I am an engineer and new to "instrument design" and "calibration curve" field. My project is to modify an existing colorimeter which presently measure several chemical compositions in water. Colorimeters as you know is a simple version of a atomic absorption spectro-photometer (as far as I understand) which uses only the visible light spectrum to detect optical density of a liquid and converts the absorbance into chemical concentration. I have to add new programs to detect new chemicals to the instrument.

We had some data with the new chemical (to be added in the newer version). I used LABFIT to find the best fit curve equation. Instead of absorbance we are measuring the RATIO (Blank/Specimen, photo sensor voltage ratios). My questions are:

1) The best fit curve I find doesn’t start at Ratio=1.0, concentration=0.0, so what I did, I added an offset correction to the equation. Is is OK? or I have to find an equation which

gives concentration=0.0 at RATIO=1.0 accurately?

2) Can I divide the curve in multiple sections and use different equations for those sections? It seems that it fits the experimental data more accurately with less error. But I would like to know can I do it? Or only one equation is to be used only changing the constant parameters of the equation in different section?

3) How do you usually find the best fit curve and which type of curve (the degree of the equation) is suitable for these sort of cases? I mean which software are good for accurate curve fitting.

4) Is there any software available which can make multi-sectioned curves and automatically calculates parameters in different sections to best fit the curve (this question is related to Question no 2, I mean to ask that using same equation finding the constants of the equation in user defined section number and region)? I need this to understand Hardware Calibration and find hardware calibration parameters.

Please suggest me any links for this or related topics if you know.

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