Hygrometers with the Hybrid Humidity Generator

1 Page 1 NIST Special Publication 250-83 Revision 1 Calibration of Hygrometers with the Hybrid Humidity Generator C.W. Meyer T . Herman W. W. Miller This publication is available free of charge from: https://doi.org/10.6028/NIST.... Open at page → 2 Page 2 NIST Special Publication 250-83 Revision 1 Calibration of Hygrometers with the Hybrid Humidity Generator C.W. Meyer T. Herman W. W. Miller Thermodynamic Metrology Group Sensor Science Division Physical Measurement Labora... Open at page → 3 Page 3 Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or... Open at page → 4 Page 4 Page 4 of 75 Abstract We describe here the calibration of customer hygrometers using t he NIST Hybrid Humidity Generator (HHG). This generator produces humidified air with water amount fractions (water mole fractions) ra... Open at page → 5 Page 5 Page 5 of 75 Calibration of Hygrometers with the Hybrid Humidity Generator Table of Contents 1. Overview of the Calibration Service ........................................................................................... Open at page → 6 Page 6 Page 6 of 75 12. Summary ................................................................................................................................... 51 Acknowledgements............................................... Open at page → 7 Page 7 Page 7 of 75 1. Overview of the Calibration Service The NIST Hybrid Humidity Generator (HHG) [1-3] provides calibration services for a variety of humidity–measuring instruments. It is operated by the Thermodynamic Metrol... Open at page → 8 Page 8 Page 8 of 75 2. Principle of Operation Generation of gas with an accurately known moisture content starts with saturating the gas with water at a known temperature and pressure. C ontrolled saturation is accomplished by... Open at page → 9 Page 9 Page 9 of 75 i FP FP c c () () e T = f T ,Px P , 3) where e i is the saturated vapor pressure for ice, as calculated by [ 8,9]. Relative humidity ( RH) is defined by the World Meteorological Organization (WMO) to be [7]... Open at page → 10 Page 10 Page 10 of 75 Computer-controlled mass flow controllers regulate the gas flow ; the maximum gas flow is 150 L/min. 4.2. Water Source The water used in the HHG is produced by a commercial water distilling system. The wate... Open at page → 11 Page 11 Page 11 of 75 4.3. Saturation System The saturation system of the HHG consists of a pre- saturator and final saturator with a temperature-controlled tube connecting them. These parts are described below. 4.3.1. Pre -satu... Open at page → 12 Page 12 Page 12 of 75 bath. The gas first enters the heat exchanger, which conditions the gas to be at the temperature of the saturation chamber; this minimizes sensible heat loading on the chamber and also minimizes latent heat... Open at page → 13 Page 13 Page 13 of 75 the saturator partition the chamber into two channels of width 3.7 cm that follow a serpentine path, as shown in Fig. 3. Each channel covers half the area of the saturation chamber, as shown. The dividers a... Open at page → 14 Page 14 Page 14 of 75 above the saturator. Therefore, when the saturator temperature is greater than the ambient temperature, any condensation occurring in the tubing is directed down to the saturator; this ensures that the tran... Open at page → 15 Page 15 Page 15 of 75 The HHG employs the two- pressure method by first using the 550 kPa pressure head of the gas source to raise the pressure in the saturator to the desired value, flowing the gas with a rate of 30 L/min. It t... Open at page → 16 Page 16 Page 16 of 75 The HHG employs the divided-flow method using a gas multiplexer. The multiplexer