The automatic calibration system of humidity fixed points at CMS

Measurement Vol. 19, No. 2, pp. 65-71,1996

PII: S0263-2241 (96) 00052-8

ELSEVIER

©1997 Elsevier Science Ltd Printed in The Netherlands, All fightsreserved 0263-2241/96 $15.00 +0.00

The automatic calibration system of humidity fixed points at CMS Yih F. Lin, Taun I. Yeh, Kuo H. Chan, Tu S. Chen Center for Measurement Standards (CMS) of Industrial Technology Research Institute, Taiwan, R.O.C.

Abstract The automatic hygrostat is designed to calibrate hygrometers by employing several humidity fixed points under thermostat conditions. The set incorporates a blowing fan, a box-like measuring vessel, a control system, and an assembly of five acrylic resin trays which contain stainless steel covers, labyrinth baffle-boards at the bottom and are all set in a reciprocating motion. The trays are successively turned on or off by operating solenoid valves to give a closed circulation with the measuring vessel, establishing the humidity at a specific value selected by the user. The desired relative humidities are attained by adding lithium chloride, potassium carbonate, sodium chloride, potassium chloride, and potassium sulfate to pure water inside the five trays and can generate levels of approximately 12, 43, 75, 86, and 97%RH respectively, which have a maximum difference value, compared to reference values of +_I%RH in the range around room temperature. After switching to another tray for the control setting, the time required to reach a new equilibrium should be no longer than about 20 minutes. © 1997 Elsevier Science Ltd. All rights reserved.

Keywords: Humidity fixed points; Saturated salt solution; Hygrostatic solution; Working standards; Regression curve

1. Introduction

hygrometers in industry or laboratory applications if close fitting of the circulatory system is ensured. A certain given saturated solution only offers a specific relative humidity because relative humidity is a function of temperature. Consequently, various humidity standards can be produced by applying other adequately saturated salt solutions. A systematic apparatus for H F P is usually classified as follows: a simple box-like hygrostatic container [1], circulatory humidity fixed point chambers [2] and a semi-automatic humidity fixed point generator [3], etc. The improved structure and functionally characteristic test of the above mentioned H F P generator will be described.

The relative humidity of air is defined by the ratio of the mole fraction of water vapor in moist air at a given temperature and at a total wet gas pressure to the mole fraction of saturated water vapor in the air under identical conditions. A relative humidity is usually expressed as a percentage. A relative humidity value in air within the confines of a tightly closed container can be obtained by adding some chemical (e.g. sodium chloride, glycerin or sulphuric acid) to pure water because the water vapor of the container space will reach a fixed and stable equilibrium state. The equilibrium for a relative humidity value utilizing a component chemical thus remains a constant under the conditions of fixed solution concentration, atmospheric pressure and ambient temperature, which is called the method of humidity fixed points (HFP). The standard of H F P with good reproducibility is suitable for the calibration of

2. Preparation and conditions of hygrostatic solution [ 3 - 5 ] The H F P method is based on maintaining a thermodynamic equilibrium state of three phases 65

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including salt, water and vapor to generate a fixed relative humidity under the conditions of specific temperature and pressure and a constant solution concentration. This saturated solution used is called the hygrostatic solution.

2.1. Reference conditions (1) The total pressure of air and water vapor is approximately equal to atmospheric pressure (101.325 kPa) above the salt solution. (2) Solid, solution, air and water vapor reach thermodynamic equilibrium. (3) The saturated solution is used at an adequate temperature. (4) Most salt solutions are quite stable; however, it is necessary both to avoid using them at temperatures near their transition points in order to prevent the formation of new hydrates, and also be aware of the flammability of some salts. The transition temperatures at which salts develop into other compounds can usually be found in general physical chemistry handbooks.

2.2. Requirements for temperature stability (1) The thermostat must be suitably used to maintain a constant temperature environment and control the temperature gradient of the testing space with an accuracy of better than + 0.2°C between 0 and 45°C. (2) The HFP generator should be adequately equipped with a good heat insulator to environmental conditions when used without a thermostat, and the change of operating temperature must be maintained below ±2°C by installing an air conditioner when applied to a nonthermostatted hygrostat.

2.3. Steps for preparation (1) The standard solutes used must be reagent grade chemicals, verified by ACS (or ISO) standards of high purity. (2) Pure water is obtained by applying either ion exchange resin, osmosis or distillation.

(3) Select proper salts to correlate with the relative humidities utilized. (4) The height of salt at the bottom of the trays for generating lower relative humidity is 2 cm, and for generating higher relative humidity, it is 0.8 cm. (5) The applied salt is added to pure water at a higher temperature. Then the solution is cooled to the required temperature. A saturated salt solution can be composed of a slushy mixture of pure salt with distilled water and formed with a thin layer of liquid solution overlaying the salt, which could thus alleviate any temperature variation within the salt solution.

