The fraction of light that is not absorbed is transmitted via the second optical fiber to the phototransistor, which converts light intensity to a voltage. The voltage is measured by the voltmeter. Samples to be analyzed for ammonia or phosphate are prepared using standard commercially available reagents. Because the blank voltage is set at 100 mV, the sample voltage, which is lower by the amount of light absorbed, can be interpreted directly as percent transmittance. Absorbance is determined by calculating the negative logarithm of the voltmeter reading.
Most scientific calculators have keys for determining logarithms.
They have found the FOP reliable and easy to use, requiring only minimal training. The fabrication procedures for the FOP are not difficult and could be accomplished by anyone with electronics skills and experience in using a soldering iron. The FOP is assembled on a Radio Shack circuit board. In addition to the components described earlier, the FOP utilizes a 5-volt voltage regulator, a trimming potentiometer, a precision potentiometer, and resistors. The majority of the parts are available from a fiber optics experimenters kit.