DUROMETER  Place stress on the first syllable DUR, from the Latin meaning Hard/Tough - the OMETER suffix generically indicates a measuring device.

The Durometer testing technique has it’s origins in instruments used for measuring metal hardness. The Rockwell, Brinell and Vickers type of instruments measure the depth of penetration of a ball or diamond pyramid pressed into the surface under known loads. Inevitably these techniques proved clumsy when applied to softer materials as they measured the indentation in-situ and whilst the indentation is permanent in metal, it is transient in an Elastomer. Paradoxically, there was another metal hardness technique that was a notable exception to the Load Indentation methods previously described. This method recorded the rebound height of a hammer dropped down a tube on to the metal sample and this instrument was called The Shore Scleroscope - it too was unsuitable for measuring rubber hardness (although the self same technique was developed to measure Resilience as the Shore Resiliometer). The Scleroscope was developed around 1908 by the man whose name is synonymous with the Durometer… Albert F. Shore.

Albert Shore was born in 1876 and died in 1936. He founded the Shore Instrument Co. Inc., and produced the first quadrant type durometers in 1915. The company was always based in or around the Jamaica area of New York City.

He was succeeded by his son Fred, who was responsible for introducing a round style durometer in 1941. The family lost control of the company and after a number of owners it eventually became a part of The Instron Corporation in 1995. Indeed the generic term ‘Shore Durometer’ belongs to that Corporation, or its successors, to this day.

Scleroscope         Quadrant Durometer                              Round Style Durometer      Albert Shore 

Shore was indisputably the originator of the durometer but other individuals and companies have contributed to the success of the durometer. Before the unification of the many separate national test methods under ISO 7619 & 868, ASTM D676 and subsequently ASTM D2240 formed the basis of most test methods. DIN 53509 incorporated most ASTM fundamentals, although it metricated the quantities and this did eventually lead to a problem (in my opinion - see chapter 'ISO make a blunder').

Durometers in Germany: Founded by Heinrich Bareiss in 1954 Bareiss Prüfgerätebau GmbH, of Oberdischingen, are a significant manufacturer and should be attributed with the introduction of the first Round Style instrument. This made the operation much easier and increased the resolution from 5 degrees Shore to 1 degree Shore.
Bareiss durometers benefited greatly from ISO 7619-1 2004 and its precision durometers meets this and the ASTM test method.

Durometers in Japan: Established in October 1956, by founder Mr. Akashi, as Kobunshi Keiki Seisakusho. The company was re-organised in 1972 as Kobunshi Keiki Co., Ltd. and is based in Kyoto. Today the company is operated by Mr Akashi's son and grandson. Their ASKER range of durometers met the JIS K6301 test method, which was also based on ASTM but with some variation of forces. JIS is fully incorporated into ISO and new Asker high precision durometers meet ISO & ASTM test methods.

The three main suppliers of Durometers (alphabetically)...
Asker (Japan) - Bareiss (Germany) - Shore/Instron (USA)

➤ Historical Durometer Contemporaries

Here are some examples of early non-metallic hardness testers, which in those days effectively meant Rubber. Many differences can be put down to national distinctions or manufacturers preferences, most instruments became extinct or where incorporated into the instruments we know today.

The Durometer survived because it was adaptable and evolved to take account of new materials as they developed e.g. Plastics, Polyurethanes & Sponge Rubber. Another major factor in it’s favour was it’s almost universal acceptance in both the USA & Germany.

Other gauges include: Strachan Piezo Micrometer - Adams Densimeter - Cussons Hardometer - Schopper Pocket Meter - Firestone Penetrometer - Lhom & Argy Meter - Admiralty Rubber Meter - ICI Gauge - Federal Hardness Gauge - Gesellschaft fur Feinmechanik - Breuil Elasto Durometer - Cambridge Hardness Gauge - Schwartz Gauge - Zet Durometer - Dunlop Gauge

➤ Historical Hardness Comparison
This illustration shows how some of the various early instruments yielded a wide variety of values. Interestingly, the BS Hardness and IRHD are effectively the same apparatus. BS Hardness measured the depth of indentation in millimetres, but industry needed to relate to Durometer values. IRHD, or International Rubber Hardness Degrees to give it it’s full name, was invented to save the day. The millimetre values of the BS test are translated by a fiddle factor chart (ISO 48 Table 5) to compare almost exactly with Durometer Hardness values. The same standard states that Shore Hardness is “apparent hardness”.

