Spectrovisc Q3000 Series Viscometer – A Portable Kinematic Oil Analysis Solution for Field-Based Users
1. Traditional Viscometer Theory and Design
Current viscosity measurement techniques rely heavily on the use of capillary, cone and plate, and concentric cylinder viscometers. These devices are mainly limited to the laboratory setting and contain obstacles to portability. While the capillary viscometer suffers from difficult and lengthy procedures for calibration, cleaning, and temperature control, the rotational viscometer is hindered by its rotating parts and delicacy. Higher sensitivity viscometers have since been developed based on differential or light scattering methods, but these are expensive and laboratory based [1-4].
Some commercial instruments have been developed to address a need for portable viscosity measurement, especially where it is essential to determine the status of critical fluids in real-time. Such viscometers include attempts at miniaturization of the differential  and rotational viscometers [5-7]. Although these devices reduce sample volume, certain components remain complicated and costly, posing a challenge for their widespread adoption.
Other devices and methods have recently developed based on MEMS technology, including membrane oscillation frequency measurement [8-10|. acoustic wave measurement [11 ], the piezoelectric actuated cantilever  and the shear resonator . Despite requiring reduced sample volumes, many of these devices lack temperature control and are not kinematic in nature, so may not yield comparable results.
2. SpectroVisc Q3000 Series Viscometer Theory and Design
The SpectroVisc Q3000 Series viscometer design includes an upper sample-loading well, microchannel. and temperature control electronics to measure fluids at a constant temperature of 40°C. Two models are available: the Q3000 which measures viscosity over the range 10-350 cSt and the Q3050 viscometer with a range of 1 -700 cSt. The SpectroVisc Q3050 also calculates oil viscosity at 100°C from the 40°C measurement with the input of the Viscosity Index for the fluid.
Operation of the device is simple; after loading ~60ul oil into the upper well of the chip, gravitational force causes the fluid sample to flow down the microchannel where a combination of emitters and detectors in the IR range detects its rate of progression. It requires no user calibration, temperature measurement, or density analysis. This viscometer operates as a Hele-Shaw cell, where Stokes flow is present between two parallel plates. The distance between plates is necessarily small relative to the width and height of the plates. As depicted in the schematic diagram of Figure 1, the presence of only two parallel plates causes the microfluidic device to be unbounded, meaning that the fluid is exposed to air on two sides.
The unbounded microchannel is very advantageous for cleaning; you just wipe the microchannel surfaces after separating the two parallel plates to clean the device. The optical detection method, where LEDs positioned on the one side of the microchannel and respective photodiodes on the other side are not obstructed by side walls, is also advantageous.
Although overflow of the microchannel might have been a problem based on the absence of side walls, surface tension generates a concave meniscus between oil and air. as seen in Figure 2. To have a positive pressure that forms this concave meniscus requires an oleophilic material.