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Humidity standards for low level water vapor sensing and measurements
Sensors and Actuators B 53 (1998) 125 – 127
Humidity standards for low level water vapor sensing and measurements P.H. Huang * Process Measurements Di6ision, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 2 October 1998; received in revised form 20 October 1998; accepted 20 October 1998
Abstract An intercomparison between the two most widely used types of low level humidity standards constituting a permeation-tube standard and an evaporation–diffusion standard is presented. Using a precision chilled-mirror hygrometer as a transfer standard, frost-point temperatures of nitrogen–water-vapor mixtures produced by the standard generators are measured in the water vapor concentration range of 5–70 nmol mol − 1. Measured differences between the standard generators are found to be within the standards’ expanded uncertainties for moisture content of greater than :15 nmol mol − 1. Below 15 nmol mol − 1, the differences are greater than the combined uncertainties of the standard generators. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Intercomparison; Moisture standards; Permeation– diffusion-tube
1. Introduction Sensing and measurements of low levels of water vapor content in molar volume ratio of the order of 10 − 9 are critical for the applications in which gases of high or ultrahigh purity are used in the microelectronics or semiconductor industries. The use of various commercially available sensors, such as electrolytic cells, dielectrics of polymer/ceramic, vibration crystals, Fourier transform infrared spectrometers, atmospheric pressure ionization mass spectrometers, and chilledmirror frost-point hygrometers, is rapidly increasing. These sensors often require a calibration using moisture standard generators. Therefore, validation of calibration standards is necessary for sensors to be used with certain accuracy and reliability. Humidity calibration standards for producing atmospheres of known water vapor content at the nmol mol − 1 levels are described in Semiconductor Equipment and Materials International (SEMI) StandardC15 [1]. Methods based on permeation of water vapor through a polymeric membrane and evaporation diffusion of water vapor through a small hole of water bottle are popular in semiconductor industry. In this
* Corresponding author. E-mail: [email protected]
paper, the results of intercomparison experiments conducted for the range from 5 nmol mol − 1 (parts per billion by volume) to 70 nmol mol − 1 of water vapor concentration in nitrogen are reported. A precision chilled-mirror frost-point hygrometer was used as a transfer standard whose output in four-wire resistance can be directly related to the measured frost-point temperature and then to the water vapor concentration in nmol mol − 1 [2]. A discussion of the standard generators, the transfer standard, and the measurement techniques are presented. The results obtained and the uncertainty analysis are discussed.
2. Standard humidity generators Standard humidity generators based on dilution methods are used for an intercomparison in this work. A single (one-stage) dilution method is used with a permeation tube and a double (two-stage) dilution method is used with a water bottle.
2.1. Permeation method The method is based on the principle that a stable amount of water in a permeation tube maintained at a
0925-4005/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 9 8 ) 0 0 3 1 3 - X
P.H. Huang / Sensors and Actuators B 53 (1998) 125–127
126
constant temperature to get permeated is fixed so as to constantly generate a standard moisture. A permeation tube of 6 mm in diameter and 20 mm in length was used. The generated water vapor concentration in the dilution gas C, in units of nmol mol − 1 is given by [1]
mirror dew-point hygrometer, which was previously calibrated by the NIST Two-Pressure Humidity Generator. The standard moisture was diluted using a twostage dilution method to obtain a range of water vapor concentration of : 5–70 nmol mol − 1. The concentration in the diluted gas C can be expressed as
(1)
C =Kg/F +B
(2)
C= C0D
where K is the molar gas constant for water vapor at 0°C and one atmospheric pressure and is equal to 1.243 l g-mol − 1; g is the permeation rate in units of ng mol min − 1 at a constant temperature and is determined by gravimetry; F is the gas flow rate in units of l min − 1 with reference to Standard Temperature Pressure (STP); and B is the background moisture concentration, in units of nmol mol − 1, measured without permeation tube.
where C0 is the standard water vapor concentration and D is the dilution factor.
2.2.1. Experimental uncertainty The relative expanded uncertainty in water vapor concentration C generated by the method described above can be determined from Eq. (2) based on the relative standard uncertainties of C0 and D. The relative standard uncertainty of C0 = 8392 nmol mol − 1 was determined to be 2.1% of the value [2]. The relative standard uncertainty of D was determined by the relative standard uncertainties of the calibrated mass flow meters. The flow meters had a maximum relative standard uncertainty of 1%. For a maximum dilution factor of 10 − 4, the maximum relative standard uncertainty of D using a double dilution method was 2%. The relative combined standard uncertainty in C based on Eq. (2) is 2.9%. The relative expanded uncertainty of C with a coverage factor of 2 is therefore 5.8%.
