This tube is volumetrically checked, serialized, and comes with a Verification Certificate supporting the accuracy of the 15 points required by the API Manual of Petroleum Measurement's Chapter 10.4. The following information is from a paper by Dr. Anne Walker Brackett for the International School of Hydrocarbon Measurement. It may clarify some of the questions you have about verified centrifuge tubes. The procedure used for testing glass volumetric apparatus is to weigh the amount of distilled water contained or delivered with reference to the graduations marked on the instrument. The volume is then computed from the density of the water. The quality of the markings and the care exercised in reading or setting the liquid level are major factors in test calibration services for glassware. Factory certified glassware is available. The accuracy of centrifuge tube graduation is stated in the API's Chapter 10.4 "Determination of Sediment and Water in Crude Oil by the Centrifuge Method- Field procedure" in the Manual of Petroleum Measurement states: The accuracy of the tube graduation marks' shall be volumetrically certified before field use of the tube, in accordance with ASTM E 542 using NIST traceable equipment. The verification or certification of the centrifuge tube shall include a calibration check at the 0.05, 0.10, 0.15, 0.25, 0.50, 1.0, 1.5, 2.0, 50.0 and 200.0 milliliter marks. (On a 200 ml tube.) The tube is not to be used if the scale error at any mark exceeds the applicable tolerances as stated in Chapter 10.4. The tolerances range from plus or minus 0.02 ml to 1.50 ml. Verified tubes' certificates state that they are within these tolerances, and certified tubes given an actual value at the stated levels. The tubes will be identified by a serial number and will be accompanied by a report to attest to the accuracy of the marks as stated.
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