The P & J (Pusey and Jones) meter is still used and quoted, but confined mainly to the rubber roller industry.

➤ Evolution of the Shore Scales
Originally, rubber testing would have simply been known as Durometer Hardness, or e.g. 50 Duro (still is in the much of the Rubber Industry today), until there came about a need to make an instrument that would measure harder rubbers.

There is no formal record of how the scales evolved. This account is formed from many discussions with the late General Manager of the Shore Instrument Company, Bill Galbraith. These took place over a ten years period between 1985 and the purchase of The Shore Instrument & Mfg. Co., by The Instron Corporation in 1995. Bill Galbraith was a leading member of the ASTM technical committee that wrote Durometer test methods. These methods formed the backbone of other National Standards such as DIN, JIS & ISO.

There is a simple logic laid behind the sequence of the early Shore scale letters and how they evolved to measure harder and harder materials. Later the need to measure materials softer than the range of Shore 'A' accuracy proved more difficult to fit into this progressive sequence of lettering.

  • The original durometer had a Truncated Cone indentor and a 8.04N spring.
  • Once extra scales were introduced the original original Scale was nominated as Shore ‘A’.
  • Albert Shore replaces the Truncated Cone indentor with a Radiused Cone one to measure samples, harder than 90 ‘A’; this scale became Shore ‘B’.
  • Then there was a need to measure even harder material, so Albert reverts to the Truncated Cone indentor but uses a stronger spring (44.45N); this scale became Shore ‘C’.
  • Later newer harder plastic materials exceeded 90 Shore ‘C’ so he combines the Radiused Cone indentor with the stronger spring; this scale became Shore ‘D’.
  • By the 1960's & 70's Shore needs to develop a Durometer to measure materials softer than Shore 'A'. They experimented again and now put the Shore ‘A’ spring with a Spherical indentor. This is a success, but what do they call it ? The new instrument is not for materials harder than ‘D’ so it's not logical to progress to the scale ‘E’ (now used to denote the ASKER C footwear durometer in ASTM). Confronted by the constrain that Albert Shore had inadvertently imposed originally they took the approach that if the Shore 'A' scale had been called Shore '1' the new instrument for measuring below Shore ‘1’ could then have been Shore ‘Ø’ (Zero) but because Ø is more commonly written as the letter ‘O’ and to keep with letter nomination it would become known as the Shore ‘O’ scale.
  • When they then developed an instruments to measure softer than Shore ‘O’, by using a very soft spring (1.111N) with the Spherical indentor; this scale became Shore ‘OO’. 
  • Bill Galbraith referred to the scales as both letters and as numbers, he wasn’t bothered if someone called it the “Zero Zero Scale” or the “OO Scale”. I never encountered any confusion between letters and numbers.
  • Later Galbraith inspired scales were Shore 'OOO', 'DO' & 'M(Micro)'.
See ISO or ASTM for dimensions and spring rates.

➤ Relation between Shore Hardness & Elastic Modulus

➤ How the test works
Shore hardness (also known as 'Durometer hardness') is determined by measuring the penetration of the Durometer's spring loaded indentor into the sample. The operator applies the instrument to the sample with sufficient contact force, in a consistent manner and without shock. During application the indentation reading may creep (decreases) over time because of the resilient nature of rubbers and plastics, so the test duration should reported along with the hardness number e.g 3 seconds. Likewise sample temperature must be controlled and recorded as it can alter the result, the colder generally the harder.

