The curves above shows 3 Infra-red absorbtion spectra for 3 different gases, Gas 1, Gas 2 and Gas 3. In order to measure the correct concentration of  Gas 1, as an example, it is necessary to measure at 3 different wavelengths, indicated with (Optical) Filter A, Filter B and Filter C.

If Gas 1 is measured with Filter A only, the measured absorbtion will be the sum of absorbtion from Gas1, Gas 2 and Gas 3. The calculated concentration will therefore be wrong. The error is called the interference from Gas 2 and Gas 3 on Gas 1. Likewise, if Gas 1 is measured with Filter C only, the calculated concentration will be wrong, due to interference from Gas 3.

To overcome this problem, the cross-compensation algorithm has been implemented in the Photoacoustic Multi-gas Monitors 1312 and 1314. A filter caroussel is installed in the instrument, containing the filters A, B and C. First the sample is measured using optical filter A. The signal measured (S_A)  can be expressed as:

S_A= a_1,A x c₁+ a_2,A x c₂+ a_3,A x c₃

Where:

S_A is Microphone signal measuring with Filter A.

a_1,N is absorption of gas 1 on Filter N.

C₁ is concentration of gas 1

In a similar way, measuring the same sample with Filter B and Filter C we obtain:

S_B= a_1,B x c₁+ a_2,B x c₂+ a_3,B x c3

S_C= a_1,C x c₁+ a_2,C x c₂+ a_3,C x c₃

We now have 3 equations with 3 unknowns, c₁, c₂ and c₃. The instrument will, when selected, automatically calculate the unknown concentrations and display the corrected result. This way, the interference from other gases can typically be reduced by more than 98 %, with a proper selection of the optical filters, compared to the gases to be measured.

A similar algorithm is available for the compensation for humidity interference.

We will be happy to assist you with the proper selection of  optical filters for exactly your application