Viscosity Measurements of Diisodecyl Phthalate Using a Vibrating Wire Instrument Operated In Free Decay Mode: Comparison with Results Obtained with the Forced Mode of Operation

Recently, several authors described measurements of liquid viscosities higher than 100 mPa.s, using the vibrating wire technique in the forced oscillation mode of operation. The use of a vibrating wire instrument to measure increasingly higher viscosities poses experimental difficulties due to the consequent decrease of the quality factor of the resonance. This problem affects both modes of operation: in the steady-state, forced oscillation mode, there is a loss of definition of the resonance curve, and in the transient, free decay mode, there is an increase of the decrement of the sinusoidal time response of the vibrating wire. In the present work, we have measured the viscosity of diisodecyl phthalate (DIDP) at 0.1 MPa with a vibrating wire of nominal radius of 190 mu m using the free decay mode of operation. The measurements covered a range of viscosities from (64 to 265) mPa.s. The results agreed with those we have obtained with a vibrating wire instrument operated in the forced mode and with the literature data, within the estimated experimental uncertainty (k = 2) of +/- 2%. The comparisons indicate that the use of the free decay mode of operation does not necessarily imply a significant increase of uncertainty of the viscosity results at moderately high viscosities.