Durometer / Hardness

Hardness, as applied to elastomers, is defined as the relative resistance of a surface to indention by an indenter of specified dimension under a specified load. The most commonly used measuring instrument is a durometer. Numerical hardness values are derived from the depth of penetration. The harder the sample, the further it will push back the indenter point and the higher the readings. On the durometer A scale, 0 is very soft, and 100 is very hard. Values are usually read immediately after firm contact has been established. The hardness range of elastomers is so broad that a single durometer cannot indicate practical measureable differences of hardness. For this reason durometers are available in more than one scale model, (eg., A and D scale durometers). The A scale durometer is widely used throughout the rubber industry. The durometer D model, which has a stiffer spring and a more pointed indenter, is used to measure the hardness of hard rubbers.

Mechanical limitation of durometers and the way people use them cause hardness measurements to be inconsistent. Also, indenter penetration is reduced immediately since elastomer deformation occurs and all elastomers inherently try to yield. It is not uncommon to find a difference of 5 points in individual hardness readings of an elastomer specimen. Tabletop durometers can measure hardness more accurately, but they are not as convenient and are not used routinely.

Frequently, hardness is assumed to correlate with stiffness (modulus), but this is not always true. Variations of a few points in hardness can show a marked difference in compression-deflection.

A statistical determination has been made of the relationship between hardness and 100% modulus of Pentathane measured with durometers on the A and D scales. As expected as shown in Table II, the A scale is more reliable for predicting the modulus of the softer stock; the D scale should be used with the harder stocks.

Hardness A 95% Confidence Limits Average 100% Modulus psi Hardness D 95% Confidence Limits
55 105-205 150 -- --
60 140-265 200 10 175-275
64 180-330 250 14 195-330
68 210-400 300 18 240-400
73 295-530 400 23 315-520
77 365-650 500 27 390-630
80 430-795 600 31 480-760
83 500-910 700 34 560-880
86 580-1060 800 36 630-1000
88 645-1200 900 38 700-1120
90 700-1350 1000 40 760-1250
92 800-1550 1150 43 900-1400
    1500 49 1200-1900
    2000 55 1650-2500
    2500 59 2000-3100
    3000 63 2400-3800
    4000 69 3200-5100
    5000 73 3900-6400

The reliability of predicting modulus from either A or D scale is approximately +/-33%, for 95% confidence limits. In the low modulus range of less than 500 psi, predictability is 25%.

The A scale should be used with moduli of 500 psi and under. The D scale is more useful for predicting moduli of more than 1000 psi. Either scale may be used for the intermediate areas as in Table II.

One hundred percent modulus can be predicted to within +/- 115 at a level of 400 psi, ranging up to +/- 315 at 1150 psi using the A scale; using the D scale predictability ranges from +/-100 at 400 psi to +/- 1200 at 5000 psi.

A linear relationship between durometer A and D does not exist. Approximate equivalent readings for durometer A and durometer D are shown in Table III.

Durometer A Durometer D
50 --
60 --
70 18
75 25
80 30
85 30
90 40
95 48

Hardnesses above 95 on the A scale should not be specified because the upper accuracy limit of the instrument is being approached. Accuracy at the lower end of the D scale is also limiting and values below 25D are questionable.