contains seven flow-metering banks. Each bank consists of a commercially made mass- flow controller and a commercially made f... Open at page → 17 Page 17 Page 17 of 75 5. Performance and Validation Tests of the Generator Performance tests were made on the HHG in two-pressure mode to determine components of an uncertainty budget for the generator. Specifically, we measured... Open at page → 18 Page 18 Page 18 of 75 Figure 7. Temperature non-uniformities in the bath containing the final saturator, as measured by differential type K thermocouples attached to the outside of the saturator. The plots display temperature di... Open at page → 19 Page 19 Page 19 of 75 after 1800 s the flow is 150 L/min. No significant difference in the non-uniformities is seen for these two flows. Note that the non- uniformity at the entrance to the heat exchanger is noticeably smaller i... Open at page → 20 Page 20 Page 20 of 75 In addition, we measured the stability of the bath over 1200 s at a series of bath temperatures. Figure 9 shows the results. At 20 °C the standard deviation was at a minimum, at 0.2 mK. As the bath temperat... Open at page → 21 Page 21 Page 21 of 75 5.3. Self-consistency of the Generator in 2-P Mode For the self-consistency tests, we generated dew points with eight different saturator temperatures and saturator pressures ranging from 500 kPa to ambient... Open at page → 22 Page 22 Page 22 of 75 5.4. Water Amount Fraction in the Dry Air in the Divided-Flow Method As stated in section 4.2, the dry air used in the divided- flow method is conditioned by passing it through a molecular sieve. The specif... Open at page → 23 Page 23 Page 23 of 75 6. Test Chamber and Its Heat Exchanger 6.1. Test Chamber Design The HHG test chamber is an accessory used for the calibration of temperature and relative humidity sensors (thermohygrometers). It is used for... Open at page → 24 Page 24 Page 24 of 75 pressure for the gas and humidity sensors during calibration; this is necessary because relative humidity is a function of temperature and pressure as well as water amount fraction. The test chamber is moun... Open at page → 25 Page 25 Page 25 of 75 Figure 14. Top: photograph of the test chamber mounted inside the environmental chamber. Bottom: blowup of the top photo, showing the chamber PRT and check standard. and outer shell are joined at the front... Open at page → 26 Page 26 Page 26 of 75 passing through the hole, the air is immediately directed radially outward by a diffuser plate of diameter 15.2 cm . The air then travels axially from the back of the chamber to the front. The air exits the... Open at page → 27 Page 27 Page 27 of 75 purpose of monitoring the temperature in the air-entrance and air-exit chambers. Two other PRTs are mounted in ports adjacent to the heat-exchange-fluid-entrance and heat-exchange-fluid-exit ports for the p... Open at page → 28 Page 28 Page 28 of 75 Figure 16. Diagram of a mounting plate in the heat exchanger Both the environmental chamber and heat exchanger can be controlled at temperatures between –34 °C and 190 °C. This allows calibration of thermoh... Open at page → 29 Page 29 Page 29 of 75 Figure 17. Temperature stability in t est chamber near a) − 40 °C, b) 20 °C, and c) 80 °C Figure 19 shows the stability of the relative humidity inside the test chamber at 23 °C at three different values of... Open at page → 30 Page 30 Page 30 of 75 Figure 18. Temperature difference between two locations in test chamber (see text for location description) separated by 30 cm, measured using differential thermocouples Figure 19. Stability of relative hum... Open at page → 31 Page 31 Page 31 of 75 7. Uncertainty Budget Based on these performance tests as well as measurement equipment specifications, we have constructed uncertainty budgets for the humidity generated by the HHG. There are budgets for t... Open at page → 32 Page 32 Page 32 of 75 When the generator is used in 2-P mode (P s ≠ Pc), ( ) ( ) ( ) 22 calc cal c 2 sccalc r 22 sc ue ue ue ee ∆≡ + , 10) ( ) ( ) ( ) 22 calc calc 2 sccalc r 22 sc uf uf uf ff ∆≡ + . 11) Here, ( ) calc s ue , (... Open at page → 33 Page 33 Page 33 of 75 7.1.5. Frost-point Temperature Generated in Divided-flow Mode In the hybrid generator, the divided flow method will only be used for generating frost points. Here, the total standard uncertainty of the fros... Open at page → 34 Page 34 Page 34 of 75 or, in terms of relative uncertainty u r(r), ( ) ( ) ( ) rr 1 () 1 ur ur ux rx = = − 17) Here, u r(x) is given by Eq. 7 when the generator is used in 1-P or 2-P mode, and by Eq. 13 when the generator is use... Open at page → 35 Page 35 Page 35 of 75 7.3. Uncertainty Plots Figure 20 shows the total expanded uncertainty for humidity generated by the HHG when the generator is operated in 1-P mode. The uncertainty is shown for the a) water amount fraction... Open at page → 36 Page 36 Page 36 of 75 are zero for the same reason. Also in this plot, the curve designated as “P ” represents the contributions from both P s and P c. The figure shows that for the 1- P mode, the dominant uncertainty is from pr... Open at page → 37 Page 37 Page 37 of 75 Figure 20. Total expanded uncertainty U for the (a) water amount fraction and (b) dew- point temperature generated by the HHG saturator when used in 1-P mode . The black curve represents the total uncertain... Open at page → 38 Page 38 Page 38 of 75 Figure 21. Total expanded uncertainty U for the (a) water amount fraction and (b) dew/frost- point temperature generated by the HHG saturator when used in 2-P mode with P s = 500 kPa. The black curve repres... Open at page → 39 Page 39 Page 39 of 75 Figure 22. Total expanded uncertainty for the a) water amount fraction (relative uncertainty) and b) frost -point temperature generated by the HHG when it is used in divided-flow mode with a saturator press... Open at page → 40 Page 40 Page 40 of 75 Figure 23. Total expanded relative uncertainty Ur(RH) for the relative humidity calibrations of a chilled-mirror hygrometer with and external temperature probe, using humid gas generated by the HHG to an en... Open at page → 41 Page 41 Page 41 of 75 Figure 24. Total expanded relative uncertainty Ur(RH) for the relative humidity calibrations of a thermohygrometer, using humid gas generated by the HHG to the test chamber with temperature T c = 20 °C and... Open at page → 42 Page 42 Page 42 of 75 Figure 25. Total expanded relative uncertainty Ur(r) for the mass ratio r generated by the HHG. In the plots the generator is used in a) 1-P mode, b) 2- P mode with Ps = 500 kPa, and c) in divided-flow mode... Open at page → 43 Page 43 Page 43 of 75 8. Comparisons with other Humidity Standards 8.1. NIST Gravimetric Hygrometer We have performed comparisons between the humidity generated by the HHG and that measured by the NIST gravimetric hygrometer [ 1... Open at page → 44 Page 44 Page 44 of 75 The values of Δx/x are all within 0.2 % and the averages of the points taken at one value of x are all within 0.12 %. This is within the combined uncertainties of the HHG and gravimetric hygrometer, showing... Open at page → 45 Page 45 Page 45 of 75 In Table 2, the DoE ’s between the HHG dew/frost-point standards and the KCRVs are shown, along with their expanded (k = 2) uncertainties (obtained from Tables 7.3 and 7.4 in [21]). The DoE values are well... Open at page → 46 Page 46 Page 46 of 75 Although there are no CCT-K6 