Verification of the TMA force - METTLER TOLEDO

Verification of the TMA force

Introduction

In thermomechanical analysis, the length of a sample is measured as a function of its temperature while the sample is subjected to a defined force. TMA measurements therefore require careful adjustment of the measured temperature, the sample length and the force exerted on the probe.

The force adjustment in the METTLER TOLEDO TMA/SDTA84x utilizes a 51 g adjustment weight incorporated in the instrument. The force adjustment is performed during the instrument installation and does not normally need to be checked afterward.

Recently, several customers have asked whether and how the force adjustment of our instrument can be verified in particular according to the ASTM E 2206-02 test method “Standard Method for Force Calibration of Thermomechanical Analyzers”.

In this article, we show how this can be done and present the results we obtained.

 

Force Calibration According to ASTM E 2206-02

Experimental Details and Results

The measurements presented here were performed with a TMA/SDTA840 and a TMA/SDTA841e using the ball-point probe. The ASTM standard specifies the use of a calibration weight between 50 and 100 g for verification of the force adjustment. This means that the internal adjustment weight of 51 g in the TMA/SDTA84x can be used.

To perform the force calibration, the internal adjustment weight is first removed from the instrument. This causes the probe to be forced upward with a force corresponding to that of the internal adjustment weight – it lifts off the sample support. If a downward force equal to that exerted by the adjustment weight is now generated by the electromagnetic force generator, the probe should once again sink down onto the sample support. According to ASTM E 2206-02, this force, F, is calculated according to the equation:

F = F a M

where 

M     is the calibration weight,

a      is the acceleration due to gravity at the equator at sea level, a = 9.8065 m/s2, and

f       is a correction factor with takes into account the height and the latitude of the location of the TMA/SDTA84x. For example for Zürich, f is 0.9997. 

In the experiment, the force exerted on the probe was increased in steps of 1 mN from 492 mN to 502 mN. The corresponding results are shown in Figure 1. If the force exerted on the probe is greater than 501 mN, the probe rests on the sample support. 

 

This force is therefore necessary to generate a force equivalent to the adjustment weight of 51 g (499.98 mN) that has been removed. 

Alternatively, you can start with a force that is greater than that exerted by the adjustment weight. At the beginning of the experiment, the probe rests on the sample support. The electromagnetic force acting on the probe is now reduced in steps until the probe lifts off the sample support (see Figure 2). 

This force should once again correspond to that of the adjustment weight calculated using the above equation. The forces observed in the two experiments agree well with each other. The resolution of the force with the TMA840 is 1.3 mN. This means that the force exerted on the probe cannot be determined to better than 1.3 mN.

Conclusions

The force adjustment of the TMA/SDTA84x can very easily be verified according to ASTM E2206-02. To do this, the 51 g adjustment weight built in the instrument is first removed from the instrument. The electromagnetic force necessary to generate the weight equivalent to the adjustment weight is then measured. For METTLER TOLEDO TMA instruments, the force calibration constant, S, lies between 0.998 and 1.002. This yields a percent conformity, C, of 0.2%, which according to ASTM E2206-02 means that the instrument is in conformance.

 Verification of the TMA Force | Thermal Analysis Application No. UC 277 | Application published in METTLER TOLEDO Thermal Analysis UserCom 27