No matter what the type ? variable-area, mass, ultrasonic or electromagnetic flowmeter - every KROHNE product is wet calibrated. Volumetric flowmeters such as electromagnetic or ultrasonic flowmeters with nominal diameters from 2.5 to 3000 are calibrated against standard volumes.
For this volumetric calibration KROHNE Altometer in the Netherlands uses eleven piston provers and two calibration towers. KROHNE Altometer has the most accurate calibration rig in the world for flowmeters up to DN 3000. Its measuring uncertainty is only 0.04%. Nevertheless, even this rig must be calibrated to international standards. Responsibility for this rests with KROHNE's quality assurance department and the Dutch Accreditation Council, Raad voor Accreditatie (RvA).
Calibrating the calibration tower
The calibration tower is fitted with 24 precision level switches at equal distances apart from top to bottom. The volume between each successive switch is a known volume. But the volume has to be regularly verified and certified to show that no changes have taken place to the tower. For this an OPTIFLUX 5300 is used. This EMF is the world?s most accurate meter with an inaccuracy of only 0.15% and a repeatability of 0.06%. It is calibrated by the Dutch Accreditation Council (RvA) to an inaccuracy of only 0.008%. The OPTIFLUX 5300 is inserted in the measuring stream and the water is gravity fed through.
The first switch activates the totalizer and the last switch deactivates the totalizer, each switch in-between emitting a measured pulse once the water level has dropped below the switch. The volumes from the original calibration and the master meter are fed into the flow computer of the calibration rig and used for volume correction during subsequent commercial meter calibrations.
The tower calibration volume at KROHNE Altometer has an uncertainty of only 0.011%. KROHNE calibration rigs are normally 5 to 10 times more accurate than the flowmeter test specimens. This safety margin guarantees that all flowmeters will actually meet the specified accuracy under reference conditions.
Further information: KROHNE Messtechnik GmbH & Co. KG, Thomas Zimmerling,
e-mail: TZimmerling@krohne.de
