is used 
to measure voltage values at two ends of the heat generating element by 
four point-terminal technique without being affected by lead wires. 
It should be understood here that not only the heat generating element 11 
but also portions of the lead wire sections 12a, 12b, 12c and 12d exposed 
from the upper surface 14U of the holder 14 are provided with coating 15 
preventing electric current from leaking into the fluid, since these 
portions are to be immersed into the fluid as will be described later. 
From a lower surface 14L project the lead wire sections 12a, 12b, 12c and 
12d in an appropriate arrangement and form together a terminal 16 to be 
connected to a connector as will be subsequently described. 
Although a combination of the voltage lead wire sections 12a, 12c and the 
current lead wire sections 12b, 12d is selective, it is also possible, for 
example, to use the sections 12a, 12c as the current lead wire sections 
and the sections 12b, 12d as the voltage lead wire sections. 
FIG. 2 shows a manner in which the terminal 16 of the disposable sensor 10 
is electrically connected to a connector 20 which includes sockets 21a, 
21b, 21c and 21d with which the lead wire sections 12a, 12b, 12c and 12d 
of the terminal 16 are mated, respectively. The sockets 21b, 21d connected 
to the lead wire sections 12b, 12d are electrically connected via lead 
wires 22b, 22d to a constant current source 31 in a measuring unit proper 
30 while the sockets 21a, 21c connected to the voltage lead wire sections 
12a, 12c are electrically connected via lead wires 22a, 22c to a voltmeter 
32 in the measuring unit proper 30. 
Reference numeral 33 designates a controller. This controller 33 and the 
voltmeter 32, and the constant current source 31 and the voltmeter 32 are 
respectively connected via communication lines such as GP-IB (general 
purpose interface bus) cables 33a. 
Then, a method for measurement utilizing the disposable sensor 10 of this 
invention will be discussed. 
As seen in FIG. 3, the heat generating element 11 of the sensor 10 
connected to the connector 20 is immersed into a quantity of fluid 35 
filling a tank 34 with said heat generating element 11 being vertically 
oriented, and is supplied with electrical current from the constant 
current source 31 via the current lead wire sections 12b, 12d and the lead 
wires 22b, 22d so that a calorific power Q is diffused from the heat 
generating element 11 into the fluid 35 through conduction and convection 
and, during such diffusion, an applied voltage is measured by the 
voltmeter 32 via the voltage lead wire sections 12a, 12c and the lead 
wires 22a, 22c. 
Although it is optimal to maintain the calorific power Q at a constant 
level by using the controller 33 when changes in fluid temperature is 
conspicuous, such control can be often effectively achieved by maintaining 
the current constant in practical use. rp Although it is optimal to 
stretch the heat generating element 11 vertically between the contacts 
13a, 13b, the heat generating element 11 may be also stretched 
horizontally as in the sensor shown by FIG. 5B in practical use or to any 
other directions. 
By measuring the average temperature .theta.w by such sensor, the heat 
transfer coefficient can be calculated as mentioned above, and thereby 
change in properties of the fluid such as the coefficient of kinematic 
viscosity .gamma. can be determined. 
As the means for measuring .theta..sub..infin., a separate sensor 
constructed similarly to the sensor 10 of the invention may be placed in 
the fluid 35 and may be supplied with feeble current in the order of 1 mA 
to also determine the fluid temperature from the resistance value of this 
separate sensor, but the thermometric means is not limited to this. 
As seen in FIG. 4 after each measurement of change in value of properties 
of fluid has been completed, the used sensor 10 is removed from the 
connector 20 and discarded, then a fresh sensor 10N is mated to the 
connector 20 for the subsequent measurement or the like. 
In this way, both the difficult, troublesome operation of washing the used 
sensor 10 and the inconvenience that the inadequate washing might prevent 
the subsequent measurement from being properly performed. 
Although the heat generating element has been described by way of example 
as comprising platinum wire, the other metals or the like may be also used 
as material for the heat generating element, for example, in addition to 
platinum, platinum-rhodium, nickel, tungsten, cobalt and molybdenum etc. 
Coating material useful for the surface of the heat generating element 
includes ceramic, various mactromolecular substances and resin. 
In the place of or onto such coating material, a suitable substance 
functioning as antibody or antigen may be used or fixed, respectively, to 
cause antibody or antigen reaction in close proximaty to the surface of 
the heat generating element immersed in the fluid and thereby to detect 
characteristics of this fluid under a special condition. 
Diemnsioning of the heat generating element is not critical, but a 
prefereble ratio of the outer diameter to the length of the heat 
generating element is in the order of 1:1000 because such dimensioning 
makes a heat loss possibly occurring from the ends of the heat generating 
element negligible and minimizes a measurement error. In practical use, 
however, the heat generating element presenting the ratio in the order of 
1:10, more specifically, the platinum wire having a diameter of 5 to 100 
.mu.m and a length of 1 to 100 mm may be used to provide a sensor which is 
sufficiently high in measurement accuracy, sensitivity and strength. 
FIG. 5A shows a second embodiment of the disposable sensor constructed in 
accordance with the present invention. 