2.1.1. Experimental uncertainty The relative expanded uncertainty in the generated water vapor concentration can be determined from Eq. (1) using the International Standard Organization (ISO) Guide to the Expression of Uncertainty in Measurement [3]. For the range in C of :5 – 70 nmol mol − 1, a mass flow meter (full scale 5 l min − 1) and a permeation tube (g=23 ng min − 1 at 60°C) were calibrated. The maximum relative standard uncertainties of flow rate F and permeation rate g were 1 and 5%, respectively. Assuming BB B Kg /F, the maximum expanded uncertainty of C with a coverage factor of 2 ( : 95% confidence level) was determined to be 10.2%.
3. Comparison of measurement results
2.2. E6aporation diffusion method
Table 1 summarizes the results of the standard intercomparison. Column 1 is the reference water vapor concentration and column 2 is the difference between the mean values of three tests obtained for the concentration Cpts using the permeation tube/single dilution and the the concentration Cwbd using the water bottle/ double dilution standard generators. A precision chilled-mirror frost-point hygrometer was used as a transfer standard capable of measuring frost-points to − 110°C. Columns 3 and 4 are the expanded uncertainty (k=2) of the permeation tube/single dilution standard generator and water bottle/double dilution standard generator, respectively.
The method is based on the principle that water vapor can diffuse from water in a glass bottle having a small hole at the top. A cylindrical glass bottle of 10 mm in diameter and 20 mm in height having a hole of 1 mm in diameter was used. The diffusion rate is a constant for the water bottle maintained at a constant temperature. In this method, a standard water vapor concentration of 8392 nmol mol − 1 was generated when the nitrogen gas was fed at the flow rate of 1 l min − 1 when the water bottle was maintained at the temperature of 30°C. It was measured using a precision chilled-
Table 1 Summary of results of the intercomparison of the permeation tube/single-dilution and the water bottle/double-dilution standard generators Nominal concentration Difference C (nmol mol−1) Cpts−Cwbd (nmol mol−1)
Expanded uncertainty
7 16 30 70
0.7 1.6 3.1 7.1
1.4 1.6 2.0 3.0
Combined uncertainty uc (nmol mol−1)
Agreement indicator Ia
1.1 2.0 3.6 8.2
1.3 0.8 0.6 0.4
Upts (nmol mol−1) Uwbd (nmol mol−1) 0.4 0.9 1.7 4.1
P.H. Huang / Sensors and Actuators B 53 (1998) 125–127
127
The measurements at NIST obtained over two successive years show that the transfer standard hygrometer was repeatable to 0.2°C. These results are based on the residuals of a least-square fit to the data comprising frost-point temperature vs. standard generator moisture concentration. Column 5 of Table 1 gives the combined uncertainty uc of the intercomparison between a permeation tube/single dilution and a water bottle/double dilution standard generators. Column 6 gives an indicator of agreement Ia defined to be the absolute value of Cpts −Cwbd divided by uc. A value of IaB 1 indicates that the two types of standard generators agree to within their combined uncertainties. Inspection of column 6 of Table 1 indicates that all the differences in the measured water vapor concentration are smaller than uc except at a nominal value below 15 nmol mol − 1.
transfer standard system. The results show that the moisture standards for low levels of water vapor above 15 nmol mol − 1 in nitrogen gas agree to within the combined uncertainties of the standards or to within 10% of a generated nominal value. At a nominal value of 7 nmol mol − 1, the standards agree to within 1.4 nmol mol − 1 or 20% of the value. The expanded uncertainty of the standard based on a double (two-stage) dilution method is significantly smaller that of single dilution method for a dilution factor of 10 − 4.
4. Conclusion
References
The humidity standard generators of a permeation tube/single dilution and a water bottle/double dilution have been compared for the range of water vapor concentration 5–70 nmol mol − 1 using a precision chilled-mirror frost-point hygrometer as the core of the
[1] SEMI Standard C15, Test Method for PPM and PPB Humidity Standards (1995). [2] P.H. Huang, Proceedings of the Third International Symposium on Humidity and Moisture, vol. 1, London, UK, 1998, p. 149. [3] ISO Guide to the Expression of Uncertainty in Measurement, ISO (1993).
.
Acknowledgements The author is grateful to Kijima Takahiko and James McKinley for their assistance in providing equipment for some parts of the experiments.
