Hydrometer Calibration by Hydrostatic Weighing With Automated Liquid Surface Positioning
Jesus J. Aguilera, John D. Wright, Vern E. Bean
We describe an automated apparatus for the calibration of hydrometers by hydrostatic weighing (Cuckow s method). The apparatus uses a laser light sheet and a laser power meter to position the liquid surface at the hydrometer scale mark to be calibrated. We selected tridecane as the liquid of known density in the apparatus for its relatively low surface tension and contact angle and also because it has stable density over time. We present an uncertainty analysis that shows that our calibration results have an expanded uncertainty of 100 parts in 106 or less. However, a sample uncertainty analysis shows that uncertainty components that arise during application of hydrometers by our customers can lead to expanded uncertainties of about 390 parts in 106. We validated our apparatus by comparisons using four liquids of known density and found agreement within 61 parts in 106. We used a NIST reference hydrometer to compare the calibration results from the new apparatus to historical NIST calibration results. The old and new results agree within 160 parts in 106, demonstrating that the new calibration method will not introduce any significant discontinuity in the results of hydrometer calibrations performed by NIST.