A method for calibrating a fluorometer comprising the steps of: a) providing a fluorometer configured to measure a fluorescent signal from a fluorescent marker in a sample of water from an industrial water system and determine from the fluorescent signal a concentration of a water treatment product in the sample of water, wherein a nominal concentration C o of the water treatment product corresponds to a nominal concentration C f of the fluorescent marker; b) withdrawing a water sample from the industrial water system; c) measuring a first fluorescent signal, S 1, from the water sample with the fluorometer; d) preparing a first calibration solution, comprising the steps of i) preparing a spike solution containing the water treatment product at a concentration of about P x C o and the fluorescent marker at a concentration of about P x C f, and ii) adding about 1 part of the spike solution to about N parts of the water sample; e) measuring a second fluorescent signal, S 2, from the first calibration solution with the fluorometer; f) calculating an approximate slope coefficient, K m, equal to C f / S 2 N + 1 P − S 1 N N + 1; g) measuring a third fluorescent signal, S 3, from a sample of a zero water sample having zero concentration of the water treatment product and/or fluorescent marker; h) setting a zero shift, Z 0, equal to S 3; and i) calibrating the fluorometer with the slope coefficient and the zero shift, wherein calibrating the fluorometer comprises calculating an approximate current fluorescent marker concentration, C m, equal to K m x (S x - Z 0), wherein S x is a current fluorescent measurement. a) providing a fluorometer configured to measure a fluorescent signal from a fluorescent marker in a sample of water from an industrial water system and determine from the fluorescent signal a concentration of a water treatment product in the sample of water, wherein a nominal concentration C o of the water treatment product corresponds to a nominal concentration C f of the fluorescent marker; b) withdrawing a water sample from the industrial water system; c) measuring a first fluorescent signal, S 1, from the water sample with the fluorometer; d) preparing a first calibration solution, comprising the steps of i) preparing a spike solution containing the water treatment product at a concentration of about P x C o and the fluorescent marker at a concentration of about P x C f, and ii) adding about 1 part of the spike solution to about N parts of the water sample; i) preparing a spike solution containing the water treatment product at a concentration of about P x C o and the fluorescent marker at a concentration of about P x C f, and ii) adding about 1 part of the spike solution to about N parts of the water sample; e) measuring a second fluorescent signal, S 2, from the first calibration solution with the fluorometer; f) calculating an approximate slope coefficient, K m, equal to C f / S 2 N + 1 P − S 1 N N + 1; g) measuring a third fluorescent signal, S 3, from a sample of a zero water sample having zero concentration of the water treatment product and/or fluorescent marker; h) setting a zero shift, Z 0, equal to S 3; and i) calibrating the fluorometer with the slope coefficient and the zero shift, wherein calibrating the fluorometer comprises calculating an approximate current fluorescent marker concentration, C m, equal to K m x (S x - Z 0), wherein S x is a current fluorescent measurement.