Solvent-free Portable Viscometer for Accurate Kinematic Viscosity Measurements

This article discusses how using the hand-held, solvent-free SpectroVisc Q3000 Series in the field delivers accurate kinematic viscosity measurements immediately, even when compared to conventional laboratory viscometers. Using new solvent-free technology, the SpectroVisc Q3000 Series delivers kinematic viscosity measurements with the need for calibration, density verification, and temperature measurement.

This article first describes the design of the SpectroVisc Q3000 Series and then explains how that design performs compared to conventional laboratory viscometers.

Traditional Viscometer Theory and Design

Existing viscosity measurement methods rely largely on the use of capillary, cone and plate, and concentric cylinder viscometers. These instruments are mainly limited to the laboratory settings and have obstacles relating to portability. While difficult and lengthy procedures for calibration, cleaning, and temperature control are the downsides of the capillary viscometer, rotating parts and delicacy hinder the rotational viscometer.

Although higher sensitivity viscometers have since been developed based on differential or light scattering techniques, these instruments are expensive and aboratory based.

Some commercial devices have been developed to meet the requirement for portable viscosity measurement, particularly where it is important to identify the status of critical fluids in real time. Such viscometers include efforts to miniaturize differential and rotational viscometers. Although these instruments minimize sample volume, some components remain complex and costly, challenging their widespread adoption.

Other instruments and techniques have recently been developed based on MEMS technology, including acoustic wave measurement, membrane oscillation frequency measurement, the shear resonator, and the piezoelectric actuated cantilever. Despite the need for reduces sample volumes, many of these instruments lack temperature control and are not kinematic in nature, so they may not deliver comparable results.

SpectroVisc Q3000 Series Viscometer Theory and Design

The SpectroVisc Q3000 Series viscometer design features an upper sample loading well, temperature control electronics, and microchannel to measure fluids at a constant temperature of 40 °C. At present, two models are available: the Q3000 with a viscosity measurement range of 10 - 350 cSt and the Q3050 viscometer with a range of 1 - 700 cSt.

The SpectroVisc Q3050 viscometer also measures 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 ~60 µl oil into the upper well of the chip, the fluid sample flows down through the microchannel by gravitational force, where the rate of progression of the sample is detected by a combination of emitters and detectors in the IR range. This instrument does not require user calibration, density analysis, or temperature measurement.

This viscometer operates as a Hele-Shaw cell, where Stokes flow exists between two parallel plates. The distance between plates is necessarily small relative to the height and width of the plates. As shown in the schematic diagram in Figure 1, the presence of only two parallel plates causes the micro fluidic device to be unbounded, which means that the fluid is exposed to air on two sides.

Figure 1. Micro fluidic kinematic viscometer - Schematic

The unbounded microchannel is beneficial for cleaning. Users can clean the instrument by simply wiping the microchannel surfaces after separating the two parallel plates. The optical detection technique, where LEDs placed on the one side of the microchannel and respective photodiodes on the other side are not hindered by side walls, is also advantageous.

Although the absence of side walls might cause overflow of the microchannel, surface tension forms a concave meniscus between air and oil, as shown in Figure 2. An oleophilic material is required to have a positive pressure that forms this concave meniscus.

Figure 2. Concave meniscus - Top view

The laminar flow condition dictated by the small gap between plates ensures that the flow can be modeled as existing only in the vertical direction. At steady state, viscous and gravitational forces are balanced under laminar flow conditions so that where u is velocity, µ is dynamic viscosity, g is gravitational acceleration, and ρ is fluid density.

From that, it is possible to determine the kinematic viscosity of the fluid using the average velocity, where U is the average velocity, d is the channel depth, and g is the gravitational acceleration.

Here the dx2 term is neglected because the microchannel has a straight geometry and the fluid movement is due to only gravitational force. This one dimensional equation is not valid near the funnel region due to transient effects of viscous forces balancing gravitational force. However, these effects are avoided by placing the optics sufficiently down the microchannel.

To successfully operate the device as a Hele-Shaw cell depends on the aspect ratio of the microchannel being large enough. However, hydrostatic considerations must be taken into account because of the unbounded design. If the hydrostatic pressure by the oil is higher than the opposing pressure caused by surface tension, the fluid will overflow through the unbounded sides.

To maximize surface tension, aluminum is used as a microchannel material because it can be easily machined and creates a small contact angle with the oils being investigated.

For instance, the contact angle between the aluminum surface and engine oil is 2.73 degrees and engine oil surface tension is approximately 31 mN/m. The surface tension induced pressure value at the unbound surface is where R1 is radius of meniscus (half of microchannel depth; 50 µm) and R2 is infinite (the plate width in relative terms is very large).

It reasons that 620 Pa is the maximum hydrostatic pressure that can be held by the surface tension when two aluminum plates are 100 microns or 100 µm apart. Therefore, the microchannel’s maximum length is based on the earlier calculation, as well as the variety in surface tension and contact angle among oils. In the Q3000 series, the microchannel length used is 42 mm.

Figures 3 and 4 show the two aluminum plates produced by an ultra precision computer machining system, and how they attach to a hinge that allows easy opening and closing.

Figure 3. Aluminum plates with hinge

Figure 4. LEDs and photodiode positions

The fluid passing between a photodiode and an LED causes a decrease in the photodiode voltage. Using the time points that mark these voltage drops, the oil’s average velocity is calculated from the elapsed time between photodiode 1 and 2 as well as photodiode 2 and 3.

Then, the average velocity is applied in Equation 2 to determine a kinematic viscosity for the measured sample. Two Resistance Temperature Detectors (RTDs) embedded within the aluminum plates allow a custom designed proportional-integral-derivative (PID) controller coupled to a heating element to effectively maintain the temperature at 40 °C.

References

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This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.