In is often desirable to obtain the integral of a signal whose amplitude is rapidly fluctuating with time. For example, the signal produced by a hydrogen flame photometer to quantify the sodium content in a sodium chloride respirator quantitative fit test instrument is representative of the type of signal that is frequently recorded on a strip-chart recorder, but whose integral is tedious to calculate. A long-term precision integrator was designed that utilizes a commercially available voltage-to-frequency integrated circuit module. The integrator is capable of integrating a 0-10 V signal for time periods greater than 1 h with an overall accuracy of 0.1%. The integrator can be built from commercially available components for less than $600.