3. HFP systematic apparatus [3,6] 3.1. Needs for the design (1) Any conditioned humidity tray with a saturated solution should have a connection with an air-tight vessel which has an adequate shape and size suitable for checking hygrometers. (2) The hygrostat should be made of nonhygroscopic material and be resistant to corrosion. (3) The hygrostat should have good tightness and should permit easy washing and cleaning. (4) The ratio of volume to inner wall surface for the measuring vessel should have the least possible value. (5) The whole hygrostatic chamber must reach a constant temperature. (6) The hygrostat should have a gas circulator, but the fan motor must be mounted outside to prevent interference from the heat produced. (7) The free surface of the saturated salt solution should be maximized in order to shorten the time to reach equilibrium. (8) The temperature of the gas above the saturated salt solution should be measured.

3.2. Construction of the hygrostat The design of an automatic hygrostat, as shown in Fig. 1 includes a two-layer box-like testing vessel. The upper or lower layer is selected by means of the 3-way valve. The top cover of the

Y.F. Lin et al.

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chosen window via the assistance of the HFP apparatus and QuickBasic programs.

4. Measurement Results and Discussions

Fig. 1. Photograph of the automatic HFP generator.

upper layer has 10 cylindrical side holes in which humidity sensors are filled up with cylindroid silicones, and hygrometers or commercial products can be put into the lower vessel to test their response and hysteresis of their dynamic characteristic or for the purposes of calibration. This lower testing vessel checks the functional display of a sample and appearance variation by using reinforced glass outside the hygrostat. Five acrylic resin trays, which have lids of stainless steel for the sake of easily transferring heat, are mainly provided for convenient manufacturing and sealing. The trays themselves are transparent to facilitate examination of whether or not the salt solutions are in saturated states. These above mentioned trays with labyrinth baffle-boards at the bottom are all set in a reciprocating motion. The scanner switches the temperature or humidity value of some test item by a digital input/output, which measures voltage by employing a multimeter with a standard GPIB bus for programmable instrumentation. Solid state relays are also indirectly controlled by a digital input/output to turn these trays on or off by operating solenoid valves to create a closed circulatory connection between the measuring vessel and the selected salt tray. A personal computer (PC) is used to link the temperature controller of this humidity generator with an RS-232 cable. Information regarding the selected salt tray, operating ambient temperature, operating time, temperature and humidity values for several test items is displayed on the PC screen through a

Various functional tests for this HFP generator, demonstrated in Fig. 1, are performed by utilizing a calibrated temperature-humidity working standard, a Rotronic-hygroskop DV-2 hygrometer with a long term stable precision of 0.1°C and less than 0.5%RH from product specifications. The working standards were calibrated by applying the calibration system of the two-pressure humidity generator at CMS, whose total uncertainties are, respectively, 1.3% for a fixed humidity reading and 0.2°C under the condition of a 95% confidence level. Add lithium chloride (LiC1), potassium carbonate ( K 2 C O 3 ) , sodium chloride (NaC1), potassium chloride (KC1), and potassium sulfate (K2SO4) to pure water inside the five trays, then keep these saturated solutions waiting until they are in equilibrium. The testing constituents for the HFP generator are indicated in Fig. 2. All values for the following graphs were modified by utilizing the regression curve of this calibrated working standard. Figure 3(a) and (b) illustrates the humidity values for K2CO3(aq ) at a temperature variation of about 17-24°C. Relative humidities decrease when the temperature rises; however, in theory, more water evaporates from a saturated salt solution inside a tray during an applied temperature increase, but this establishment of the water vapor pressure difference between the testing vessel and the salt tray occurs by slow heat conduction, which is primarily due to manufacturing the material for an acrylic resin tray and leads to the equilibrium delay. In an inverse situation, with decreasing temperature, a similar consequence still holds true. The value of this equilibrium relative humidity for the HFP method is the functional relationship between solution concentration, pressure and temperature inside an air-tight space. The temperature of the saturated salt solution affects its equilibrium therefore the measured value will not follow the expected value as the temperature changes. The humidity will then attain equilibrium and the measured value should equal the acknowl-

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oltimoter [! 1 : D I/O

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3(a)

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edged value after the temperature allows stabilization of the saturated salt solution. Humidities roughly reaching equilibrium in Fig. 3(a) and (b)

are 43.0%RH and 43.8%RH respectively (the reference value is 43.2%RH [7]). Figure 4(a) and (b) indicates the humidities for

Y.F. Lin et al.

69

value is about 86.0%RH for Fig. 5(c), by switching to the KCl~aq) tray from the K2CO3~aq) tray and through the medium of the NaCl~aq~ tray at an average temperature around 20.8°C.

