Flowmeter sensor

A flowmeter sensor includes a sensor tube, and a heater/resistor through which the sensor tube passes. The heater/resistor includes a substantially cylindrical sleeve fitted around the sensor tube and made of ceramic. A thin film of platinum is deposited on the sleeve and is then partially removed by a laser to provide a helical resistance pattern. A pair of electrically conductive rings are tightly fitted around opposite ends of the ceramic sleeve and serve as bases to be connected with a corresponding pair of leads.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a flowmeter sensor for measuring the flow 
of a gas or other fluid used in the manufacture of semiconductors. 
2. Description of the Related Art 
A conventional flowmeter sensor includes a conduit (sensor tube) through 
which a fluid flows, and a temperature-sensitive resistance wire wound on 
the outer periphery of the sensor tube as exemplified in U.S. Pat. No. 
4,815,280. 
In the prior art flowmeter sensor, the sensor tube has quite a small 
diameter, approximately 1 mm, and the temperature-sensitive resistance 
wire has a diameter of approximately 0.02 mm. As such, it is cumbersome to 
wind the temperature-sensitive resistance wire on the sensor tube. It is 
also cumbersome to adjust the resistance value of a heater/resistor since 
it is determined by the number of turns of the heat-sensitive resistance 
wire. Another problem with the prior art flowmeter sensor is that the 
resistance wire is loosened when the heater/resistor is heated at a high 
temperature for a substantial length of time. This results in a change in 
the resistance value of the heater/resistor. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the problems 
encountered in the prior art and to provide a flowmeter sensor which can 
easily and accurately adjust the resistance value of a heater/resistor, 
and which is highly reliable if it is heated at a high temperature for a 
substantial length of time. 
According to the present invention, there is provided a flowmeter sensor 
which comprises a substantially cylindrical sensor tube through which a 
fluid flows, and a heater/resistor including at least one substantially 
cylindrical sleeve fitted around the sensor tube and made of an 
electrically nonconductive material and a heating element formed on the 
sleeve. The heating element includes a thin film of resistance material. 
The film of resistance material is deposited on the sleeve and thereafter 
is scraped off or otherwise removed so as to provide a predetermined 
resistance value. 
A pair of electrically conductive rings may be fitted tightly around 
opposite ends of the sleeve and electrically connected to the film of 
resistance material to facilitate the supplying of electrical current to 
the heater/resistor. 
A corresponding pair of leads may be attached to the electrically 
conductive rings by resistance welding, Each lead may be in the form of a 
tape and has substantially the same width as the electrically conductive 
ring. 
A plurality of sleeves may be fitted around the sensor tube and spaced a 
predetermined distance away from each other so as to enhance the 
sensitivity of the flowmeter sensor. The distance between adjacent sleeves 
may be changed according to the types of fluids to be sensed. 
The flowmeter sensor of the present invention is capable of providing a 
desired resistance value by providing an appropriate width and the length 
of a resistance pattern formed in the film, Since each heater/resistor has 
a tubular base, two heaters/resistors can be moved on the sensor tube and 
spaced a given distance away from each other. The flowmeter sensor 
includes a pair of electrically conductive rings fitted tightly around 
opposite ends of the sleeve and adapted to be connected with a 
corresponding pair of leads, This arrangement eliminates the need for 
silver paste and allows the heater/resistor to have a constant resistance 
value. 
According to the present invention, the film of resistance material is 
scraped or otherwise removed so as to provide a helical resistance pattern 
and cooperates with the rings fitted tightly around the opposite ends of 
the sleeve. This arrangement enables an easy and fine adjustment of the 
resistance value of the heater/resistor and thus, accurate measurement of 
the flow rate of a fluid. 
The heater/resistor comprises a tubular element or sleeve and fitted around 
the sensor tube such that it can be moved to any desired position so as to 
adjust the sensitivity of the flowmeter sensor. Further, the film of 
resistance material is deposited on the sleeve which is made of ceramic so 
as to insulate the sensor tube from the heater/resistor. This allows the 
flowmeter sensor to be used at a high temperature for a substantial length 
of time. 
These and other objects and advantages of the present invention will be 
apparent from the following description of preferred embodiments when 
taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings and particularly to FIG. 1, a heater/resistor 
1 includes a substantially cylindrical sleeve 2. The sleeve 2 is made of 
ceramic (alumina by 96 percent) and, for example, has an inner diameter of 
0.5 mm, an outer diameter of 0.7 mm, and a length of 7 mm. The inner 
diameter, the outer diameter and the length of the sleeve 2 may, of 
course, be changed according to the diameter of a sensor tube and the 
types of fluids to be sensed. A thin film of platinum has a thickness of 
approximately 1 micron, and is deposited on the outer periphery of the 
ceramic sleeve 2. A pair of rings 5, 5 are made of nickel (or stainless 
steel) and each has a width of 0.5 mm, and a thickness of 0.7 mm. The ring 
5 is forced in the direction of the arrow X and is tightly fitted around a 
respective end of the sleeve 2. These rings serve as bases and allow the 
heater/resistor to more easily and firmly be connected with leads than 
electrically conductive paste. Such paste is subject to uneven 
application. This results in an undesirable change in the temperature 
coefficient of the heater/resistor. The film is, then, scraped or 
otherwise removed by a laser (not shown) to form a void 3 providing a 
helical resistance pattern 4. The helical resistance pattern 4 extends 
between the opposite ends of the ceramic sleeve 2. 
A corresponding pair of platinum foils 6 (only one is shown) extend around 
the rings and each has a width of 0.5 mm, and a thickness of 0.1 mm. Each 
foil 6 is attached as at P by resistance welding and acts as an electrical 
lead. The heater/resistor 1 thus made has a resistance value of 
200.OMEGA., and a temperature coefficient of 3600 PPM. Since a laser is 
employed to provide a resistance pattern, heater/resistors of identical 
properties can be fabricated on a mass production basis. 
A stainless steel sensor tube 11 passes through two heaters/resistors 1A, 
1B. As shown in FIG. 2, after the sensor tube 11 has been appropriately 
positioned, a polyimide resin 12 is applied between the sensor tube 11 and 
the sleeves and is then cured at a temperature of at least 100 .degree. C. 
Electrical current is then passed to heat each heater/resistor. 
Temperature differentials between the two heaters/resistors allows of the 
flow rate of a fluid flowing through the sensor tube 11 to be measured. 
These heaters/resistors 1A, 1B form part of a bridge circuit. It will be 
appreciated that the sensitivity of the flowmeter sensor of this type 
varies depending on the distance between the two heaters/resistors. As 
shown in FIG. 3, the two heaters/resistors 1A, 1B may be spaced a distance 
(a) away from each other to obtain a desired sensitivity. This arrangement 
is advantageous in that fluids of different specific heats can be measured 
by appropriately selecting the distance. 
The thin film of resistance material may be made of any resistance 
materials other than platinum. Also, the sleeve may be made of any 
electrically nonconductive materials other than ceramic. 
A minimum quantity of electrically conductive paste may be applied between 
the electrically conductive rings and the film of resistance material. 
While the invention has been described in its preferred form, it will be 
obvious to those skilled in the art that it is not intended to limit the 
scope of the invention, and that various changes and modifications may be 
made therein without departing from the spirit and scope of the invention.