This embodiment is characterized in that the holder 14 is rectangular and 
the heat generating element is provided within a cylindrical container 40. 
Correspondingly, the lead wire sections 12a, 12b, 12c, 12d are in the form 
of rectangular plates. The voltage lead wire section 12a and the current 
lead wire section 12b are respectively L-shaped, vertically extend from 
the bottom of the holder 14 at opposite sides thereof, jointed together 
within the cylindrical container 40 1nd form together a gate-like 
configuration. The voltage lead wire section 12c and the current lead wire 
section 12d vertical extend from the bottom of the holder 14 at 
intermediate positions thereof and jointed together at the bottom of the 
cylindrical container 40. The contact 13a defined by the junction point of 
the voltage lead wire section 12a and the current lead wire section 12b is 
vertical opposed to the contact 13b defined by the junction of the voltage 
lead wire section 12c and the current lead wire section 12d. The heat 
generating element 11 is connected vertically between these contacts 13a, 
13b. 
The lead wire sections 12a, 12b, 12c, 12d define together the rectangular 
terminal 16 projecting from the bottom of the holder 14. 
The construction such that the heat generating element 11 is mounted within 
the cylindrical container 40 is advantageous in that the fluid can be 
sampled into this cylindrical container 40 for measurement. 
Preferably, the cylindrical container 40 is provided with a lid 42, sheet 
film or the like to prevent any contaminant often adversely affecting the 
measurement from entering into the container 40 before every actual 
measurement is started and to allow the container 40 containing therein 
the sampled fluid to be transported without spilling the content. 
FIG. 5B shows a third embodiment of the disposable sensor 10 constructed in 
accordance with the present invention, which is similar to the embodiment 
shown by FIG. 5A except that the heat generating element 11 is 
horizontally stretched within the cylindrical container 40. Such 
construction is advantageous in that an effective measurement is possible 
even when the sampled fluid is of an extremely small amount. 
The voltage lead wire section 12a and the current lead wire section 12b are 
respectively L-shaped, vertically extend from the bottom of the holder 14 
at the opposite ends thereof, jointed together within the holder 14 and 
form together a gate-like configuration. The contact 13a defined by the 
junction of said sections 12a, 12b vertically extends and is, within the 
cylindrical container 40, provided with the coating 15. The voltage lead 
wire section 12c and the current lead wire section 12d are identical to 
said voltage lead wire section 12a and said current lead wire section 12b. 
The contact 13b defined by the junction of said section 12c, 12d is 
opposed to the contact 13a. The heat generating element is horizontally 
stretched between these contacts 13a, 13b. 
Reference numeral 44 designates a cover adapted to be mounted around the 
location at which the disposable sensor 10 has been mated with the 
connector 20, not only to assure this mating relationship but also to 
prevent the fluid from entering into this location and causing a current 
leakage. More specifically, this cover 44 is provided in consideration of 
the fact that the terminal 16 comprising the lead wire sections 12a, 12b, 
12c, 12d is relatively poor in its strength and the terminal is not 
provided with insulating coating. 
It should be understood that the sensor 10 according to this embodiment 
also may be provided with the lid 42 or the like to prevent any 
contaminant from entering into the cylindrical container 40. 
FIG. 6 shows an embodiment in which the sensor 10 is connected by the 
intermediate connectors 17 to the connector 20 provided on the measuring 
unit proper 30. 
The disposable sensor of the present invention inevitably requires frequent 
removal of the sensor and this otherwise would progressively reduce a 
resistance which is necessary for a reliable contact with the connector 20 
on the measuring unit proper 30. 
To overcome such problem, the present invention provides the intermediate 
connector 17 on the measuring unit proper 30 so that frequent removal of 
the sensor directly from the connector 20 on the measuring unit proper 30 
may be avoided. 
Such intermediate connector 17 may be single, but a plurality of the 
intermediate connectors 17 may be interposed between the sensor and the 
connector 20 on the measuring unit proper 30 as shown in FIG. 6 to avoid 
said undesirably frequent removal of the sensor from the connector 20 on 
the measuring unit proper 30 more effectively. 
FIG. 7 shows an embodiment in which a plurality of the connector 20 are 
provided on a side of the measuring unit proper 30 containing the various 
components such as the constant current source and the volt meter so that 
a result of measurement can be read on a display 36. 
This construction including a plurality of the sensors respectively mated 
on a series of the connectors 20 permits properties of various fluid 
samples to be measured at once. 
Although the insulating coating for the lead wires and the heat generating 
element is provided principally in order to prevent a short-circuit due to 
the current leak, the demand for such coating depends on a situation at 
which the sensor is to be used and therefore such coating may be provided 
on any locations so far as such locations require the coating. 
Although the embodiments of the invention have been described hereinabove 
as employing the voltage lead wire and the current lead wire of 
four-terminal type, it is also possible to mate the sensor with the 
connector in the two or three-terminal fashion and to distribute the lead 
wires from the connector in four-terminal fashion. 
While the invention has been particularly shown and described with 
reference to preferred embodiment thereof, it will be understood by those 
skilled in the art that the foregoing and other change in form and details 
can be made therein without departing from the spirit and scope of the 
invention.