KCRVs at −70 °C and −80 °C, it is nevertheless useful to show the DoE between the NIST HHG and NMIJ/AIST standards at those point for validation of their standards. The DoE valu... Open at page → 47 Page 47 Page 47 of 75 9. Calibration of Chilled -Mirror Hygrometers and Cavity Ringdown Spectrometers 9.1. Initial Procedures Before connecting the hygrometers to the hybrid generator, they are checked for noticeable damage and... Open at page → 48 Page 48 Page 48 of 75 9.3. Calibration Measurements The calibrations are done at specific humidity points as requested by the customer. The order in which the calibration points are acquired is at the discretion of the operator.... Open at page → 49 Page 49 Page 49 of 75 10. Calibration of Thermohygrometers and Dataloggers 10.1. Initial Procedures Before beginning calibration of the thermohygrometers and dataloggers, they are checked for noticeable damage and for proper ope... Open at page → 50 Page 50 Page 50 of 75 example, with dataloggers, where the controller and sensors are together in one unit, the measurement values can be transmitted after the measurements are completed and the datalogger is taken out of the te... Open at page → 51 Page 51 Page 51 of 75 Once the data is acquired for all requested humidity points, the results are assembled into a workbook. The new check standard data is compared to previous check standard data to validate the performance of... Open at page → 52 Page 52 Page 52 of 75 Acknowledgements The authors thank Dr. Joseph T. Hodges for conceiving the humidity generator described in this document, obtaining the funding to develop it, and performing preliminary calculations on its... Open at page → 53 Page 53 Page 53 of 75 References [1] G.E. Scace et al., “An Overview of the NIST Hybrid Humidity Generator”, in Proceedings of the 5 th International Symposium on Humidity and Moisture (Rio de Janeiro, Brazil: INMETRO, 2007). [2... Open at page → 54 Page 54 Page 54 of 75 [13] BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP and OIML, “ Evaluation of measurement data--Guide to the expression of uncertainty in measurement”, JCGM 100:2008, https://www.bipm.org/utils/common/documents/j... Open at page → 55 Page 55 Page 55 of 75 [26] P.H. Huang et al., SIM.T- K6.2 Final Report, Metrologia 51 Tech. Suppl., 03002 (2014) [27] P. Huang, C. Meyer, and J.D. Brionizio , SIM.T- K6.3 Final Report, Metrologia 52 Tech. Suppl., 03001 (2015) [2... Open at page → 56 Page 56 Page 56 of 75 Appendix I. Derivation of the Uncertainty Equations for the Hybrid Generator The total uncertainty of a quantity z is related to the n individual uncertainty components y i through the general law of error... Open at page → 57 Page 57 Page 57 of 75 calc calcs s ss s s s ss s2 ss s s sf de e f f e dx dT dP de df P dT P P P = −++ . A6) The uncertainty in the amount fraction may then be expressed as ( ) ( ) ( ) ( ) 2 22 2 2222 2 calc calcs s ss s s s ss... Open at page → 58 Page 58 Page 58 of 75 Recognizing that x is the same in both the saturator and chamber, ss s cc c //ef P ef P= , and so A12 may be written [ ] calc calc c s ss c DP s c DP s s s s c calc calc sc c sc c 1 / e de dP de de dT dT de... Open at page → 59 Page 59 Page 59 of 75 ( ) ( ) ( ) 22 calc calc 2 sccalc r 22 sc uf uf uf ff ∆≡ + . A20) 3. Relative Humidity When the Generator is U sed in 2-P Mode or 1-P Mode Here we consider the total uncertainty for the relative humidity RH... Open at page → 60 Page 60 Page 60 of 75 Inserting Eqs. A14 and A15 yields: cs s ss ssc ssc c sc s c 22 scc s scc ccs c cs c calcssc c ssc ccs c ccs calccs s s c s c sc sc c s s c s c c () 11 Pf de ef efP efP de d RH dT dP dP dT Pef dT Pef efP e f... Open at page → 61 Page 61 Page 61 of 75 ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 22 2 2 2 calc cal c 22 ssss rs 22 ss s s s 22 22 pp ss 2 22 s 1 1 ue u fux uPde u x uT x e dT P e f un u x unx Nx x N   = ≅ ++ +    + +− +    ... Open at page → 62 Page 62 Page 62 of 75 Here, we once again use the conventional method for estimating ( ) 2 calc r ue∆ and ( ) 2 calc r uf∆ , which employs Eq. A1 9-A20. 