NaCl~aq) at temperatures near 22.2 and 24.4°C respectively. Relative humidities gradually approach equilibrium from the originally lower point. The values at a steady state for Fig. 4(a) and (b) are 75.9 and 75.2% (the values referred to related data are 75.6 [1], 75.4 1-7] and 75.5%RH I-8]). Figure 5(a)-(c) shows humidity diagrams for switching salt trays. The new equilibrium value is 97.3%RH from the LiCl~q) equilibrium value (11.3%) to the KzSO4(aq ) equilibrium state at an average temperature of around 20.2; the values of which are 97.59 [7] and 97.1%RH [-8] from the related references. Another equilibrium value is roughly 86.1%RH (the reference value being 85.1%RH [7]) for Fig. 5(b) when changing to KCl~aql from the original mean value (44.1%RH) for K 2 C O 3 ( a q ) under the condition of approximately average temperature, 20.3°C. Then, its equilibrium value is about 12.0% (reference values being 11.8 [1], 11.3 [7] and 12.4%RH [8]) when finally switching to the LiCl~q) tray. This steady

5. Conclusions

The method of humidity fixed points has the advantages of being both economically feasible and easily operated, and also employing a system with little noise when in operation. Thus, humidity standards with good reproducibility may be generated if the ambient temperature, saturated solution concentration and this systematic closeness extent are precisely controlled. However, this method has been capable of offering only discrete points for humidity standards because of the limitations of temperature determination and on the various salts which are used. Consequently, users were not satisfied with this proposed method if they wanted

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Y.F. Lin et al.

Humidity (%) ...... Temperaturc(°C)

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Fig. 5. Equilibrium humidities attained by switching trays at temperatures of a b o u t 20.2-20.8°C.

to calibrate some fixed humidity point. However, it has enhanced the traditional method in the light of simultaneously performing five diverse humidity standard measurements. If the temperature control of the H F P generator is not precise, the following salts can be used: ZnBrz, LiC1, KzCOa and NaC1, whose nominal values are 8, 11, 43 and 75%RH respectively; and the equilibrium relative humidities for different concentrations are less related to the temperatures inside an air-tight vessel. The maximum difference value compared with the refer-

ences is + I % R H in the spectrum around room temperature for the five various salt solutions, and the time required to reach a new equilibrium for this established system with good reproducibility is no longer than about 20 minutes when switching to another salt tray after slowly adding salt to pure water inside the tray for a few days. The humidity equilibrium time and value should reduce and approach to uniformity, respectively, if (1) the total amount of inner circulatory tubing in this humidity generator is increased, (2) solutions are frequently

Y.F. Lin et al.

stirred to avoid concentration gradient formation, and the purity of these applied solutes and the water is ensured and maintained.

Acknowledgements This study was sponsored by funding support from the National Bureau of Standards, Ministry of Economic Affairs in Taiwan, Republic of China. The authors are very grateful to Dr Daesung Chi at Chang Min Technology Co., Ltd, Seoul, Korea, Dr George R. Waller at O.S.U., U.S.A. and Mr David Larson and our colleagues at CMS for their enthusiastic assistance in revising this whole paper and providing suggestions for us.

References [.1] K. Hiroshi, C. Takahashi and T. Inamatsu, A method of realizing humidity fixed points by saturated solutions,

1-2]

[3]

[-4]

[5]

[.6] [7]

[.8]

71 Bulletin of National Research Laboratory of Metrology 37(2) (April 1988) 31-38 (in JapaneseL K. Hiroshi and T. Inamatsu, An air--circulation humidity generator by saturated salt solutions, Bulletin of National Research Laboratory of Metrology 39(3) {July 1990), 19- 24 (in Japanese). The Scale of Relative Humidity of Air Certified against Saturated S a l t Solution, OIML Report SP30-Sr3 I December 19871. Maintaining Constant Relative Humidity by Means of Aqueous Solutions, American Society for Testing and Materials E104-85. Y.F. Lin, T.I. Yeh and K.H. Chan, A study on the approximate polynomials of humidity fixed points, in Proceedings of the SimTec'93 (SCS), November 1993, pp. 153-159. J.F. Young, Humidity control in the laboratory using salt solutions - - a review, J. Appl. Chem. 17 (1967) 241-245. L. Greenspan, Humidity fixed points of binary saturated aqueous solutions, Journal of Research of the National Bureau of Standards 81(A. 1) (1977) 89-96. W. Arnold and S. Hasegawa, Relative humidity-temperature relationships of some saturated salt solutions in the temperature range 0-50°C, Journal of Research of the National Bureau of Standards 53.1 (19541 19-26.