Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2002-05-29
2004-12-14
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06829930
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved gas flow velocity and temperature sensor system and more particularly to a gas flow velocity and temperature sensing system utilizing a processor configured to use an empirically derived equation which more accurately calculates the gas flow velocity and temperature proximate a sensor.
BACKGROUND OF THE INVENTION
Gas velocity and temperature sensors are used to monitor the gas (e.g., air) flow velocity and the temperature proximate sensitive electronic components, and also in refrigeration systems, gas conditioning systems, biocontainment systems, gas supply applications, industrial process control of gas mixing, weather applications and any application which requires monitoring of gas flow. For example, in electronic systems with heat generating components, failure to maintain sufficient gas flow within the system can result in damage to the sensitive electronics of the system. In biocontainment systems, failure to maintain the correct gas flow within the system can result in overheating or overcooling the biocontainment area killing the organisms within the system. In gas conditioning systems, gas supply applications, gas mixing, and weather applications, measuring the gas flow velocity and temperature within the system is key to operation of the system.
The inventors hereof have invented gas flow and temperature sensors/probes and circuits which facilitate easy access to even difficult locations by employing a small sensor comected over a long, flexible, small cross-sectional area of cable for providing a measurement of the gas flow velocity and temperature along with calibration data characterizing the response of the particular probe sensor and circuitry. See U.S. Pat. Nos. 5,929,333, 5,792,951, 5,511,415, and 4,733,541, incorporated herein in their entirety by these references.
These gas velocity sensors and probes, as well as other prior art gas velocity sensors/probes, typically employ two thermistors to calculate gas flow velocity and temperature. A thermistor is a thermally sensitive resistor which exhibits a change in electric resistance due to a change in temperature. One thermistor is typically maintained at the temperature of the gas flow being measured and a circuit connected to this thermistor is configured to output a temperature signal proportional to the gas temperature. The other thermistor is maintained at a chosen temperature which is significantly higher than the temperature of the gas being measured (e.g., a “hot” thermistor 100° C. above the temperature of the gas being measured). Because the resistance and temperature of a thermistor are related by a characteristic curve, a specific chosen temperature of the hot thermistor relates to specific resistance of the thermistor. A constant temperature servo connected to the hot thermistor maintains the hot thermistor at a constant resistance representative of the chosen temperature and outputs a measure of the power required to maintain the thermistor at the chosen resistance. When the hot thermistor is subjected to an increase or decrease in gas flow, it causes an increase or decrease in the power requirements of the constant temperature servo to maintain the hot thermistor at the constant resistance (representative of the chosen temperature). Typically, the constant temperature servo is configured to output a signal representative of the power dissipated as a function of gas velocity and the temperature proximate the thermistor.
Prior art gas flow velocity sensor systems may then employ a processor which receives the signal representative of the power dissipated as a function of gas velocity, the temperature signal representative of gas temperature proximate the hot thermistor, and the temperature signal representative of temperature of the gas being measured (often called the “ambient temperature”). The microprocessor of these prior art gas flow velocity sensors then calculates the gas flow velocity proximate the sensor using King's law as shown below:
P
=
E
v
2
/
R
v
=
(
Ak
+
Bk
(
μ
C
P
k
)
0.33
Re
n
)
(
T
v
-
T
A
)
(
1
)
where P and E
v
2
/R
v
is the power dissipated in a hot wire of infinite length, A, B, and n are constants derived via flow calibration, k is the fluid's thermal conductivity, C
P
is heat capacity, R
e
is the Reynolds number, T
v
is the temperature of the wire, and T
A
is the ambient temperature. The Reynolds number in expanded form is:
Re
=
ρ
vd
μ
,
where p is the gas density, v is fluid (gas) velocity, d is the diameter of the wire, and &mgr; is the gas viscosity. Equation (1) is solved for v to calculate the gas velocity as follows:
gas
velocity
=
v
=
K
2
·
[
[
E
v
2
-
K
0
·
(
Tv
-
Ta
)
]
K
1
·
(
Tv
-
Ta
)
]
2
37
(
2
)
Based on the measured gas velocity, a feedback loop can be used to control gas velocity, and thus the temperature of, for example, an equipment cabinet or biocontainment system.
Prior art sensors and probes which rely on King's law to calculate gas velocity, however, produce inaccurate readings because King's law is based on the approximation that the hot thermistor is a hot filament of infinite length when in fact it is not.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an improved gas velocity and temperature sensor system.
It is a further object of this invention to provide such a sensor system which accurately measures the gas flow velocity and temperature proximate a sensor.
It is a further object of this invention to provide such a sensor system which refines the approximation used to calculate gas flow velocity.
It is a further object of this invention to provide such a sensor system which calculates gas flow velocity and temperature proximate a sensor without the errors associated with the approximation associated with King's law.
The invention results from the realization that a truly effective gas flow and temperature sensor can be effected by providing a first thermistor driven at a constant temperature higher than the gas temperature being measured and which outputs a signal representative of the power dissipated as a function of gas velocity, a second thermistor which measures the gas temperature and which outputs a signal representative of the gas temperature, and a microprocessor configured to calculate a more accurate representation of the gas flow velocity and temperature, not by using King's law which relies on the erroneous approximation that the thermistor is a hot wire of infinite length, but, instead, by utilizing a innovative and significantly more accurate empirically derived equation which reduces the error of approximation associated with King's law to yield a significantly more accurate measurement of gas flow velocity and temperature proximate the sensor.
This invention features a gas velocity and temperature sensor system comprising a first thermistor driven at a constant temperature and configured to output a flow signal representative of the power dissipated as a function of the gas velocity and a temperature signal representative of the temperature of the first thermistor, a second thermistor configured to output a gas temperature signal representative of the gas temperature proximate the second thermistor, and a processor responsive to the flow signal and the temperature signals. The processor is configured to calculate gas velocity using an empirically derived equation in which gas flow velocity is a function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor, the processor deriving a signal representing the gas velocity. Ideally, the processor derives a signal representing the temperature of the gas proximate the second thermistor.
In one preferred embodiment the empirically derived equation is
v
≅
[
kP
Δ
T
Bailey Clarke
Hiramoto Karl Y.
Ismail Raouf A.
Degree Controls, Inc.
Iandiorio & Teska
Lefkowitz Edward
Mack Corey D.
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