Photometer Calibration: The Case for Two-Point Calibration and Multi-Point Verification, January 25, 2024

Date: January 25, 2024

Authors:
Meng Hu, PhD, Applications Engineer, ATI
Sylvain Masset, Principal Engineer, ATI
Jim Power, Director of Engineering, ATI
Ron Adkins, President, ATI

Abstract:
Aerosol photometers are calibrated at two extreme ends of the intended concentration range. The two-point calibration has been in practice since the advent of portable photometers in the 1960s. This technical note outlines the reasons for the validity of the calibration practice both in theory and in practice. Specifically, the principle of light scattering in the Mie regime dictates that the photometric response to a polydisperse aerosol is linearly proportional to its mass concentration, and that it is on the condition of constant aerosol properties (refractive index, particle density, and particle size distribution (PSD)) that the linear relationship stands. The dictation establishes the validity of the two-point calibration. There are also practical considerations that favor the two-point calibration over a multi-point calibration, in which one or more aerosol calibration standards (i.e., standards of measurement) are obtained by diluting the aerosol used to set the span (the high end of the concentration measurement range) of the photometer. We identify and discuss two significant sources of error during multi-point calibration—one originating from the changes in aerosol PSD when adjusting aerosol concentration and the other concerning gravimetric measurements of aerosol calibration standards. Such errors are avoided (in the case of changes in aerosol PSD among multiple concentrations) or minimized (in the case of aggregating errors in each aerosol standard) in the two-point calibration, making it the preferred calibration approach in practice. To complement the two-point calibration, a verification step is strongly advised to confirm the linearity of photometric responses to aerosol mass concentrations throughout the measurement range. Herein, calibration refers to the process of adjusting the response of the device under test to an established standard of measurement, whereas verification means the comparison between the device response and the standard without adjusting the device response. The combination of the two-point calibration and multi-point verification ensures photometer performance and validity of testing results.