Sample size is important and 6mm of thickness is generally accepted as the norm, samples can be plied but results may be softer. The sample should be have a surface area such that it permits at least 5 test points each being at least 12mm apart and from the edge.

The results are a useful measure of relative resistance to indentation of various grades of polymers. However, the Shore hardness test does not serve well as a predictor of other properties such as strength or resistance to scratches, abrasion, or wear, and should not be used alone for product design specifications. Shore hardness is often used as a proxy for flexibility (flexural modulus) for the specification of elastomers. The correlation between Shore hardness and flexibility holds for similar materials, especially within a series of grades from the same product line, but this is an empirical and not a fundamental relationship.

Remember polymers will have a specification which allows for a tolerance, ± 3 Shore for example. This means that you will need to accumulate the errors in the durometer (usually ± 1) and the durometer operation.

Avoiding simple mistakes
It's easy to make a simple mistake when testing. If you combine one or more you can compound the error. Repeatability, reproducibility and reliability are the 3 R's of testing.

Here is a list of basic errors to avoid when testing by hand, it is by no means comprehensive.
  • Wrong scale of durometer.
  • Result < 20 Shore or > 90 Shore.
  • Sample too warm or too cool (a).
  • Incorrect surface contact (b).
  • Sample to thin (c).
  • Sample surface irregular.
  • Taking a measurement from, or near, a previous location.
  • Measuring too close to the edge.
  • Test time period inconsistent.
  • Durometer out of specification.

➤ Does the test test the operator or the sample?   
One problem with applying a durometer by hand is the operators ability to manipulate the result sub-consciously by applying a little too much, or slightly insufficient, pressure.

Here a couple of simple experiment to test the effectiveness of durometer testing in your company. Have a mixture of operators, regular operators and random staff, around a table. Give a basic demonstration of how to use a durometer. Pass the durometer around with a selection of samples, keep the results a secret until the end and compare the variability of results. Repeat the test with the hardness known and see how much closer the results are.

Alternatively try some blind testing. We have established how the application of the durometer onto the sample is imperative in achieving repeatable results. If your testing using a digital durometer with a hold function, carry out the test but without watching the display - lift the durometer off and read the held result. This can also be done with an analogue durometer with the help of a college to record the value before lifting the durometer off (just don't copy the technique below).

These exercises are useful to illustrate the repeatability, reliability and accuracy of the test - the differences that experience brings to the test procedure - the likelihood of disputes.

They also highlight the importance of mounting the durometer onto a stand to eliminate variability's.

➤ Testing irregular shaped samples
Testing these products it's possible but with such a wide variety it would be impossible to cover them all here. Rubber Rollers are a typical example of a non standard sample shape. Here the technique is to rotate the durometer, with a maximum hand fitted, around the circumference.

➤ Micro Hardness - Product Testing
As discussed there are constraints on the size of the sample used in durometer testing. The ideal sample is a laboratory plaque (see set illustrated lower down) and the further a test piece strays from these constraints, so the probability for errors increases. The need to accurately measure O-Rings led directly to the design and incorporation of Shore "A" Micro (It must be noted that a Micro Shore "D" or any other scale for that matter, does not exist).

Micro's are not hand-held instruments.

The relationship between all micro and macro tests is not definitive but the Shore scales are closer than the IRHD scales mainly because of the history of it's origins. The original, non ASTM, Shore Micro was calibrated to read within ± 1 against a set of Shore 'A' certified reference blocks. Later versions, to ASTM, differs very little in specification but are calibrated in the conventional way.

Rubber compounds will undergo the full battery of traditional testing to determine their characteristics. But of course the compound is then moulded or extruded and it would be wrong to assume the component will have maintained those characteristics throughout the manufacturing process - nevertheless, such assumptions happen often.
Does a 70 ± 3 Shore 'A' Nitrile compound produces a 70 ± 3 Shore 'A' Nitrile O-Ring? If the answer was 'Yes', then there would be no need to test. The truth is you must test, the question is how, because you cannot measure a 70 ± 3 Shore Nitrile O-Ring with a Durometer of any description.

