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TruGap™ (US-Patent 6,499,336), developed by Anton Paar, controls and adjusts the rheometer’s actual measuring gap of specific parallel-plate and cone-plate measurement geometries. It is a temperature-independent method, which recognizes the smallest changes in the measuring gap and immediately adapts its size to the desired constant value.


The accuracy of gap setting is essential for reliable rheological measurements. In simplified terms: In rheological tests with significant temperature changes, the gap size increases or decreases due to thermal expansion or shrinkage of the measurement geometry. Usually, this temperature-related gap dimension change is compensated for by using an automatic gap control (AGC) function. AGC corrects the gap mathematically, according to a thermal expansion coefficient, which depends basically on the material of the measuring geometry and the heating rate. For accurate results in temperature sweeps, the expansion coefficient must be determined for every test in advance – a time-consuming procedure. If the expansion coefficient is not exactly calibrated, the instrument sets wrong gap sizes, which leads to incorrect evaluation of rheological properties. Alternatively, parallel-plate and cone-plate measurement geometries from Anton Paar can be equipped with the TruGap™ feature. TruGap™ is a temperature-independent method, which recognizes the smallest changes in the measuring gap and immediately adapts its size to the desired constant value.

Working principle

TruGap™ measuring systems are based on a magnetic induction principle. An AC current flows through the primary coil in the lower plate, which induces a voltage in the secondary coil, since the circuit is closed by an iron disk in the upper measuring plate. The magnetic field strength and thus the induced voltage at the secondary coil change with the variation of the gap between the upper and lower measuring system: A larger gap means lower induced voltage, while a smaller gap leads to higher induced voltage. Based on this working principle, the real gap size is consistently measured and adjusted which is a major benefit for measurement accuracy and documentation. In addition, TruGap™ helps to save time since determination of only one zero gap at any temperature is required.  

Figure 1 Scheme for the working principle of TruGap™

Measurement example (temperature sweep)

The following figure depicts temperature-dependent viscosity curves of silicone oil measured with a cone-plate geometry at a constant shear rate. For this experiment, three different gap control settings (1 – no gap control (red curve), 2 – AGC (blue curve) and 3 – TruGap™ (green curve) were used.

Figure 2 Temperature-dependent viscosity curves of silicone oil using various gap control settings

It can be clearly shown that the viscosity values differ significantly at elevated temperatures. In this connection, no gap control reveals the highest viscosity values at 90 °C because the temperature-induced expansion of the measuring system is not corrected, and consequently the changed gap dimension is not considered for the calculation of the viscosity values. The most precise values can be achieved with the TruGap™ setting, revealing the advantages of the in situ measurement/compensation working principle compared to the mathematical correction of the AGC. AGC in the default case is based on a steady temperature and can therefore be faulty with temperature ramps. Find a detailed description of this experiment in the following application report: TruGap™: Direct Measurement of the Real Gap