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At facilities having a research-training reactor, such as the University of Missouri-Rolla Reactor (UMRR), one finds it necessary to perform a large number of rod calibrations during the course of the year. In practice rod worths are determined by measuring the reactor period created by an incremental withdrawal of the rod under calibration. Period is then related to reactivity thru the use of a publication such as the AEC publication, IDO16485. This frequent measurement of period makes it desirable to have a simple, automatic and accurate method to make such measurements. At UMRR we have designed, constructed and installed such an instrument. The instrument measures doubling time rather than period but, thru the use of an internal time base conversion, displays a four bit decimal number that is the reactor period in seconds. The instrument is simple in concept and utilizes the 7400 Series integrated circuits in the largest portion of the unit. The instrument is easy to operate and once initiated, will automatically complete the measurement of the period displaying the results. Error in the instrument can be shown to be less than 1.5%. Thus the unit meets the three requirements of simplicity, accuracy and ease of operation and in addition is moderately inexpensive, less than $120.