The National Institute of Standards and Technology (NIST) Low Pressure Manometry Laboratory maintains national pressure standards ranging from 1 mPa to 360 kPa through the operation of four ultrasonic interferometer manometer (UIM) pressure standards. NIST s newest UIM standard operates with Di-2-ethylhexyl sebacate (DEHS)oil as the working fluid over the range of 1 mPa to 140 Pa in absolute mode. The relative change in density of DEHS oil as a function of nitrogen gas pressure was determined using a vibrating-tube densimeter with measurement repeatability of 1 10 6 g/cm3 and measurement accuracy of 5 10 6 g/cm3 given by the instrument manufacturer. The densimeter was modified to operate at pressures below one atmosphere. The density of DEHS oil was observed to follow a linear dependence with the pressure of nitrogen gas exposure. The difference or change in density of DEHS oil after vacuum exposure (≤ 8 10 4 Pa) and pressures ranging from 138 Pa to 101 kPa was evaluated. For the pressures up to 1.367 kPa, there was no statistically significant change in the oil density observed. However, exposure to pressures between 10 kPa and 101 kPa showed significant density changes. The density of DEHS oil decreased by 46 ppm 5 ppm after 2 h of nitrogen gas exposure at 101 kPa. The density of DEHS oil decreased by 42 ppm 3 ppm after more than 24 h air exposure at 101 kPa. The results of this study show that the effect of dissolved gas on the density of DEHS oil does not contribute significantly to the pressure dependent uncertainty of the NIST oil UIM when operated over its range of 1 mPa to 140 Pa in absolute mode. Additionally, the results demonstrate that the oil density change is small enough that an oil manometer with an extended range of 1.4 kPa (10 torr) would not be adversely impacted by this gas density effect.
Citation: Measure: The Journal of Measurement Science
Pub Type: Journals
CDG, capacitance diaphragm gauge, CAS 122-62-3, degassed, densimiter, density, Evangelisti Torricelli, invoile-s, liquid column manameters, manometry, nitrogen gas, octoil-s, pressure standards, sebacate, UIM, ultrasonic interferometer manometer, vibrating tube densimiter, 2-ethylhexyl sebacate