Chemical Science and Technology Laboratory Design and Performance of the New NIST Hybrid Humidity Generator C. W. Meyer, W. W. Miller, D. C. Ripple, and G. E. Scace Thermometry Group, Process Measurements Division National Institute of Standards and Technology, USA NCSLI 2008 Symposium, August 5, 2008

Chemical Science and Technology Laboratory Purpose for Generator Generator will replace current generator (Two-Pressure Humidity Generator Mark II) Criteria for new generator Improve safety Reduce operating costs Reduce operating complexities Expand capabilities Reduce uncertainties

Chemical Science and Technology Laboratory Concept of Humidity Generator Saturate flowing gas with water in chamber with known temperature and pressure xmole fraction of H 2 O vapor e w water vapor pressure ƒ enhancement factor P s Saturator pressure T s Saturator temperature water gas f (T s, P s ) reflects: a)departures from ideal solution behavior b)Non-deal gas effects

Chemical Science and Technology Laboratory We use two methods for lowering the generated humidity when the saturator is at temperature T s :

Chemical Science and Technology Laboratory Two-Pressure (2-P) Method

Chemical Science and Technology Laboratory Divided Flow Method

Chemical Science and Technology Laboratory Hybrid Method Combines 2-P Method and Divided Flow Method

Chemical Science and Technology Laboratory Hybrid Humidity Generator Schematic

Chemical Science and Technology Laboratory Heat-Exchanger/Saturator Heat Exchanger Saturator InletOutlet Temperature- controlled bath Bath container

Chemical Science and Technology Laboratory Gas Flow Through Final Saturator

Chemical Science and Technology Laboratory Control and Measurement Equipment Temperature –Control: temperature controlled bath –Measurement: SPRT (calibrated at NIST, used with bridge) in bath Pressure –Control: butterfly throttle valve –Measurement: digital pressure transducer (calibrated at NIST) Flow –Control: mass flow controllers –Measurement: laminar flow elements

Chemical Science and Technology Laboratory Final Saturator Heat Exchanger Saturator

Chemical Science and Technology Laboratory 2-Pressure Generator Pre-saturator Final Saturator (inside bath) Temp controlled connector tube Throttle Valve Gen. Exit

Chemical Science and Technology Laboratory Divided Flow System

Chemical Science and Technology Laboratory Bath Stability at 15 °C (measured by SPRT) Standard deviation: 0.4 mK

Chemical Science and Technology Laboratory Temperature non-uniformities in the bath containing the final saturator Temperature differences referenced to that at saturator exit; measurements made using differential type K thermocouples

Chemical Science and Technology Laboratory Influence of pre-saturator dew-point  T = T flow – T no flow Flow rate: 50 slm

Chemical Science and Technology Laboratory Saturator Bath Temperature Stability

Chemical Science and Technology Laboratory Pressure Stability inside Saturator (pressure controlled using butterfly valve) Flow rate: 100 slm Standard deviation: 9 Pa

Chemical Science and Technology Laboratory Comparison with uncalibrated DP Hygrometer

Chemical Science and Technology Laboratory Uncertainty elements for the Hybrid Humidity Generator and their uncertainty values. u(X) is the standard uncertainty for element X.

Chemical Science and Technology Laboratory Mole FractionDew/Frost Point Expanded Uncertainty (k=2) of the HHG when P s = ambient

Chemical Science and Technology Laboratory Mole FractionDew/Frost Point Expanded Uncertainty (k=2) of the HHG when P s = 500 kPa

Chemical Science and Technology Laboratory Mole FractionFrost Point Expanded Uncertainty (k=2) of the HHG in Divided-Flow Mode

Chemical Science and Technology Laboratory Comparison with other NIST Humidity Generators using an uncalibrated DP Hygrometer HHG in 2-P mode

Chemical Science and Technology Laboratory Comparison with NIST LFPG HHG in Divided-Flow Mode, high end Comparison made using Dew Point Hygrometer

Chemical Science and Technology Laboratory Comparison with NIST LFPG HHG in Divided-Flow Mode, low end Comparison made using uncalibrated Cavity Ring Down Spectrometer

Chemical Science and Technology Laboratory Comparison with NIST Gravimetric Hygrometer

Chemical Science and Technology Laboratory Still to be done Documentation (SP-250, completion of operator’s manual, paper in archival journal) Calibrate check standard Write Report Templates HHG Expected to be in operation by 10/08!