Tu-4 Cup Viscosity Measurement: Principles, Procedures, and Applications

Tu-4 cup viscosity measurement is a widely used outflow viscometer method for determining the conditional viscosity of paints, coatings, inks, and related liquids, as specified in the Chinese national standard GB/T 1723. This article explains the working principle of the Tu-4 flow cup, the standard test procedure, applicable sample types, and how the measured efflux time is used for quality control and specification compliance. It also compares the method with international flow cup standards such as ISO 2431 and ASTM D1200.

Technical Overview

The Tu-4 viscosity cup (also written as T-4 cup or "coating No. 4 cup") is a gravity-type outflow viscometer that has become one of the most common viscosity measurement tools in the coatings industry in China. It is defined and standardized in GB/T 1723, the Chinese national standard for the determination of paint viscosity. The instrument consists of a cylindrical or slightly conical cup with a precisely machined 4 mm orifice at the bottom, and the test result is expressed as the time, in seconds, required for a fixed volume of sample to flow out of the cup under gravity. This parameter is known as conditional viscosity or efflux time, and it provides a fast, low-cost, and reproducible indication of a liquid's flow behavior.

The Tu-4 cup is functionally comparable to international flow cup instruments covered by ISO 2431 (flow time by use of flow cups) and ASTM D1200 / ASTM D5125 (Ford viscosity cups), although the cup geometry, volume, and calibration differ, so results between the various cup types are not directly interchangeable. The Tu-4 method is best suited for transparent or opaque, low- to medium-viscosity liquids that are essentially Newtonian or near-Newtonian, such as ready-to-use coatings, lacquers, varnishes, and dilute resin solutions.

• Cup capacity: approximately 100 mL of test liquid
• Orifice diameter: 4 mm
• Result expression: efflux (flow-out) time in seconds
• Governing standard: GB/T 1723
• Recommended measuring range: approximately 10 s to 150 s flow time
• Reference temperature: typically 25 °C ± 1 °C, or 23 °C ± 2 °C when following ISO 2431 practice

Testing Samples

Architectural and Decorative Coatings

Water-based latex paints, primers, sealers, and solvent-borne decorative enamels are routinely checked with the Tu-4 cup. The method allows painters, site supervisors, and quality inspectors to quickly confirm that the product is at the correct application consistency before brushing, rolling, or spraying, and to control the amount of dilution added on site.

Industrial and Protective Coatings

Alkyd, epoxy, polyurethane, chlorinated rubber, and other industrial coating systems are tested during manufacturing and before application. The Tu-4 cup is used to verify consistency between production batches, to check materials after storage or transport, and to confirm that spraying or dipping viscosities fall within the technical specification.

Printing Inks and Related Fluids

Low- to medium-viscosity liquid inks, ink vehicles, overprint varnishes, and wash solvents can be evaluated with the Tu-4 cup, provided their flow time lies within the usable range of the instrument. Heavier paste inks require rotational or cone-plate viscometers instead.

Adhesives, Resin Solutions, and Other Liquids

Thin adhesives, resin solutions, lacquer thinners, and similar Newtonian liquids may also be measured. For samples containing large particles, highly thixotropic materials, or liquids with flow times outside the cup range, alternative viscosity methods such as rotational viscometry (for example, in accordance with GB/T 9751 or ISO 2884) should be selected.

Testing Items

The core deliverable of a Tu-4 cup viscosity determination is the efflux time of the sample at a specified temperature, expressed in seconds. This result is often accompanied by supporting information that makes the test meaningful and traceable:

• Flow-out (efflux) time in seconds, reported as the mean of duplicate determinations
• Test temperature and its deviation from the specified reference temperature
• Pass or fail judgment against the viscosity range stated in the product specification or technical data sheet
• Optional approximate conversion of flow time to kinematic viscosity (mm²/s) using empirical conversion tables
• Instrument verification status, including cup calibration records against standard viscosity liquids

Testing Methods

Testing is performed in accordance with GB/T 1723 using a calibrated Tu-4 viscometer, a clean flat leveling surface, an accurate thermometer, and a stopwatch or electronic timer readable to 0.1 s or better. The method is straightforward, but strict control of temperature, cup cleanliness, and filling technique is essential for accurate and repeatable results.

Sample Conditioning

The sample is brought to the specified test temperature, normally 25 °C ± 1 °C, in a constant-temperature room or water bath. If the sample contains entrained air or surface skin, it is gently stirred and, where necessary, filtered or degassed before testing. Vigorous stirring that introduces bubbles must be avoided.

