  We report on measurements of relative beta-decay rates of Na-22, Cl-36,
Co-60, Sr-90, Cs-137 monitored for more than one year. The radioactive samples
are mounted in an automated sample changer that sequentially positions the five
samples in turn, with high spatial precision, in front of each of four
Geiger-M\"uller tubes. The sample wheel, detectors, and associated electronics
are housed inside a sealed chamber held at constant absolute pressure,
humidity, and temperature to isolate the experiment from environmental
variations. The statistical uncertainty in the count rate approaches a few
times 0.01% with two weeks of averaging. Other sources of error are on a
similar scale. The data are analyzed in variety of ways, comparing count rates
of the various samples on one or more detectors, and comparing count rates of a
particular sample across multiple detectors. We observe no statistically
significant variations in the ratios of decay rates, either annual or at
higher-frequency, at a level above 0.01%.
