durometer readings on rubber lined pipe 1

[ytse]

Can anyone tell me how accurate are durometer readings on rubber lined pipe ?
According to ASTM D2240, durometer hardness test should be performed on FLAT and parallel surfaces "to permit the presser foot to contact the specimen over an area of at least 6 mm (0.24") from the indentor point". All pipe surfaces are curved.
Even for 24" pipe, the curvature still mean that there is 0.0026" between the high and low points within an area of 0.24" radius. Will this cause an error in the durometer reading ?
If this curvature is not significant to cause a problem, what is the minimum pipe size that a durometer reading will yield an accurate result ?
If the surface must be perfectly flat, then how do we perform a QA check on the hardness of rubber lining in a pipe ?

 

[corrosionman]

ytse presumably the rubber lining continues onto the flat flange face so here is where you do the tests. But even if it is not like this, you will find it ok to test inside the bore down to about 4 inches. your Durometer reading is usually a tolerance of maybe 5 Degrees and the curvature will not cause readings to exceed this value. regards CM

 

[RichGeoffroy]

Ytse:

Unless your coating is 0.25 inches thick, the influence of the substrate will significantly affect the results of the test, as well as the radius of curvature of the test specimen.

The best way that I know of to take measurements on a pipe coating is the remove the coating from the pipe, and layer sections of the coating on a flat surface until the appropriate minimum thickness is attained, then take your measurements. If the coating is not relatively smooth on both sides, the sample may need to be "skived" to remove the gross surface imperfections prior to layering.

Hope this helps.

Rich Geoffroy
Polymer Services Group
polyserv@cox.net