Humidity standards for low level water vapor sensing and measurements P.H. Huang * Process Measurements Di6ision, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 2 October 1998; received in revised form 20 October 1998; accepted 20 October 1998
Abstract An intercomparison between the two most widely used types of low level humidity standards constituting a permeation-tube standard and an evaporation–diffusion standard is presented. Using a precision chilled-mirror hygrometer as a transfer standard, frost-point temperatures of nitrogen–water-vapor mixtures produced by the standard generators are measured in the water vapor concentration range of 5–70 nmol mol − 1. Measured differences between the standard generators are found to be within the standards’ expanded uncertainties for moisture content of greater than :15 nmol mol − 1. Below 15 nmol mol − 1, the differences are greater than the combined uncertainties of the standard generators. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Intercomparison; Moisture standards; Permeation– diffusion-tube
1. Introduction Sensing and measurements of low levels of water vapor content in molar volume ratio of the order of 10 − 9 are critical for the applications in which gases of high or ultrahigh purity are used in the microelectronics or semiconductor industries. The use of various commercially available sensors, such as electrolytic cells, dielectrics of polymer/ceramic, vibration crystals, Fourier transform infrared spectrometers, atmospheric pressure ionization mass spectrometers, and chilledmirror frost-point hygrometers, is rapidly increasing. These sensors often require a calibration using moisture standard generators. Therefore, validation of calibration standards is necessary for sensors to be used with certain accuracy and reliability. Humidity calibration standards for producing atmospheres of known water vapor content at the nmol mol − 1 levels are described in Semiconductor Equipment and Materials International (SEMI) StandardC15 [1]. Methods based on permeation of water vapor through a polymeric membrane and evaporation diffusion of water vapor through a small hole of water bottle are popular in semiconductor industry. In this
* Corresponding author. E-mail: [email protected]
paper, the results of intercomparison experiments conducted for the range from 5 nmol mol − 1 (parts per billion by volume) to 70 nmol mol − 1 of water vapor concentration in nitrogen are reported. A precision chilled-mirror frost-point hygrometer was used as a transfer standard whose output in four-wire resistance can be directly related to the measured frost-point temperature and then to the water vapor concentration in nmol mol − 1 [2]. A discussion of the standard generators, the transfer standard, and the measurement techniques are presented. The results obtained and the uncertainty analysis are discussed.
2. Standard humidity generators Standard humidity generators based on dilution methods are used for an intercomparison in this work. A single (one-stage) dilution method is used with a permeation tube and a double (two-stage) dilution method is used with a water bottle.
2.1. Permeation method The method is based on the principle that a stable amount of water in a permeation tube maintained at a
0925-4005/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 9 8 ) 0 0 3 1 3 - X
P.H. Huang / Sensors and Actuators B 53 (1998) 125–127
126
constant temperature to get permeated is fixed so as to constantly generate a standard moisture. A permeation tube of 6 mm in diameter and 20 mm in length was used. The generated water vapor concentration in the dilution gas C, in units of nmol mol − 1 is given by [1]
mirror dew-point hygrometer, which was previously calibrated by the NIST Two-Pressure Humidity Generator. The standard moisture was diluted using a twostage dilution method to obtain a range of water vapor concentration of : 5–70 nmol mol − 1. The concentration in the diluted gas C can be expressed as
(1)
C =Kg/F +B
(2)
C= C0D
where K is the molar gas constant for water vapor at 0°C and one atmospheric pressure and is equal to 1.243 l g-mol − 1; g is the permeation rate in units of ng mol min − 1 at a constant temperature and is determined by gravimetry; F is the gas flow rate in units of l min − 1 with reference to Standard Temperature Pressure (STP); and B is the background moisture concentration, in units of nmol mol − 1, measured without permeation tube.
where C0 is the standard water vapor concentration and D is the dilution factor.
2.2.1. Experimental uncertainty The relative expanded uncertainty in water vapor concentration C generated by the method described above can be determined from Eq. (2) based on the relative standard uncertainties of C0 and D. The relative standard uncertainty of C0 = 8392 nmol mol − 1 was determined to be 2.1% of the value [2]. The relative standard uncertainty of D was determined by the relative standard uncertainties of the calibrated mass flow meters. The flow meters had a maximum relative standard uncertainty of 1%. For a maximum dilution factor of 10 − 4, the maximum relative standard uncertainty of D using a double dilution method was 2%. The relative combined standard uncertainty in C based on Eq. (2) is 2.9%. The relative expanded uncertainty of C with a coverage factor of 2 is therefore 5.8%.
