Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2001-02-14
2003-04-01
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06539792
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to resistance balancing, and more particularly to a mass flow sensor that is capable of detecting the mass flow rate of a fluid by balancing the resistance of upstream and downstream temperature sensors.
DESCRIPTION OF RELATED ART
Mass flow sensors are used in a wide variety of applications to measure the mass flow rate of a gas or other fluid. One application in which a mass flow sensor may be used is a mass flow controller. In a conventional mass flow controller, the mass flow rate of a fluid flowing in a main fluid flow path is regulated or controlled based upon a mass flow rate of a portion of the fluid that is diverted into a typically smaller conduit forming a part of the mass flow sensor. Assuming laminar flow in both the main flow path and the conduit of the sensor, the mass flow rate of the fluid flowing in the main flow path can be determined (and regulated or controlled) based upon the mass flow rate of the fluid flowing through the conduit of the sensor.
Two different types of mass flow sensors have traditionally been used, constant current mass flow sensors, and constant temperature mass flow sensors. An example of a constant current mass flow sensor is illustrated in FIG.
1
. In
FIG. 1
, a fluid flows in a sensor pipe or conduit in the direction of the arrow X. Heating resistors or “coils” R
1
and R
2
having a large thermal coefficient of resistance are disposed about the sensor conduit on downstream and upstream portions of the sensor conduit, respectively, and are provided with a constant current I from a constant current source
901
. As a result of the constant current I flowing through the coils R
1
and R
2
, voltages V
1
and V
2
are developed across the coils. The difference between voltages V
1
and V
2
(V
1
−V
2
) is taken out of a differential amplifier
902
, with the output of the amplifier
902
being proportional to the flow rate of the fluid through the sensor conduit.
At a zero flow rate, the circuit of
FIG. 1
is configured so that the resistance value (and thus, the temperature) of coil R
1
is equal to the resistance value (and temperature) of coil R
2
, and the output of the amplifier
902
is zero. As fluid flows in the sensor conduit, heat that is generated by coil R
2
and imparted to the fluid is carried towards R
1
. As a result of this fluid flow, the temperature of coil R
2
decreases and that of coil R
1
increases. As the voltage dropped across each of these resistors is proportional to their temperature, voltage V
1
increases with an increased rate of fluid flow and voltage V
2
decreases, with the difference in voltage being proportional to the mass rate of flow of the fluid through the sensor conduit.
An advantage of a constant current mass flow sensor is that it can operate over a wide range of temperatures, is relatively simple in construction, and is responsive to changes in the ambient temperature of the fluid entering the sensor conduit. In this regard, as the ambient temperature of the fluid entering the sensor conduit changes, so does the resistance of each of the coils R
1
and R
2
. However, it takes a relatively long time for the temperature (and thus, the resistance) of the coils R
1
and R
2
to stabilize in response to a change in the rate of flow of the fluid.
The other type of mass flow sensor that is frequently used is a constant temperature mass flow sensor, examples of which are illustrated in
FIGS. 2-4
. As shown in the constant temperature mass flow sensor of
FIG. 2
, heating resistors or “coils” R
1a
and R
1b
are respectively disposed about the downstream and upstream portions of a sensor conduit through which a fluid flows in the direction of the arrow X. As in the constant current mass flow sensor of
FIG. 1
, each of the downstream and upstream coils R
1a
and R
1b
has a large thermal coefficient of resistance. The resistance (and thus the temperature) of each of the coils R
1a
, R
1b
is fixed by separate and independent circuits to the same predetermined value that is governed by the value of resistors R
2a
, R
3a
, R
4a
, and R
2b
, R
3b
, R
4b
, respectively. Control circuitry is provided to maintain each of the coils R
1a
, R
1b
at the same predetermined value of resistance (and thus, temperature) independently of the rate of fluid flow through the sensor conduit.
In the absence of fluid flow, the circuit of
FIG. 2
is configured so that the resistance (and temperature) of each of the downstream and upstream coils R
1a
and R
1b
is set to the same predetermined value and the output of the circuit is zero. As fluid flows in the sensor conduit, heat from the upstream coil R
1b
is carried towards R
1a
. As a result, less energy is required to maintain the downstream coil R
1a
at the fixed temperature than is required to maintain the upstream coil R
1b
at that same fixed temperature. The difference in energy required to maintain each of the coils R
1a
, R
1b
at the predetermined temperature is measured and is proportional to the mass flow rate of fluid flowing through the sensor conduit.
The constant temperature mass flow sensor described with respect
FIG. 2
is also relatively easy to construct. In addition, the circuit of
FIG. 2
stabilizes more quickly in response to changes in the mass flow rate of the fluid entering the sensor conduit than the constant current mass flow sensor described with respect to FIG.
1
. However, because each of the coils R
1a
and R
1b
is set and maintained at a predetermined temperature independently of the ambient temperature of the fluid flowing into the sensor conduit, a problem arises when the ambient temperature of the fluid flowing into the sensor conduit increases. In particular, when the ambient temperature of the fluid flowing in the sensor conduit approaches the predetermined temperature that is maintained by the upstream and downstream coils, the circuit loses its ability to discern differences in the flow rate of the fluid, and when the ambient temperature of the fluid increases beyond this predetermined temperature, the sensor is rendered inoperable.
To overcome these disadvantages, a number of alternative constant temperature mass flow sensors have been provided. For example, the circuit of
FIG. 3
provides a constant temperature mass flow sensor that is capable of responding to changes in the ambient temperature of a gas or fluid, at least to a certain degree. Once again, R
1b
and R
2b
are downstream and upstream temperature sensing coils with a large temperature coefficient of resistance. However, rather than maintaining the temperature of the coils at a predetermined constant value as in the circuit of
FIG. 2
, the circuit of
FIG. 3
maintains the temperature of the sensor coils R
1b
, R
2b
at a temperature that is above the ambient temperature of the fluid flowing into the sensor conduit. This is achieved by the insertion of an additional coil R
3b
, R
4b
having a coefficient of resistance similar to that of the sensor coils R
1b
, R
2b
in each of the downstream and upstream circuits. As the ambient temperature of the fluid changes, the series addition of coil resistance R
3b
, R
4b
to the temperature setting resistors R
5b
, R
6b
results in raising the temperature to which the upstream and downstream resistance coils are maintained above the ambient temperature of the fluid flowing into the sensor conduit. As in the circuit of
FIG. 2
, the difference in energy supplied by each of the downstream and upstream circuits to maintain the temperature of the coils R
1b
, R
2b
at the same temperature is proportional to the mass flow rate of the fluid through the sensor conduit.
As should be appreciated by those skilled in the art, for the circuit of
FIG. 3
to operate properly, it is critical that the values and thermal characteristics of each element in the downstream circuit match that of the corresponding element in the upstream circuit. Thus, the resistance of the downstream and upstream coils R
1b
, R
2b
must have the same value, and the same thermal coefficien
Lull John Michael
Urdaneta Nelson
Mack Corey D.
Unit Instruments
Wolf Greenfield & Sacks P.C.
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