This is a hardness tester for the International Hardness Degree scale normal, IRHD-N. It was manufactured by Wallace Co in UK.

When asking Wallace about the manufacturing year, they replied that they never seen a photo of such an old instrument. They estimated the manufacturing to mid-60-ies.

The hardness tester also has dial gauges for the scales IRHD-L and IRHD-H with corresponding indenters.

The instrument measures Rubber hardness by measuring the indentation of a ball with the diameter of 2,5 mm pressed into the rubber. The total stroke is 2,5 mm, so if the indenter is pressed 1,25 mm into the test piece, the instrument shows a hardness of 50 IRHD.

The table where the test piece is placed is vibrating to eliminate eventual friction in the instrument.

The IRHD hardness instrument was developed to get a more accurate reading of rubber hardness than the Shore durometer. To get a more accurate result a constant load is used, the instrument is mounted on a stand and the measuring time is 30 s. Compared to the Shore instrument using a spring, originally handheld and a measuring time originally specified to within 1 s.

The IRHD hardness tester was designed to show values equivalent to the Shore A scale. This is valid for natural rubber but may differ for other rubber polymers.

The IRHD hardness method was standardised very early and have the number ISO 48.

This instrument was earlier very common in the rubber laboratories.

I worked at SP between 1978 to 1989 and used this tester. Now we have it in our museum at Elastocon.

Owner history: SP, Statens Provningsanstalt (now RISE – Research Institutes of Sweden)

Göran Spetz