A digital readout-type period meter has been used to monitor period information during critical mass experiments. Its range of measurement extends from to 1000 sec. It can be used to measure the slope of any linearly changing signal. By using a logarithmic amplifier in conjunction with the meter, it will measure an exponential slope. The instrument provides a greater readout range and is less susceptible to noise than the conventional differentiating operational amplifier types presently used at the Rocky Flats Plant Nuclear Safety Facility. Noise frequency periods are much shorter than the periods measured during an experiment. Typically, the experiment periods are on the order of 1 min and greater. The input stage of the instrument has a cutoff frequency allowing these typical periods to be measured, but above the cutoff frequency response decreases at 20 db per decade. Contrary to this, the conventional differentiating operational amplifier-type period meter increases in response above its cutoff frequency. The error in period readings was found to be less than 5% probable error.