Filter monitoring system using a thermistor

A filter monitoring system for a filter-fan for monitoring a percentage of filter life remaining of a filter assembly, the filter-fan device having a fan that generates an airflow that passes through the filter assembly and exits through an air outlet, the airflow having a temperature, said filter monitoring system comprising:

• a thermistor positioned in the airflow within the filter-fan adjacent to the air outlet;
• a controller, the controller:
• (a) applying a first current to the thermistor;
• (b) determining the temperature of the airflow from a first thermistor voltage, the first thermistor voltage being responsive to the first current applied to the thermistor;
• (c) applying a second current to the thermistor to heat the thermistor, the second current being greater than the first current;
• (d) determining the temperature of the thermistor from a second thermistor voltage, the second thermistor voltage being responsive to the second current applied to the thermistor;
• and
• (e) determining the percentage of filter life remaining of the filter assembly from the temperature of the airflow and the second thermistor voltage.

A filter monitoring system for a filter-fan for monitoring a percentage of filter life remaining of a filter assembly, the filter-fan device having a multiple speed fan that generates an airflow that passes through the filter assembly and exits through an air outlet, the airflow having a temperature, said filter monitoring system comprising:

• a thermistor positioned in the airflow within the filter-fan adjacent to the air outlet;
• a controller, the controller:
• (a) applying a first current to the thermistor;
• (b) determining the temperature of the airflow from a first thermistor voltage, the first thermistor voltage being responsive to the first current applied to the thermistor;
• (c) applying a second current to the thermistor to heat the thermistor, the second current being greater than the first current;
• (d) determining the temperature of the thermistor from a second thermistor voltage, the second thermistor voltage being responsive to the second current applied to the thermistor;
• and
• (e) determining the percentage of filter life remaining of the filter assembly from the temperature of the airflow, the second thermistor voltage and the speed of the fan.