This paper describes an improvement in the method of setting vibration amplitudes for calibrating vibration pickups by the disappearance of an interferometric fringe pattern. In the usual method, one plate of an interferometer is stationary, the other vibrates with the pickup, and the fringe pattern disappears at zeros of the Bessel function J0[' (4•r//)/X-], where//is the amplitude of vibration and X is the wavelength of light used. In the improved method, the fringes are observed by a photomultiplier, the previously stationary plate is vibrated at a modulating frequency much lower than the calibrating frequency applied to the pickup, and the signal from the photomultiplier, filtered at the modulating frequency, has minima at the zeros of J0['(4•r•)/X-]. Observation of the nulls on a meter allows faster calibration with greater precision and less observer fatigue.
This paper analyses the data having been gathered from interlaboratory comparisons of passive radon instruments over 10 y with respect to the measurement accuracy. The measurement accuracy is discussed in terms of the systematic and the random measurement error. The analysis shows that the systematic measurement error of the most instruments issued by professional laboratory services can be within a range of ±10 % from the true value. A single radon measurement has an additional random measurement error, which is in the range of up to ±15 % for high exposures to radon (>2000 kBq h m(-3)). The random measurement error increases for lower exposures. The analysis especially applies to instruments with solid-state nuclear track detectors and results in proposing criteria for testing the measurement accuracy. Instruments with electrets and charcoal have also been considered, but the low stock of data enables only a qualitative discussion.
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