Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2001-01-12
2003-01-14
Lieu, Julie (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S608000, C055S307000, C055S309000, C055S311000
Reexamination Certificate
active
06507282
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a filter monitoring system for filter-fan products and more particularly relates to a filter monitoring system using a thermistor that monitors the airflow through the product to determine ultimately when a filter requires replacement. The system can also provide an indication of percentage of filter life remaining.
2. Brief Description of the Prior Art
Filter-fan products such as some types of portable fans, air purifiers, humidifiers and dehumidifiers include filters for removing airborne particles from the homes or offices in which they operate. Such filters include fine particle high efficiency particulate air (HEPA) filters, filters for trapping relatively large particles and carbon filters to remove odors. Typically, a fan is positioned adjacent a removable filter to force air through the filter thereby trapping airborne particles therein.
FIG. 2
illustrates a cross section through new a filter-fan device
12
that includes a housing
14
, a fan
16
, and a filter assembly
18
. The fan
16
has a motor
20
and a fan blade
22
. In operation, the rotation of the fan blade
22
causes air to be drawn into the air inlet
24
through the filter assembly
18
. The filtered air then moves through to enter the fan
16
. The fan
16
expels the air into a scroll
26
which then exits through the air outlet
28
. This illustrates the basic operation of the filter-fan device
12
that uses replaceable filter assemblies
18
. The air inlet
24
is preferably formed as a perforated grill in a door of the filter-fan device
12
.
As the efficiency of these type of products depends upon the replacement of he filter when spent, the ability to easily determine when the filter is spent is important. Accordingly, it is desirable to provide such fan-filter products with a system to monitor the remaining life of a filter and to indicate when the filter should be replaced.
U.S. Pat. No. 4,050,291 to Nelson discloses a filter condition responsive apparatus for a stove vent system. The stove vent system consists of a duct having two filters located in series with a fan for creating the airflow. The patent indicates that the filters have similar pressure drop characteristics although the first filter is designed to collect grease and the second filter is a charcoal filter for removing odors. The filter condition responsive device includes a first bypass containing a thermistor which is constantly heated by an electrical heater. The bypass is essentially a small duct connected to the duct of the stove vent system with an inlet on one side of the grease filter and the outlet on the other side of the grease filter. The electrical heater appears to be a separate resistor that generates heat. A second bypass is included across the charcoal filter along with a thermistor which is also constantly heated by an electrical heater. The thermistors are electrically coupled and are connected to an amplifier which in turn is connected to a filter condition indicator. When the grease filter becomes clogged, the bypass flow rates through bypasses are different. This difference is measured by a bridge network circuit and indicated on the filter condition indicator.
U.S. Pat. No. 5,014,908 to Cox discloses a control circuit for use in a humidifier. The patent discloses using a pair of thermistors for checking the condition of a wick in a humidifier. The filter check includes a first thermistor TRM
1
located adjacent to the air inlet before the wick W and a second thermistor TRM
2
located adjacent to the air outlet AO. When the humidifier is operating properly with a good wick, the air absorbs water from the wick causing the temperature to drop. The control circuit compares the air temperatures at the air inlet and air outlet through the thermistors and provides an indication to replace the filter when there is not a substantial difference.
U.S. Pat. No. 5,429,649 to Robin discloses a device for the detection of the clogging of an air filter. The device includes first and second sensing means, an auxiliary air duct, and an electronic circuit. The first and second sensing means are thermistors. The first sensing means is located in the air duct for measuring the air flow velocity in proximity to the air filter. The second sensing means is located in the auxiliary air duct and measures the air flow velocity therein. The electronic circuit compares the velocities measured by the sensing means to determine whether the air filter is clogged.
U.S. Pat. No. 5,668,535 to Hendrix et al. discloses a filter condition sensor and indicator that includes a heated thermistor located in a “small-by-pass air flow path” and a circuit having an indicator light. The device experiences an increase in air velocity through the “small by-pass air flow path” due to clogging of the filter. The indicator light is illuminated when the thermistor is cooled to a predetermined value. The device includes means for adjusting the current through the thermistor depending upon the setting of a multi-speed fan. The patent discloses two embodiments of the “small by-pass air flow path”. In both embodiments the by-pass is “connected to the output air flow path separate from the filter.” (See Column 3, Lines 43-45). That is, the air that flows by the thermistor is not filtered by the filter. The first embodiment is a separate compartment and the second embodiment is an extension through the filter.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for monitoring a percentage of filter life remaining of a filter assembly of filter-fan device. The filter-fan device generally include a fan that generates an airflow that passes through the filter assembly and exits through an air outlet. The invention uses a single thermistor that is operated in two separate modes for monitoring the filter assembly.
The method of operating the thermistor within a filter-fan device to monitor a percentage of filter life remaining of a filter assembly includes positioning the thermistor in the airflow within the filter-fan adjacent to the air outlet. A first current is applied to the thermistor for determining the temperature of the airflow from a first thermistor voltage. The first thermistor voltage is responsive to the first current applied to the thermistor. A second current that is greater than the first current is applied to heat the thermistor. The temperature of the thermistor is determined from a second thermistor voltage which is responsive to the second current applied to the thermistor. The percentage of filter life remaining of the filter assembly is determined from the temperature of the airflow and the second thermistor voltage. Preferably an indication of the percentage of filter life remaining of the filter assembly is provided on a visual display. Preferably the thermistor is positioned about 3 inches from the air outlet of the filter-fan.
The system for monitoring the percentage of filter life remaining of a filter assembly in accordance with the present invention includes a thermistor and a controller. The thermistor is positioned in the airflow within the filter-fan adjacent to the air outlet. The controller is configured to operate the thermistor as noted above in connection with the method of the invention. Preferably the controller includes a microprocessor having a first look-up table and a second look-up table. The first look-up table correlates the first thermistor voltage with the temperature of the airflow. The second look up table correlates the temperature of the airflow and the second thermistor voltage with the percentage of filter life remaining of the filter assembly. Preferably the system includes a display for providing a visual indication of the percentage of filter life remaining. Preferably the controller is configured to account for different line voltages to the filter-fan.
In an alternative embodiment where the filter-fan includes a multiple speed fan, the controller is.configured to determine the percentage of
Hoffmann & Baron , LLP
Lieu Julie
The Holmes Group, Inc.
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