Disputes happen right down the chain from compounder to the end user of the O-Ring (I'm using the O-Ring as an example, the principle applies to any small product). The O-Ring end user has only one way to perform a QC check of incoming stock - a micro hardness tester and he has the right to expect to be given a real specification against which he can check.

If I haven't worried you enough, there is an even worst case scenario... where the 70 ± 3 Shore 'A' Nitrile compound is actually measured with an IRHD Deadload but sold as a 70 ± 3 Shore 'A' Nitrile compound (or visa versa of course). Probably using a cross reference chart like the one I illustrated earlier.

Testing at Top Dead Centre is paramount or low readings will result. 
It's also important to support small samples (like O-Rings) correctly, to eliminate deformation

The 3 main suppliers of Shore Micro Hardness Testers (alphabetically)...
Q-tech offer an alternative Nano-Shore-method for measuring small product related samples and is said by the manufacturer to be a "Down-scale method of the Shore-A measurement".

➤ Testing v Checking  
We should not compare hand-held durometers with a stand mounted Shore Hardness Tester. The hand-held durometer will check hardness, when used by a competent operator, and is perfectly valid form of first-line quality control. A stand mounted Shore Hardness Tester (which may or may not incorporate a durometer) will give a definitive hardness value.
It would be folly to stop production based on the result of a hand-held check, but it would be prudent to pause it and apply a definitive test.

➤ Calibration v Referencing
The Shore Hardness Tester attached to a stand requires an absolute calibration, a hand-held one does not. It is neither cost effective, or practical, to apply the same calibration regime to a hand-held instrument, where the operator is by far and away the biggest influence in the degree of result accuracy.
Referencing a hand-held durometer against certified reference blocks is cost effective, relevant and practical. By reference blocks I mean a set of elastomer/polymer pieces, covering the range 10 - 90 degrees, traceable to a  National Standard source such as RAPRA, NIST or BAM. I do not mean the metal block supplied by some manufacturers, that only checks the indentor extension, not it's shape or the characteristics of it's spring loading - it's about as useful as a chocolate teapot.
Shore/Instron Hardness Block set.

➤ ISO make a blunder - In our opinion.
At the turn of the millennium the International Standards Organisation and TC45 in particular, made a fundamental blunder in issuing a revised version of ISO 7619.
For 90 years the extension of the durometer worldwide was set at 0.100 inch and each 100th increment was equal to 1 degree shore.
When TC45 re-wrote ISO 7619 as ISO 7619-1 2004, they eliminated imperial measurements and in the process of metrication re-set the indentor extension to 2.5mm. But 2.5mm is not 0.100 inch it actually equals 0.098 inch. In placing a tighter tolerance on the extension, many existing durometers fell out of specification.
This was especially true of the many instruments that incorporated a conventional Imperial Dial Gauge Indicator movement. Many of these instruments were being mass produced in the far east and were able to be sold at significantly lower costs than many traditional instruments manufacturers could match.
This oversight muddies the calibration status of many existing durometers and, given that the vast majority of durometers are hand-held, does little to improve their usefulness. It also goes against the general principal of ISO test methods, which are usually designed to be inclusive rather than exclusive.
ASTM & ISO are the main test methods, others (e.g. DIN & JIS) are affiliated.

Low priced hand-held durometers based on Imperial Dial Gauges seem to have 
been targeted unfairly.They clash with ISO 7619 but can meet ASTM D 2240.
➤ So, what hardness tester do you need?
Shore A, B, C, D, O, OO? IRHD? Micro/Macro? Analog? Digital?
Cost is not always a pointer to accuracy, reliability and repeatability. A quick look around will show how much durometers vary in price. Buying the wrong durometer can be a costly mistake and lead to worse quality control than not having one at all.

The 3 main suppliers of Shore Hardness Testers (alphabetically)...