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  • Stephen Biss

Bandwidth of Filters Changes Over Time


Purpose:

To explain the basic methodology of IR quantitative analysis using an approved instrument.

To explain calibration.

To explain the function of the linearizer.

To explain drift over time.

To explain calibration curve.

The pencil is the sample in the sample chamber.

The paperclips are the instrument's electronic response to the transmittance of IR light through the sample chamber. An electrical signal is produced by the detector at the end of the sample chamber that has a relationship to transmittance.

The instrument has been calibrated at the factory such that the calibration curve recorded in the instrument's calibration software settings says 4 paperclips = 2 gms and 2 paperclips = 1 gm.In other words, 4 paperclips of electronic response in the detector is made to be equal to 1 gm in indication. That making of a relationship is the calibration of the instrument at the factory during a manual tweaking (5000C) or an auto calibration sequence (8000C or 5000EN).

The circuit that does that is called the linearizer.

That relationship established with a screwdriver on a 5000C or a software adjustment on the 8000C does not change until the next re-calibration at the factory or the Canadian authorized Service Centre.

But what if the electronic response changes "over time" because the filter changes colour or gets dirty? Perhaps the bandwidth changes. Perhaps the properties of the filter and the optical system change "over time". What happens if the IR light bulb dims?

The calibration curve becomes wrong. It needs changing. The instrument needs re-calibration. The adjustment require may be non-linear - it may be a different amount at each of 50, 100, 150, 200, and 250 mg/100mls. The new calibration curve relative to the old calibration curve may be a movement up, down, left, right, a stretch, or a rotation on any axis.

#bandwidth #overtime #calibrationcurve #linearizer #linearity

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