Photoacoustic detector with long term drift compensation

A method of compensating a photoacoustic detector comprising:

• providing a photoacoustic detector;
• calibrating the detector, and obtaining an initial transfer function indicative of detector response;
• storing the transfer function;
• exposing the detector to airborne ambient conditions and determining at least one gas concentration;
• obtaining another transfer function for the detector subsequent to the exposing;
• comparing the initial and the another transfer functions;
• responsive to the comparing, establishing at least one compensating indicium for the detector;
• and
• carrying out measurements and compensating performance of the detector for output loss from a radiant energy source and changes in an acoustic generator, carried in the detector,
• wherein the initial transfer function is based upon a measurement of an absolute signal level of the acoustic detector over time and at several frequencies that are selected based upon a fundamental performance characteristic of the acoustic detector,
• wherein the several frequencies are sequentially measured by driving the acoustic generator at a predetermined target frequency prior to measuring a response of the acoustic detector including a signal amplitude and phase of the acoustic detector over several complete cycles, and
• wherein the response of the acoustic detector to the several frequencies is compared by the control circuits with the initial transfer function of the acoustic detector to detect changes over time.

A multi-channel photoacoustic detector comprising:

• a multi-channel photoacoustic sensing cell which has first and second acoustic output channels;
• an acoustic generator coupled to the cell;
• a source of radiant energy coupled to the cell and which includes a closed loop control system to at least intermittently energize the source of radiant energy;
• and
• control circuits, coupled to the output channels, the generator and the source,
• wherein the control circuits compensate for output loss from the source and changes in the acoustic generator,
• wherein the acoustic generator is used by the control circuits to track an absolute signal level of at least one microphone over time and at several frequencies that are selected based upon a fundamental performance characteristic of the at least one microphone,
• wherein the several frequencies are sequentially measured by driving the acoustic generator at a predetermined target frequency prior to measuring a response of the at least one microphone including a signal amplitude and phase over several complete cycles, and
• wherein the response of the at least one microphone to the several frequencies is compared by the control circuits with an initial performance of the at least one microphone to detect changes over time.