6. Relative Humidity When the Generator is U sed in Divided-flow Mode Here... Open at page → 63 Page 63 Page 63 of 75 ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 22 22 2 sc 2sc r sc 22 ss s cc c 22 22 pp scalc calc s rr 22 2 s 11 () 1 uRH uP uPde de u RH u T u T RH e dT P e dT P ux ununx ue uf x xN N     = = ++ + ... Open at page → 64 Page 64 Page 64 of 75 Appendix II. Uncertainty Element Subcomponents for the HHG and its Test Chamber Source of uncertainty Standard Uncertainty Unit Saturator Temperature T s Calibration 0.5 mK SPRT Drift 0.3 mK Self-heating 0.... Open at page → 65 Page 65 Page 65 of 75 Appendix III. Sample Calibration Reports ______________________________________________________________________________________________________ This publication is available free of charge from: https://doi... Open at page → 66 Page 66 Page 66 of 75 Sample Calibration Report REPORT OF CALIBRATION CHILLED-MIRROR HYGROMETER Michell Instruments, Model S4000 TRS, S/N 12345 Submitted by DPM, Inc. Somewhere, NC 98765 The chilled-mirror hygrometer was calibra... Open at page → 67 Page 67 Page 67 of 75 Order No: O-0000009876 Service ID: 36070C Date: August 6, 2021 Table 1 Michell Instruments, Model S4000 TRS, S/N 155515 Calibration Values Calibration Uncertainties NIST HHG T DP, TFP (°C) DUT T DP, TFP (°C... Open at page → 68 Page 68 Page 68 of 75 Sample Calibration Report REPORT OF CALIBRATION CHILLED-MIRROR HYGROMETER (with external thermometer) RH Systems Dew Point Mirror RH-473-RP2, S/N 12-3456 Submitted by DPM, Inc. Somewhere, NC 98765 The dew/f... Open at page → 69 Page 69 Page 69 of 75 Order No: O-0000009876 Service ID: 36070 S Date: August 6, 2021 RH Systems Dew Point Mirror RH-473-RP2, S/N 12-3456 Table 1 Dew/Frost Point Temperature Pt. Calibration Values Calibration Uncertainties NIST... Open at page → 70 Page 70 Page 70 of 75 Order No: O-0000009876 Service ID: 36070 S Date: August 6, 2021 RH Systems Dew Point Mirror RH-473-RP2, S/N 12-3456 Table 2 External Thermometer Temperature T Pt. Calibration Values Calibration Uncertaintie... Open at page → 71 Page 71 Page 71 of 75 Order No: O-0000009876 Service ID: 36070 S Date: August 6, 2021 RH Systems Dew Point Mirror RH-473-RP2, S/N 12-3456 Table 3 Relative Humidity Pt. Calibration Values Calibration Uncertainties RH NIST (%) RH... Open at page → 72 Page 72 Page 72 of 75 Sample Calibration Report REPORT OF CALIBRATION THERMOHYGROMETER Fluke Model 1620 “DewK” Thermohygrometer, S/N 12345 with Fluke Thermo-hygrometer Sensor 2626-S, S/N 98765 Submitted by DPM, Inc. Somewhere, N... Open at page → 73 Page 73 Page 73 of 75 Test No: 0000009876 Service ID: 36070S Date: August 6, 2021 Table 1 Fluke Model 1620 “DewK” Thermohygrometer, S/N 12345 with Fluke Thermo-hygrometer Sensor 2626-S, S/N 98765 a. Test Chamber Temperature T Pt... Open at page → 74 Page 74 Page 74 of 75 Sample Calibration Report REPORT OF CALIBRATION CAVITY RING DOWN HYGROMETER Tiger Optics, Model LaserTrace 3 H 2O S/N 12345 Submitted by DPM, Inc. Somewhere, NC 98765 The cavity ring down hygrometer was cal... Open at page → 75 Page 75 Page 75 of 75 Order No: 0000009876 Service ID: 36070S Date: August 6, 2021 Table 1 Tiger Optics Cavity Ring Down Hygrometer LaserTrace 3 H 2O; S/N 12345 Calibration Values Calibration Uncertainties NIST HHG x (micromol/m... Open at page →