2.1.1. Experimental uncertainty The relative expanded uncertainty in the generated water vapor concentration can be determined from Eq. (1) using the International Standard Organization (ISO) Guide to the Expression of Uncertainty in Measurement [3]. For the range in C of :5 – 70 nmol mol − 1, a mass flow meter (full scale 5 l min − 1) and a permeation tube (g=23 ng min − 1 at 60°C) were calibrated. The maximum relative standard uncertainties of flow rate F and permeation rate g were 1 and 5%, respectively. Assuming BB B Kg /F, the maximum expanded uncertainty of C with a coverage factor of 2 ( : 95% confidence level) was determined to be 10.2%.
3. Comparison of measurement results
2.2. E6aporation diffusion method
Table 1 summarizes the results of the standard intercomparison. Column 1 is the reference water vapor concentration and column 2 is the difference between the mean values of three tests obtained for the concentration Cpts using the permeation tube/single dilution and the the concentration Cwbd using the water bottle/ double dilution standard generators. A precision chilled-mirror frost-point hygrometer was used as a transfer standard capable of measuring frost-points to − 110°C. Columns 3 and 4 are the expanded uncertainty (k=2) of the permeation tube/single dilution standard generator and water bottle/double dilution standard generator, respectively.
The method is based on the principle that water vapor can diffuse from water in a glass bottle having a small hole at the top. A cylindrical glass bottle of 10 mm in diameter and 20 mm in height having a hole of 1 mm in diameter was used. The diffusion rate is a constant for the water bottle maintained at a constant temperature. In this method, a standard water vapor concentration of 8392 nmol mol − 1 was generated when the nitrogen gas was fed at the flow rate of 1 l min − 1 when the water bottle was maintained at the temperature of 30°C. It was measured using a precision chilled-
Table 1 Summary of results of the intercomparison of the permeation tube/single-dilution and the water bottle/double-dilution standard generators Nominal concentration Difference C (nmol mol−1) Cpts−Cwbd (nmol mol−1)
Expanded uncertainty
7 16 30 70
0.7 1.6 3.1 7.1
1.4 1.6 2.0 3.0
Combined uncertainty uc (nmol mol−1)
Agreement indicator Ia
1.1 2.0 3.6 8.2
1.3 0.8 0.6 0.4
Upts (nmol mol−1) Uwbd (nmol mol−1) 0.4 0.9 1.7 4.1
P.H. Huang / Sensors and Actuators B 53 (1998) 125–127
127
The measurements at NIST obtained over two successive years show that the transfer standard hygrometer was repeatable to 0.2°C. These results are based on the residuals of a least-square fit to the data comprising frost-point temperature vs. standard generator moisture concentration. Column 5 of Table 1 gives the combined uncertainty uc of the intercomparison between a permeation tube/single dilution and a water bottle/double dilution standard generators. Column 6 gives an indicator of agreement Ia defined to be the absolute value of Cpts −Cwbd divided by uc. A value of IaB 1 indicates that the two types of standard generators agree to within their combined uncertainties. Inspection of column 6 of Table 1 indicates that all the differences in the measured water vapor concentration are smaller than uc except at a nominal value below 15 nmol mol − 1.
transfer standard system. The results show that the moisture standards for low levels of water vapor above 15 nmol mol − 1 in nitrogen gas agree to within the combined uncertainties of the standards or to within 10% of a generated nominal value. At a nominal value of 7 nmol mol − 1, the standards agree to within 1.4 nmol mol − 1 or 20% of the value. The expanded uncertainty of the standard based on a double (two-stage) dilution method is significantly smaller that of single dilution method for a dilution factor of 10 − 4.
4. Conclusion
References
The humidity standard generators of a permeation tube/single dilution and a water bottle/double dilution have been compared for the range of water vapor concentration 5–70 nmol mol − 1 using a precision chilled-mirror frost-point hygrometer as the core of the
[1] SEMI Standard C15, Test Method for PPM and PPB Humidity Standards (1995). [2] P.H. Huang, Proceedings of the Third International Symposium on Humidity and Moisture, vol. 1, London, UK, 1998, p. 149. [3] ISO Guide to the Expression of Uncertainty in Measurement, ISO (1993).
.
Acknowledgements The author is grateful to Kijima Takahiko and James McKinley for their assistance in providing equipment for some parts of the experiments.