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I’m getting the feeling that unless you have a “official” conversion chart; the ones that are free online are no good? 
How about this one? http://www.gordonengland.co.uk/hardness/hardness_conversion_3c.htm
How about this one? http://www.gordonengland.co.uk/hardness/hardness_conversion_3c.htm
"The conversion values, whether from the tables or calculated from the equations, are only approximate and may be inaccurate for specific applications".
Most folks seem to have, and use, variations of Table 1 in E-140. The first problem that comes up is that the material being tested may not be that for which the table is intended, in this case Non-Austenitic Steels.
Other problems are the ignoring of the column headings. For example, the Knoop column specifies 500 gram loads or
greater, yet the facility will be testing at 200 grams.
Another is with the Vickers column. That column is referring to the English Vickers Test (known as Macro or Heavy Load Vickers in the US), and a load of 10 Kg. Many facilities using microindentation testers and Vickers indenters will use this column for their conversions.
The problem with the two examples above is that the test results are "Load Dependent" at lesser loads than those specified, i.e. if you test a part with 50 gm Knoop and 300 gm Knoop loads, the 50 gm result will always tend to give a higher Knoop number.
The only reliable way to determine the conversion factors for your particular application is to actually do the testing on the different scales. You then develop a "reliable basis for the conversion" for your material.
There are nine tables given in E-140. Make sure you pick the one that closest matches your material, and then take the numbers with a grain of salt.
Or better yet, do the actual testing (or have it done outside) on the different scales and develop the
conversion data. Then YOU know that you are RIGHT.
(True, scarey story: Cust A tests at 200gm Knoop and converts to HRC, wrongly. Cust B tests at 500gm Knoop and converts to HRC, correctly within the constraints of conversions) Cust B notices that parts from Cust A are testing lower upon receipt than A specified. A + B meet to discuss. Result of discussion is that Cust B changes their method to match that of Cust A. Parts are now actually coming in softer than specified, but A + B are now getting the same (wrong) result so they are happy!
The parts go in aircraft engines.:mg:
