Abstract: A monitoring device for use in monitoring the life of a filter used in the operating unit of an air conditioning system. The monitoring device has a support for use in attaching the device to the housing of the operational unit in the air conditioning system, the unit having a least one air conditioning filter holding filter material. At least one tubular arm attached to the support, each arm having an air inlet and an air outlet. The support is mounted in a manner to have a portion of the air passing through the air conditioning unit pass through the arm on the support as well. A sample filter is provided along with mounting means for removably mounting the sample filter on the arm to have air passing through the arm also pass through the sample filter. The sample filter holds the same filter material as the air conditioning filter.