Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1993-10-27
2003-04-22
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C338S225000
Reexamination Certificate
active
06550325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric device and a method of driving the same. Particularly, the present invention relates to a measuring instrument for fluids such as gases, liquids, and fog-like fluids (including atomized fluids and gases and fluids containing solid powder) and to a method of operating the measuring instrument. More particularly, the invention relates to a measuring instrument for measuring flow rates or flow velocities of fluids, discriminating between fluids, measuring the ratio of one constituent of a mixed fluid to the other, and measuring the concentration (e.g., humidity) of a substance contained in a fluid. Also, the invention relates to a method of operating this measuring instrument.
2. Prior Art
A known instrument for measuring a flow rate makes use of a thermistor. In particular, a fluid takes heat away, thus lowering the temperature of the thermistor. This instrument makes use of this phenomenon. Generally, when the thermistor is in contact with the fluid, the amount of heat taken away from the thermistor depends on the flow rate and so the output from the thermistor has a certain correlation with the flow rate. Utilizing this correlation, the flow rate can be calculated from the output from the thermistor.
A flow rate is the product of the cross-sectional area of a fluid and the flow velocity. It is assumed that a fluid flows through a circular pipe having an inside diameter r at a flow velocity of v. The flow rate is given by v&pgr;r
2
. Accordingly, we will concentrate on the flow rate in the description made below. If the cross-sectional area of the fluid is known, then the flow rate and the flow velocity can be found simultaneously.
Generally, a thermistor refers to a semiconductor having a negative temperature coefficient of resistance. Instead of such a thermistor, a metal such as platinum having a positive temperature coefficient of resistance can be used. That is, any material can be used as long as its resistance changes with temperature. Devices using a material whose resistance changes with temperature are collectively known as thermally sensitive resistors, temperature sensing elements, or resistance thermometers. Thermally sensitive resistors referred to herein mean materials whose resistances change with temperature.
Another structure uses a heating resistor exposed to a fluid. The amount of heat taken away from this resistor depends on the flow rate. This structure makes use of this phenomenon. In this system, the flow rate can be calculated by measuring the electrical current flowing through the heating resistor.
A further structure employs a heating element in contact with a fluid. Heat is taken away from the heating element by the fluid. A temperature sensing element (e.g., a platinum sensor) disposed independent of the heating element measures the amount of heat carried away by the fluid. The flow rate is calculated from this amount of heat.
In these methods, a higher sensitivity is effectively obtained by increasing the amount of heat taken away by the fluid. In order to enhance the response speed, it is necessary to reduce the heat capacity of the temperature sensing element as small as possible.
The flowmeters constructed as described above have the problem that only a narrow range of flow rates can be measured. That is, their dynamic ranges are narrow. Specifically, these flowmeters are capable of precisely measuring flow rates within ranges only from 20 to 300 sccm or from 200 to 2000 sccm.
It is considered that these problems are chiefly due to the following reasons.
(1) Since the temperature sensing element is in a thermally quite unstable state, the linearity of the response to the heat is poor. Hence, the flowmeter cannot respond to thermal changes occurring in a wide range.
(2) In association with (1) above, heating is especially difficult to perform. It is impossible to do effective heating over a wide range of flow rates.
(3) If the heat capacity of the temperature sensing element is reduced to increase the response speed, it is impossible to treat large amounts of heat.
The item (1) described above occurs because it is difficult to build a structure which permits the fluid to effectively take heat away from the temperature sensing element over a wide range of flow rates and which, at the same time, effectively supplies heat to the sensing element.
The item (3) described above means that increasing the response speed conflicts with increasing the dynamic range, or the range of flow rate of the investigated fluid.
Accordingly, the present situation is that the prior art flowmeter is designed to measure a narrowed range of flow rates so that the heat amount treated by the temperature sensing element may not vary greatly. More specifically, the measured range of flow rates is narrowed. Within this range, the heat capacity of the temperature sensing element, the positional relation between the temperature sensing element and the heating element, their thermal relationship, and the electrical current flowing through the temperature sensing element are subtly adjusted to obtain the required sensitivity and measuring accuracy.
A further known structure is designed so that the heat amount supplied to a heating element changes according to the flow rate. The amount of heat taken away from a temperature sensing element is in proportion to the flow rate, irrespective of the flow rate value.
Temperature sensing elements sensitively detect temperature variations (i.e., temperature variations in the fluid) in the environment as well as flow rates. Therefore, where the temperature sensing elements are used in an environment where temperature varies, a problem takes place. Various proposals have been made to solve this problem. In actual usage, however, the measured flow rate is affected greatly by the fluid temperature.
Where a temperature sensing element or a heating element is exposed to a fluid, the material of the sensing element or heating element is corroded, depending on the kind of the fluid. As a result, the electrical and thermal characteristics are varied. To solve this problem, a method of coating the surface with a protective film may be contemplated. However, where the thermistor surface is coated with a protective film, the speed of the response to heat deteriorates. Also, the measuring accuracy is deteriorated by refraction of heat. Hence, this scheme is undesirable.
Flowmeters using thin diamond films are described in Sensors and Materials,
2
,
6
(1991), pp. 329-346 and in Applications of Diamond Films and Related Materials, Y. Tzeng, M. Murakawa, A. Feldmand (Editors), Elsevier Science B. V., 1991, pp. 311-318. These two flowmeters are essentially the same in structure. In these flowmeters, a thin film of diamond is formed on a silicon substrate by CVD. Resistors formed inside the silicon substrate are in contact with two opposite ends, respectively, of the thin film. One of the resistors acts as a heating element, while the other serves as a temperature sensing element, or a sensor. When heat is conveyed from one end to the other end of the diamond film in the direction of the plane of the film, the fluid carries away heat from the surface of the thin film. This amount of heat is detected by the temperature sensing element mounted at the other end. However, these flowmeters do not have any characteristics which deserve attention.
SUMMARY OF THE INVENTION
Accordingly, in the present invention, a thermistor is formed on the surface of a thin film of diamond. This film is operated as a heat storage layer for supplying heat to the thermistor. A thermal equilibrium state which quickly satisfies Eq. (8) if a flow rate change occurs is realized. The temperature of the thermistor is settled in such a way that the flow rate of the fluid is constantly reflected.
W=K
(
T−T
0
)+
G
(
T−T
0
) (8)
First Invention
A first invention of the present application lies in a flow detector using a thin film of diamond which has a layer
Hiroki Masaaki
Inushima Takashi
Rimantas Vaitkus
Sato Eiji
Teramoto Satoshi
Patel Harshad
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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