The ORID itself is a tube with a specific weight inside it. The weight falls a controlled distance. To determine an O-ring's rubber compound, place the O-ring on a stable table or surface. Place the ORID® over the O-ring and raise the weight all the way to the top of the tube (as far as it will go). Release the weight and let it drop onto the O-ring. The weight will fall and bounce. The height of the first bounce will rise to one of the four calibrated ranges. The highest calibration is EP (Ethylene Propylene). The next is Nitrile. The next is Kalrez®. The lowest calibration graduation is Viton®. This indicates your rubber compound.
The ORID® 70-C will indicate the material composition of 100, 200, 300 and 400 series O-rings of 60 to 80 Durometer which are already known to be either Viton®, Kalrez®, Nitrile or EPDM. The O-rings must be within their rated shelf life and be unused and undamaged.
The height of the FIRST bounce indicates the O-ring: