Illustration of blood cells flowing through vessels being measured by a Deer Rheometer

Unlock Your Blood's Secrets: The Variable Shear Stress Viscometer Explained

Beau Callahan in Health & Wellness August 2025 • 4 min read.

"Dive into the world of blood viscosity and discover how a specialized viscometer can reveal critical insights into your health."

Understanding blood viscosity, or how easily blood flows, is crucial for grasping various health conditions. Unlike simple fluids, blood's viscosity changes under different forces, a property known as non-Newtonian behavior. This is why specialized instruments called rotational viscometers are essential for accurate measurements. These devices ensure a constant shear rate across the fluid, providing a more precise analysis compared to linear or capillary viscometers.

The core principle behind viscosity measurement involves calculating the force applied to a material divided by the resulting deformation or flow. The force is the 'shear stress,' and the deformation is the 'shear rate.' In a rotational viscometer, shear stress is directly proportional to the angular velocity, or rotation, of the material. By knowing one term and measuring the other, viscosity can be accurately determined.

Traditionally, viscometers apply a known rotation and measure the resulting stress. However, this approach can mask the 'yield stress,' the force needed to overcome blood's inertia and initiate flow. A more insightful method involves applying a known stress and measuring the resulting rotation, allowing for direct determination of yield stress.

The Deer Rheometer: A Window into Blood's Viscoelastic Properties

Illustration of blood cells flowing through vessels being measured by a Deer Rheometer

The Deer Rheometer is designed to apply a 'known stress' and measure the resulting rotation, ensuring operation only when the set stress is met. It uses an electronically controlled induction motor and an air-supported bearing system to minimize friction. This setup reduces any interference, maintaining the precision of the measurement. This system allows for a direct determination of the yield stress, a critical factor in understanding blood flow dynamics.

Torque settings on the Deer Rheometer are controlled by two calibrated potentiometers, covering ranges from 0 to 9.8 x 10^-4 Nm and 0 to 9.8 x 10^-5 Nm. The shear stress generated is directly proportional to the applied torque. Angular velocity, and therefore shear rate, is measured using either a constant displacement or a continuous rotation system. The continuous rotation system outputs a signal proportional to the shear rate, useful for measuring shear rates outside the viscoelastic region. The displacement system, utilizing an eccentric cam and proximity transducer, is particularly suited for studying elasto-viscous and pseudoplastic properties of blood, capturing both positive and negative changes in flow rate.

Modifications to improve precision include:

Mechanical Micrometer: Measuring the end gap between the cup and cylinder, or the cone and plate.
Transducer Replacement: Enhancing accuracy to better than ±0.005mm.
Thermistors: Monitoring temperature at shear faces.
Air Pressure Controller: Giving constant pressure at air bearing.

Performance testing showed a coefficient of variation of 2% for silicon oils and distilled water over a shear stress range of 0.00865 to 8.65 Nm and a shear rate range of 5 to 500 s^-1. The normal range for blood viscosity, compiled from 20 donors, aligns with other research findings. The yield stress is determined by identifying the shear stress required to initiate continuous flow. Sub-yield stress forces produce deformation that reverts to the original position once the stress is removed. Applying stress beyond the yield value results in permanent deformation and flow, with an elastic response upon stress removal.

The Clinical Significance of Low Shear Rate Analysis

In conclusion, the Deer Rheometer excels at analyzing blood at low to zero shear rates, which is particularly relevant for patients with hyperviscosity states who suffer from stasis in the microcirculation. Determining yield stress is critical, and although values can be derived by extrapolation, mathematical interpretations introduce uncertainty. Devices like the Haake CV100 and Contraves 100 struggle to approach these low-stress values, but the Contraves low shear 30 offers a potential comparison point. Addressing air bearing biasing system issues is key to making the Deer Rheometer a leading tool in biorheology.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.3233/ch-1982-2415, Alternate LINK

Title: A Variable Shear Stress Viscometer

Subject: Physiology (medical)

Journal: Clinical Hemorheology and Microcirculation

Publisher: IOS Press

Authors: P. Davenport, S. Roath

Published: 2016-12-22

Everything You Need To Know

What is blood viscosity, and why is it important to measure it accurately?

Blood viscosity refers to the measure of how easily blood flows, a crucial factor in understanding various health conditions. Unlike simple fluids, blood exhibits non-Newtonian behavior, meaning its viscosity changes under different forces. To accurately measure this, rotational viscometers are used to ensure a constant shear rate across the fluid, providing a more precise analysis than linear or capillary viscometers.

How does the variable shear stress viscometer work to measure blood viscosity?

The variable shear stress viscometer calculates viscosity by determining the force applied to a material divided by the resulting deformation or flow. The 'shear stress' represents the force, and the 'shear rate' indicates the deformation. In a rotational viscometer, shear stress is directly proportional to the angular velocity of the material. By knowing one term and measuring the other, viscosity can be precisely determined. The Deer Rheometer applies a known stress and measures the resulting rotation, which allows for direct determination of yield stress.

What is 'yield stress' in the context of blood viscosity, and why is it important to measure it accurately?

The 'yield stress' represents the force needed to overcome blood's inertia and initiate flow. Measuring yield stress is important for understanding how blood behaves at very low flow rates, such as in the microcirculation. The Deer Rheometer is specifically designed to measure yield stress by applying a known stress and measuring the resulting rotation. Instruments like the Haake CV100 and Contraves 100 may struggle to accurately measure these low-stress values, highlighting the Deer Rheometer's significance.

What are the key features and components of the Deer Rheometer that make it suitable for measuring blood viscosity?

The Deer Rheometer uses an electronically controlled induction motor and an air-supported bearing system to minimize friction and ensure precision. Torque settings are controlled by calibrated potentiometers, and shear stress is directly proportional to the applied torque. Angular velocity, and therefore shear rate, is measured using either a constant displacement or a continuous rotation system. Modifications such as a mechanical micrometer, transducer replacement, thermistors, and an air pressure controller further enhance precision.

What is the clinical significance of low shear rate analysis using the Deer Rheometer, especially for patients with hyperviscosity?

The Deer Rheometer is particularly useful for analyzing blood at low to zero shear rates, which is highly relevant for patients with hyperviscosity states who experience stasis in the microcirculation. Determining yield stress is critical in these conditions. While yield stress values can be derived by extrapolation, mathematical interpretations can introduce uncertainty. Addressing air bearing biasing system issues is crucial for making the Deer Rheometer an even more effective tool in biorheology and enhancing our understanding of blood flow dynamics in various health conditions.