The hardness is determined by the difference in penetration depth of a specified dimension ball under two conditions of contact with the rubber:
- with a small initial force, and
- with a much larger final force.
The differential penetration is measured at a specified time and converted into a hardness scale value.
The IRHD Tester is equipped with a vertical plunger. The lower end of the plunger has an indenter with a ball-shaped tip of specified dimensions. The upper end of the plunger has a good electric conductive tip. The weight of the plunger and fittings attached to it provide the minor force., while a large brass weight provides the specified major force.
A dial gauge is attached to the instrument to measure the penentration depth of the indenter into the rubber.
An electrically operated buzzer is mounted under the cover of the instrument to gently vibrate the apparatus and overcome slight friction.
The tester is finished in Nitrocellulose (NC)-based painting and bright chrome plating to provide a corrosion-resistant finish.| Minor Force | 30.6+/- 2gm |
| Major Force | 550.5+/-1gm |
| Radius of Indenter Dial Gauge | 1.25mm |
| Range | 0 – 10 X 0.01mm |
| ASTM D 1415 – 81 –Rubber property – International Hardness | 3400 |