Tu-4 Cup Viscosity Testing: Principle, Procedure, Standards, and Applications

Tu-4 cup viscosity testing is a widely used efflux-type viscosity measurement method for paints, coatings, inks, and related liquids, based on the Chinese national standard GB/T 1723. This article explains the working principle of the Tu-4 viscosity cup, the standard test procedure, applicable sample types, and how the measured efflux time is used for quality control and specification compliance.

Technical Overview

The Tu-4 cup (also written as "Tu-4 viscosity cup" or "flow cup No. 4") is an efflux viscometer commonly used in China for the rapid determination of the viscosity of coatings and related liquid materials. The instrument consists of a cup with a defined internal volume of 100 mL and a calibrated orifice with a diameter of 4 mm at the bottom. The test is performed by filling the cup with the conditioned sample and measuring the time, in seconds, required for the liquid to flow out under gravity until the stream first breaks.

The primary reference standard for this method is GB/T 1723, "Determination of viscosity of paints." The Tu-4 cup is functionally comparable to other international flow cups, such as the Ford cup described in ASTM D1200 and ASTM D5125, and the ISO flow cup described in ISO 2431, although the cup geometry, orifice size, and efflux time scales differ. DIN-type cups with a 4 mm orifice are also closely related in principle.

The measured efflux time is a "conditional viscosity" value, expressed in seconds, rather than a fundamental rheological parameter. For liquids exhibiting Newtonian or near-Newtonian behavior within the measured range, the efflux time can be empirically converted into an approximate kinematic viscosity in mm²/s using published conversion tables or regression formulas. The method is considered valid and reproducible for samples with an outflow time typically between 10 s and 150 s.

• Standard basis: GB/T 1723, with reference to ISO 2431, ASTM D1200, and ASTM D5125 for related flow cup methods
• Cup volume: 100 mL
• Orifice diameter: 4 mm
• Result: efflux time in seconds (conditional viscosity)
• Effective measuring range: approximately 10 s to 150 s of outflow time
• Temperature control: typically 23 °C ± 1 °C or 25 °C ± 1 °C, as specified by the product standard

Testing Samples

Paints and Coatings

Solvent-borne and waterborne architectural paints, industrial coatings, primers, topcoats, varnishes, and lacquers are the most common samples tested with the Tu-4 cup. These materials are routinely controlled by efflux time in production and incoming inspection.

Inks and Printing Materials

Printing inks, overprint varnishes, and ink vehicles can be evaluated with the Tu-4 cup when their viscosity falls within the valid efflux time window, providing a fast check of batch-to-batch consistency.

Resins, Emulsions, and Adhesives

Liquid resins, polymer emulsions, latex binders, and low-to-medium viscosity adhesives may also be tested, provided the material flows freely through the orifice without gelling, skimming, or containing coarse particles that could block the orifice.

Other Newtonian or Near-Newtonian Liquids

Any transparent or opaque liquid with suitable flow behavior, such as oils, diluents, and chemical solutions, can be measured as long as the sample is homogeneous, free of entrained air, and stable at the test temperature.

Testing Items

The core deliverable of Tu-4 cup viscosity testing is the efflux time of the sample, from which several related parameters can be reported:

• Efflux time (flow time) in seconds, measured at the specified temperature
• Conditional viscosity value expressed as "Tu-4 cup, seconds" for specification comparison
• Estimated kinematic viscosity in mm²/s (cSt), obtained through standard conversion tables or empirical formulas
• Viscosity stability assessment, comparing measured values against product technical data sheets or acceptance limits
• Temperature-annotated results, since efflux time is highly sensitive to temperature

Testing Methods

Sample and Instrument Preparation

The sample is conditioned to the specified test temperature, typically 23 °C ± 1 °C or 25 °C ± 1 °C, in a constant-temperature environment. Any skin, crust, or coarse particles are removed by filtering if required. The Tu-4 cup, its orifice, and the overflow groove are cleaned with a suitable solvent and dried so that no residue affects the flow. The cup is mounted on a stand and leveled so that the rim and orifice are horizontal, and a level bubble or spirit level is used to verify orientation.