Step-by-Step Procedure

• Place the clean, dry Tu-4 cup on its stand and verify that it is level.
• Block the orifice with a finger and pour the conditioned sample into the cup until it overflows slightly at the rim, taking care not to trap air bubbles.
• Scrape the surface flush with the top edge of the cup using a straight-edge glass plate to remove excess liquid and foam.
• Release the orifice and simultaneously start the timer.
• Stop the timer at the first break (interruption) in the liquid stream at the orifice, which marks the endpoint of the determination.
• Record the efflux time in seconds and the sample temperature.
• Repeat the determination on a fresh portion of sample; the two results should agree within the repeatability limits of the standard, and the mean is reported.

Instrument Verification

The cup should be periodically verified using certified standard viscosity liquids with known flow times. A cup whose verification result drifts beyond the permitted tolerance must be cleaned, inspected for orifice damage, recalibrated, or replaced. Accredited third-party laboratories maintain traceable calibration programs for flow cups, comparable to the verification principles in ISO 2431 and ASTM D1200.

Precautions and Error Sources

• Temperature deviation is the largest error source; viscosity can change by several percent per degree, so tight temperature control is mandatory.
• A dirty, scratched, or partially blocked orifice directly alters the flow time.
• Bubbles, skin, or undispersed gels in the sample cause erratic stream break and false endpoints.
• An unleveled cup changes the effective hydrostatic head and biases the result.
• Non-Newtonian or very highly viscous samples should be tested with appropriate rheological methods rather than extrapolated from cup data.

Applications

Tu-4 cup viscosity measurement is used throughout the coatings value chain, from raw material acceptance to field application. Its speed, simplicity, and low equipment cost make it the preferred routine method for production and construction environments, while rotational viscometers are reserved for research-grade rheological characterization.

• Incoming inspection of resins, binders, solvents, and semi-finished intermediates
• In-process viscosity control during dispersion, milling, letdown, and tinting operations
• Finished product release testing against internal standards and customer specifications
• On-site adjustment of application viscosity before spraying, rolling, brushing, dipping, or curtain coating
• Batch-to-batch consistency comparison and shelf-life or storage stability studies
• Compliance verification in trade disputes and third-party laboratory test reports

FAQ

What is the measuring range of the Tu-4 cup?

The Tu-4 cup is suitable for samples with flow times of roughly 10 s to 150 s. Below about 10 s the timing uncertainty becomes too large for meaningful comparison, while above 150 s the stream break endpoint becomes vague and evaporation or temperature drift can distort the result. Samples outside this range should be tested with other cup sizes or a rotational viscometer.

Are Tu-4 cup results interchangeable with Ford cup or ISO flow cup results?

No. Although the instruments work on the same outflow principle, the cup geometry, internal volume, and orifice length-to-diameter ratio differ between the Tu-4 cup, the Ford cups of ASTM D1200 and ASTM D5125, and the ISO cups of ISO 2431. Published correlation tables give only approximate conversions, and contractual specifications should always name the exact cup and standard to be used.

Why is temperature control so critical?

Viscosity is strongly temperature dependent; coatings can change viscosity by 3 % to 10 % per degree Celsius depending on composition. A sample measured at 20 °C instead of 25 °C can show a noticeably longer flow time, leading to incorrect dilution decisions or false specification failures. The sample, the cup, and the surroundings should all be at the specified test temperature.

Can the Tu-4 cup measure non-Newtonian fluids?

Only as a comparative, empirical indicator. Because flow cups apply a single, changing shear condition, they cannot characterize shear-thinning, thixotropic, or yield-stress behavior. For such materials, and for viscosity specification purposes involving non-Newtonian products, rotational or cone-plate viscometry under defined shear rates is the appropriate method.

How is the endpoint of the test determined?

The clock is stopped at the first interruption of the liquid stream emerging from the orifice, when the stream first breaks close to the cup outlet. This visual endpoint is well defined for properly conditioned samples within the recommended flow time range.

How often should a Tu-4 cup be calibrated or verified?

Cups in daily production use should be verified at regular intervals with certified reference liquids, and always after suspected mechanical damage or orifice blockage. Verification frequency is defined by each laboratory's quality system, and accredited third-party laboratories typically maintain annual traceable calibration schedules for flow cup instruments.