The Krebs viscometer, also called a Stormer viscometer, is a rotational viscometer designed specifically for measuring the “Krebs Units” (KU) viscosity of paints, coatings, adhesives, inks, and other high-solids materials. It was standardized as ASTM D562 and is the industry standard for architectural paints. Caltech India offers two models CES 370 Krebs Viscometer and CES 371 Krebs Viscometer
1. Basic Principle
It operates on the principle of a constant torque driving a paddle at a constant speed, and measuring the resistance of the fluid. Viscometers that measure torque at set speeds, the Krebs/Stormer measures the speed required to maintain a fixed driving torque. The paddle is immersed in the sample and rotated by a falling weight or spring-loaded motor. The fluid’s viscosity creates drag on the paddle. The more viscous the material, the slower the paddle rotates. The result is reported in Krebs Units KU, grams, or centipoise cP after conversion.
2. Mechanical Construction
A standard Krebs viscometer consists of:
- Drive System: Two main types exist.
- Weight-driven: A 500g weight falls and drives the spindle through a pulley system. This is the classic “Stormer” design.
- Motor-driven: A synchronous motor provides constant torque. Digital versions use a load cell to measure torque at 200 RPM.
- Measuring Paddle: The standard paddle is a T-bar or 4-blade paddle made of stainless steel. Dimensions per ASTM D562: blade length 3.5 in, blade width 0.75 in, thickness 0.020 in. The paddle is attached to a shaft that rotates in the sample.
- Sample Container: A 500 mL metal or plastic can, typically 3.25 in diameter x 4.25 in height. The can sits in a holder to prevent rotation.
- Speed Sensor / Timer: In manual units, a stopwatch measures time for 100 revolutions. In digital units, an optical encoder measures RPM directly.
- Temperature Control: Viscosity is temperature dependent. Most labs use a 25°C ±0.5°C water bath or jacketed sample cup.
3. Technical Specifications
| Parameter | Typical Value |
| Measurement Range | 40 KU to 141 KU, approx. 0.1 to 100 Pa·s |
| Speed | 200 RPM nominal for digital units |
| Torque | 54.9 mN·m constant for weight-driven units |
| Paddle Type | T-bar, 4-blade per ASTM D562 |
| Sample Volume | 400 mL to 500 mL |
| Accuracy | ±2% of reading or ±0.5 KU |
| Repeatability | ±0.5 KU |
| Operating Temperature | 5°C to 40°C ambient, sample at 25°C standard |
4. Test Procedure per ASTM D562
- Bring sample to 25°C.
- Pour 400 mL into the test container and place it in the viscometer.
- Immerse paddle to the marked depth, typically 1.25 in from bottom.
- Start rotation. For manual units, time 100 revolutions. For digital, read KU directly after stabilization.
- Report as KU at 25°C. Conversion charts convert KU to cP or Pa·s, though the conversion is non-linear because the flow is not purely Newtonian.
5. Why Krebs Units for Paints?
Architectural paints are non-Newtonian and shear-thinning. The Krebs measures viscosity at a low-to-mid shear rate that mimics brushing/rolling. 70-80 KU is typical for wall paint. Below 60 KU = too thin, runs. Above 90 KU = too thick, hard to apply. The paddle geometry creates a wide gap and turbulent flow, which is less sensitive to particle size than cone-plate geometries. This makes it robust for pigmented systems.
6. Advantages & Limitations
- Direct reading in industry-standard KU units
- Rugged, simple, and fast — 30 seconds per test
- Works well with high-solids, high-viscosity coatings
- Glass body versions available for solvent resistance
- Limitations:
- Not suitable for low-viscosity liquids <50 cP — poor accuracy
- Non-Newtonian materials require reporting at specific shear
- Shear rate is not well-defined, so not for fundamental rheology
- Cleaning is critical — dried paint on paddle causes errors
7. Modern Versions
Digital Stormer viscometers now provide direct KU, cP, and RPM readouts, data logging, and auto-temperature compensation. Some models also allow variable speed to generate flow curves.
Summary:
The Krebs/Stormer viscometer remains the QC workhorse for the paint industry because it correlates well with application properties. It trades fundamental rheological precision for speed, ruggedness, and direct correlation to how painters experience the material.
Details are available at caltechindia offers two models CES 370 Krebs viscometer and advance model CES 371 Krebs viscometer with magnetic spindle coupling for high accuracy, LCD Display & Built in sensor for temperature measurements.
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