The vibrating-wire technique has been applied to design a viscometer f
or precise measurements on gases in the temperature range 25 to 250 de
grees C at pressures from 0.1 to 40 MPa employing two Chromel wires wi
th different radii. The technique has been improved to avoid the influ
ence of higher harmonic modes and the degeneracy of perpendicular mode
s, to eliminate electromagnetic noise From the signal, and to minimize
the influence of the magnetic damping. The decrement and frequency of
the oscillation have to be determined by extrapolation to zero displa
cement, and wires with a perfectly smooth surface are needed to meet t
he requirements of the measuring theory. The viscosity measurements ar
e characterized by a precision of +/- 0.05% at ambient temperature. Co
nsidering the uncertainty of the reference data used for calibration,
the total uncertainty amounts to +/- 0.2% within the calibrated range
of the boundary-layer thickness.