An evaluation test has been made of a fixed frequency, variable tension vibroscope in which the gauge length was established by passing the fiber over one pin support and under the other. Two lengths of very uniform terylene fiber of linear density ∼0.95 Tex were used in tests all of which were done at 17.8°C and 65% RH. A method was developed for accurately evaluating the total mass of long lengths of filament from a succession of readings of the vibroscope taken at intervals of half of the gauge length along the fiber. The discrepancies, approximately 3 μg in both instances, between the weighed masses of 735 and 535 μg of fiber are slightly larger than the prime source of uncertainty, that of the weighings, and are both in the direction implying an underestimation of linear density by the vibroscope. This result is consistent with an error due to underestimation of the correction for stiffness of the fiber, the latter being based on an assumption that the supports behave as ideal pinned ends. Despite the elementary nature of the principle underlying the free or forced vibrations of a tensioned filament supported at its ends, its use as an instrument requires sophisticated corrections for a number of departures of real vibroscopes from the idealized model. These corrections can be quite significant in some configurations, and, as discussed in the paper, the theoretical bases for two types of correction have yet to be fully developed. For these reasons a metrological study such as this is important in order to validate the use of the vibroscope as an absolute instrument. Since the form of vibroscope employed was such that the effect of the less theoretically certain corrections were minimized, some reserve should be exercised in accepting this study as a validation for forms of vibroscope employing a variable frequency. The results should be equally applicable, however, to vibroscopes operating at a fixed frequency but with span or gauge length varied for resonance.