Viscosity measurement of small volume liquid samples in a falling body viscometer employing compact electromagnetic actuation and sensing

Falling body (FB) methods have unique advantages in the viscosity measurement of high-temperature and high-pressure liquids because of the durability of the sensing parts and their suitability for carrying out measurements in a metal closed cylindrical chamber (CCC). However, the current methods usually use two coils fixed away from each other to sense the position and speed of the FB, which leads to a long travel distance of the FB, a large CCC, and the inability of the FB to automatically reset. This paper proposes a novel compact liquid viscosity measurement device using only one coil outside a stainless steel CCC to sense the moving speed of the iron FB and uses another coil fixed close to the sensing coil to reset the FB. Both the FB travel distance and the CCC size are very small. The stable velocity of the FB is directly measured according to the linear relationship between the impedance of the sensing coil and the position of the FB, and then the velocity is used to calculate the liquid viscosity. Viscosity tests were conducted on four standard viscosity liquids at different temperatures. The results show that for liquids with a viscosity range of 4.417 mPa·s∼88.44 mPa·s, the descent time of FBs is between 2 and 19 s. The relative error is concentrated within 5 %, and the standard deviation falls between 0.01 mPa·s and 1.46 mPa·s. Within the 95 % confidence interval ( k = 2), the expanded uncertainty of the four liquids viscosity measurement at different temperatures ranges from 0.74 mPa·s to 2.93 mPa·s, and the relative expanded uncertainty ranges from 6.32 % to 10.6 %. The measurement results have high accuracy and repeatability. Fig. 1 Viscosity measurement apparatus • A novel compact device for measuring liquid viscosity in a closed cylindrical chamber is proposed. • Velocity of falling body(FB) is proportional to coil impedance’s change rate and inversely proportional to liquid viscosity. • The FB's travel distance is short, so the chamber is small and the FB can be reset by another coil fixed close. • The relative error is concentrated within 5 %, and the standard deviation is between 0.01mPa·s-1.46mPa·s.