The Tu-1 cup viscosity test is a flow-cup (efflux viscometer) method widely used in the coatings industry to determine the conditional viscosity of paints, varnishes, and related liquid products. Specified in the Chinese national standard GB/T 1723, the test measures the time required for a defined volume of sample to discharge through a calibrated orifice, offering a fast, low-cost, and highly repeatable quality-control indicator. This article explains the principle, apparatus, operating procedure, applicable materials, and typical applications of Tu-1 cup viscosity testing.
Technical Overview
The Tu-1 viscosity cup (also written as T-1 flow cup) is a gravity-type efflux viscometer defined in the Chinese national standard GB/T 1723 for the determination of paint viscosity. In this method, a sample is poured into the cup, and the time required for the liquid to flow out through a calibrated orifice is measured with a stopwatch. The outflow time in seconds is reported as the "conditional viscosity" of the material, providing a simple and reproducible single-point viscosity index for quality control.
Physically, the Tu-1 cup consists of a smooth-walled metallic vessel with an upper cylindrical section, a lower conical section, and a bottom discharge orifice with a nominal diameter of approximately 5.6 mm. The cup holds a defined volume of sample (approximately 100 mL) and must be maintained in a clean, undamaged, and calibrated condition to ensure valid results.
Compared with the more commonly used Tu-4 cup, the Tu-1 cup features a larger orifice and is intended for products of relatively higher viscosity. According to GB/T 1723, the Tu-1 cup applies to coating materials whose outflow time is not less than 20 seconds when measured with this instrument. The test is normally performed at a controlled temperature of 25 ± 1 °C, because viscosity is strongly temperature-dependent.
As with other flow-cup methods such as ISO 2431 (flow time by use of flow cups) and ASTM D5125 (Ford viscosity cups), the Tu-1 cup measures conditional viscosity rather than absolute rheological properties. It is best suited to Newtonian or near-Newtonian liquids; results for strongly shear-thinning or thixotropic products depend on the sample preparation and history, and such materials require standardized pre-treatment before measurement.
- Measured quantity: outflow time (conditional viscosity), expressed in seconds (s)
- Governing standard: GB/T 1723
- Test temperature: 25 ± 1 °C
- Applicable range: outflow time not less than 20 s
- Orifice: calibrated bottom hole of nominally 5.6 mm diameter
- Result type: relative (conditional) viscosity, suitable for comparison and quality control
Testing Samples
Solvent-Borne Paints and Enamels
Alkyd enamels, primers, anti-corrosion paints, industrial finishes, and other solvent-borne coating products are typical candidates for Tu-1 cup testing. These materials usually exhibit near-Newtonian flow behavior after proper mixing and thinning, which makes the outflow-time measurement stable and meaningful for batch control.
Varnishes, Lacquers, and Clear Coatings
Transparent coatings such as varnishes, nitrocellulose lacquers, and wood finishes are frequently characterized with flow cups. The Tu-1 cup is selected when the product viscosity is high enough that thinner cups would produce excessively long flow times or an unsteady stream.
Inks, Adhesives, and Related Liquids
Printing inks, liquid adhesives, resins solutions, and other free-flowing chemical products may also be evaluated with the Tu-1 cup, provided they are homogeneous and free of coarse particles. Regardless of sample type, the specimen must be homogeneous, free of skins and large particles (filtered through a fine mesh if necessary), free of entrained air bubbles, and conditioned to the specified test temperature before measurement.
Testing Items
The primary testing item is the outflow time of the sample at 25 ± 1 °C, expressed in seconds, which directly represents the conditional viscosity of the material. Depending on customer or specification requirements, the following deliverables are typically included in a test report:
- Outflow time of each determination and the arithmetic mean, in seconds
- Test temperature and humidity conditions during measurement
- Identification of the cup used (Tu-1) and its calibration status
- Sample preparation steps, such as filtration or stirring history
- Optional conversion of outflow time to approximate kinematic viscosity using recognized conversion tables or empirical formulas
- Assessment of conformity against the product specification or technical data sheet limits
Testing Methods
Apparatus and Test Conditions
The test requires a calibrated Tu-1 viscosity cup, a precision thermometer, a stopwatch or electronic timer readable to 0.1 s, a leveling platform, and a constant-temperature environment or water bath capable of holding the sample and cup at 25 ± 1 °C. The cup interior and orifice must be clean, dry, and free of any residue, coating skin, or mechanical damage, since even minor deposits or burrs at the orifice will alter the flow time.
Step-by-Step Procedure
- Condition the sample, the cup, and the accessories to 25 ± 1 °C, and stir the sample gently to ensure homogeneity without entraining air.
- Filter the sample through a suitable fine mesh if skins or coarse particles are present, and allow any bubbles to rise and escape.
- Mount the cup level and close the bottom orifice with a finger or stopper.
- Pour the sample into the cup in a thin, continuous stream along the wall until the cup is completely full, taking care to avoid air bubbles.
- Level the surface with a glass plate or scraper so that the cup contains exactly its defined volume with no meniscus above the rim.
- Release the orifice and start the timer simultaneously; keep the cup level and do not disturb the instrument during the test.
- Stop the timer at the moment the liquid stream first breaks at the orifice, and record the outflow time to the required precision.
- Perform at least two determinations on fresh sample portions and verify that the results agree within the repeatability tolerance allowed by the standard.
Result Expression and Repeatability
The result is reported as the mean of the replicate determinations, expressed in seconds at 25 ± 1 °C. Duplicate values should agree within a few percent of each other; a larger discrepancy indicates operational problems such as temperature drift, bubbles, partial orifice blockage, or non-uniform sample preparation, and the test should be repeated. Outflow time may additionally be converted to an approximate kinematic viscosity value through recognized conversion relationships when a rough comparison with other viscosity units is required.
Precautions and Sources of Error
- Temperature control is critical; a deviation of only one degree can shift the flow time measurably.
- Air bubbles entrained during pouring shorten the measured time and scatter the results.
- A dirty, scratched, or deformed orifice changes the effective flow area and invalidates the measurement.
- Evaporation of fast solvents during or before the test increases the apparent viscosity; testing should be performed promptly.
- Strongly thixotropic or shear-thinning materials require a defined, standardized stirring and resting schedule to obtain comparable values.
Applications
Tu-1 cup viscosity testing is used throughout the coatings value chain as a rapid, equipment-light quality-control tool. Typical applications include:
- In-process quality control in paint and coating production lines, ensuring batch-to-batch consistency of viscosity
- Incoming inspection of purchased coatings, resin solutions, and semi-finished materials against supplier specifications
- Adjustment of application viscosity before spraying, brushing, dipping, or roller coating, including controlled thinning with the correct solvent
- Verification of conformity with product technical data sheets and contractual viscosity limits
- Formulation screening and comparative evaluation in research and development laboratories
- Quality monitoring in architectural paints, wood coatings, industrial finishes, automotive refinishing materials, protective and marine coatings, and printing inks
Accredited third-party laboratories routinely perform Tu-1 cup viscosity tests as part of coatings conformity assessment, providing independent verification of viscosity values for buyers, manufacturers, and regulatory purposes.
FAQ
What is the difference between the Tu-1 cup and the Tu-4 cup?
Both cups are specified in GB/T 1723 and operate on the same efflux principle, but the Tu-1 cup has a larger discharge orifice (approximately 5.6 mm) and is intended for relatively viscous products with outflow times of not less than 20 seconds, whereas the Tu-4 cup has a smaller 4 mm orifice and is generally used for lower-viscosity materials. The two cups are not interchangeable, and results from one cannot be directly substituted for the other without an established conversion.
Which standard specifies the Tu-1 cup viscosity test?
The Tu-1 cup method is specified in the Chinese national standard GB/T 1723, which covers the determination of paint viscosity using flow cups. Internationally comparable flow-cup methods include ISO 2431 and ASTM D5125, although these use different cup geometries and temperatures, so their results are not directly equivalent.
At what temperature should the test be performed?
The test is performed at 25 ± 1 °C according to GB/T 1723. Because viscosity decreases as temperature rises, the sample, cup, and environment must all be conditioned to the specified temperature before measurement, and the temperature must be recorded in the report.
Can Tu-1 cup results be converted into dynamic viscosity units?
Flow cups measure conditional viscosity, not absolute viscosity. Approximate conversions from outflow time to kinematic viscosity (mm²/s) are available through recognized empirical tables and formulas, but any converted value should be treated as an estimate. For precise absolute or rheological characterization, a rotational viscometer or rheometer should be used.
When should the Tu-1 cup be chosen instead of other flow cups?
The Tu-1 cup is preferred when the material is too viscous for smaller-orifice cups, that is, when its Tu-1 outflow time remains at or above 20 seconds and when specifications are written in terms of Tu-1 cup seconds. For thin, low-viscosity liquids, a smaller-orifice cup such as the Tu-4 cup is more appropriate.
How are bubbles and particles in the sample handled?
The sample should be stirred gently to achieve homogeneity, filtered through a fine mesh to remove skins or coarse particles, and allowed to stand briefly so that entrained air escapes. Pouring slowly along the inner wall of the cup also minimizes bubble formation during filling.