## 3 Seal-Material Compatibility Testing
### Test Parameters and Significant Changes in Seal Properties  
Testing involves measuring the change in hardness, volume swell and shrinkage and the percentage change in tensile strength, elongation, and work function change when it is immersed in a specified test fluid, for a specified time, at a specified temperature. As an example of the scope of testing of an individual parameter, the determination of the change in the volume of the seal due to the swelling or shrinkage of the material and the change in hardness will be used as baseline tests for further judgements about the compatibility of the material. The evaluation of the change in all three dimensions of the seal is obtained on the basis of measurements of the mass of the seal before and after testing. Before testing, each test specimen must be weighed in air, \(M_{1}\), (and the value rounded to the nearest 1 mg) and then immersed (as Mohr-Westphal balance) in water, \(M_{2}\), at room temperature. This is followed by the thermal loading of the sample in the test liquid at a certain temperature (\(\pm 1\)\({}^{\circ}\)C) and for a certain time (24, 70, 100 h and more), after which, after cooling, the sample is re-weighed in air, \(M_{3}\), and in water, \(M_{4}\). At the end, the volume change is calculated according to Equation (1):  
\[\Delta V=\ \frac{(M_{3}-M_{4})-(M_{1}-M_{2})}{(M_{1}-M_{2})}\ 100\ \ \ [\%] \tag{1}\]  
wherein:  
\(M_{1}\) = the initial mass of the specimen in air [g];  
\(M_{2}\) = the initial mass of the specimen in water [g];  
\(M_{3}\) = the mass of the specimen in air after immersion [g];  
\(M_{4}\) = the mass of the specimen in water after immersion [g];  
The same specimen that was used for the volume-swelling test may also be used for the shrinkage test, provided that this specimen has not been used before for any other mechanical test, e.g., stress and strain tests. The shrinkage test is performed similarly to the swelling test, and has a similar range of measurements. Also, the change in volume due to shrinkage is calculated in a similar way--Equation (2):  
\[\Delta V=\ \frac{(M_{5}-M_{6})-(M_{3}-M_{4})}{(M_{3}-M_{4})}\ 100\ \ \ [\%] \tag{2}\]  
wherein:  
\(M_{3}\) = the initial mass of the volume-swell specimen in air [g];  
\(M_{4}\) = the initial mass of the volume-swell specimen in water [g];  
\(M_{5}\) = the mass of the specimen in air after drying out [g];  
\(M_{6}\) = the mass of the specimen in water after drying out [g];  
Apart from the tests of the change in the volume of the gasket due to swelling or shrinkage, the test of the change in the hardness of the seal material is the next important basic test. Hardness is measured in accordance with the ASTM D1414 Standard using a micro-hardness meter, whereby measurements must be made before and after the exposure of the seal material to the influence of the fluid. The change in hardness is calculated according to Equation (3):  
\[\Delta H=H_{1}-H_{2}\ \ [\%] \tag{3}\]where \(\Delta H\) is the hardness change, \(H_{1}\) is the hardness before fluid exposure, and \(H_{2}\) is the hardness after fluid exposure. The units are given as Shore A points. The differences between the original properties and the properties obtained after testing the sample in the test fluid are usually expressed as percentage changes, although both the percentage change and the actual values are often included in the test report. Typically, testing, including ageing testing, is performed at a higher temperature than the gasket will be exposed to during service. In order to evaluate the obtained result, it is definitely necessary to know the values of the permitted changes. After all, this is the only way we can judge whether the values obtained after measurements are within some suitable limits.