#1 What exactly is the IRHD measurement?
IRHD is a hardness testing method, whereby the abbreviation “IRHD” stands for “International Rubber Hardness Degree”. In addition to the Shore hardness test, the IRHD is another way of defining the hardness of an O-ring.
Definition of the hardness of a material: The resistance of a body to the penetration of a harder material.
In the IRHD test method a small steel ball (e.g. 2.5 or 5 mm diameter) is pressed into a test specimen. The deeper the ball penetrates into the material, the softer the test specimen is. Based on the measured values, which can be between 0 (hard) and 100 (soft), the hardness value can then be read from a table (ISO 48).
#2 What are the differences between IRHD and Shore hardness?
It is very interesting to note that the IRHD hardness test is superior to the generally more popular Shore hardness measurement in many respects:
1st advantage: Shore hardness measurements may only be carried out on test specimens in the laboratory. The IRHD measurement can be carried out on finished parts (e.g. O-rings) as well as on test specimens.
In practice, users often measure the Shore hardness of their O-rings with a pocket gauge and find deviations from the specifications on a data sheet. This is because the Shore hardness can only be meaningfully measured on test specimens and under laboratory conditions.
The pocket gauges for determining Shore hardness are only intended to measure a guide value for the hardness of an O-ring in the short term. So the O-ring is more likely to have 50, 70 or 90 Shore.
With the IRHD measurement, the result of the measurement applies to both test specimens and finished parts.
2nd advantage: A Shore hardness measurement requires a 6 mm thick test specimen. An IRHD test can also be carried out with much thinner test specimens or prefabricated parts.
Conclusion: The Shore hardness measurement is the inferior measurement method compared to the IRHD measurement, especially for O-rings. However, since IRHD measuring devices are not available in pocket format, Shore hardness measurement is more popular and therefore recognized worldwide.