Test Procedure

With the orifice closed by a finger or stopper, the sample is poured into the cup until it completely fills the cup and overflows into the surrounding groove, ensuring that no air bubbles are trapped. The surface is struck off flush with the rim. A timer is started at the instant the orifice is opened, and timing is stopped at the first break in the efflux stream, when the stream changes from a continuous flow to discrete drops. The elapsed time is recorded in seconds.

Repetition and Calculation

The measurement is repeated, usually on a second fill, and the reported result is the mean of the determinations. The difference between repeated determinations should not exceed the tolerance stated in the standard, commonly a small percentage of the mean value. If conversion to kinematic viscosity is required, the mean efflux time is entered into the conversion relationship or lookup table referenced by GB/T 1723.

Calibration and Quality Control

The cup is periodically verified using standard viscosity oils with certified flow times, and the orifice is inspected for wear, damage, or deposits that would alter the effective diameter. Only calibrated cups in good condition should be used, because even minor orifice deviation produces significant timing errors.

Key Precautions

• Maintain strict temperature control, since viscosity changes measurably with every degree of temperature variation
• Ensure the sample is homogeneous, bubble-free, and free of skins or agglomerates before filling
• Keep the cup perfectly level during the test to guarantee a consistent hydraulic head
• Start the timer simultaneously with the opening of the orifice and stop it at the first break of the stream, not at the last drop
• Avoid the method for highly thixotropic, fast-evaporating, or very high viscosity materials outside the 10 s to 150 s efflux window
• Clean the cup immediately after each test to prevent dried material from altering the orifice

Applications

Tu-4 cup viscosity testing is used extensively across the coatings and chemical industries because it is fast, low-cost, and directly aligned with product specifications written in "seconds" units:

• Manufacturing quality control of paints, varnishes, and coatings, ensuring each batch meets the declared viscosity range
• Incoming inspection of purchased coatings, resins, and inks before production use
• Construction and engineering site acceptance testing of architectural paints and protective coatings
• Formulation and research and development work, where dilution curves are mapped by efflux time versus thinner addition
• Automotive refinish and industrial maintenance coating shops for on-site viscosity adjustment before spraying
• Specification compliance verification in accredited third-party laboratories reporting against GB/T 1723
• Comparative studies with Ford cup (ASTM D1200), ISO flow cup (ISO 2431), and DIN cup measurements for international documentation

FAQ

What does the Tu-4 cup result actually mean?

The result is the time in seconds for 100 mL of the sample to flow through the 4 mm orifice under gravity. It is a conditional, instrument-defined viscosity value, meaningful only when the cup type, temperature, and method are quoted together with the number.

How is the Tu-4 cup different from a Ford cup or an ISO cup?

All three are efflux cups based on the same gravity flow principle, but they differ in cup geometry, orifice diameter, and calibration. Ford cups follow ASTM D1200 and ASTM D5125, ISO cups follow ISO 2431, and the Tu-4 cup follows GB/T 1723. Results in seconds are not directly interchangeable and must be converted through established correlations.

Can the efflux time be converted into kinematic viscosity?

Yes. For Newtonian liquids within the valid range, published conversion tables and empirical formulas relate the Tu-4 efflux time to an approximate kinematic viscosity in mm²/s. The conversion is an approximation and should be identified as such in test reports.

Why is temperature so important?

Viscosity is strongly temperature-dependent; most coatings become noticeably thinner as temperature rises. A deviation of only one or two degrees can shift the efflux time beyond specification tolerance, which is why the standard requires tight temperature control of both the sample and the cup.

Is the Tu-4 cup suitable for all liquids?

No. The method is intended for liquids that flow as essentially homogeneous Newtonian fluids within roughly 10 s to 150 s of efflux time. Highly thixotropic, shear-sensitive, rapidly evaporating, or particle-laden materials, and very viscous products, should be evaluated with rotational viscometers or other appropriate rheological methods instead.

How often should the cup be calibrated?

The cup should be verified at regular intervals with certified viscosity reference oils, and immediately re-checked whenever the orifice shows signs of wear or damage. Routine visual inspection and cleaning after every test help preserve calibration validity and measurement consistency.