Measurement Techniques, Vol. 52, No. 6, 2009
PHYSICOCHEMICAL MEASUREMENTS HIGH PRECISION DEVICE FOR REPRODUCING CONCENTRATION UNITS FOR OXYGEN DISSOLVED IN WATER
A. A. Ukolov, O. V. Karpov, and L. N. Bryanskii
UDC 544.355-13
A high precision device for reproducing concentration units for oxygen dissolved in water and its operating principles are described. Results are given for studies of the technical and metrological characteristics of the device. Key words: installation, water, oxygen, concentration, certification, calibration.
There are a number of difficulties in the delivery and operation of instruments for determining the content of oxygen dissolved in water connected with the lack of equipment comprising a basis for metrological provisions in certification and calibration of oxygen analyzers. Currently they are calibrated for a solution of deoxidized distilled water and for water saturated with oxygen at constant temperature and normal atmospheric pressure [1]. In order to obtain intermediate concentrations of oxygen dissolved in water [1], fulfilling the role of reference points, previously certified nitrogen-oxygen mixtures dissolved in distilled water are used [2]. The aim of developing a high precision device (HPD) is an increase in the accuracy of reproducing concentration units C of oxygen dissolved in water. The device is intended for checking (calibrating) analyzers used in enterprises of thermal and atomic power engineering, metallurgy, the food, chemical, and oil processing industries, in biotechnology and medicine, housing, in fisheries, in stations for biological purification of waste water, and in Goskompriroda organizations [3–5]. The high precision device reproduces a concentration unit C within a hermetically sealed volume of the working chamber. For this purpose, a distilled water solution is used with a different equilibrium concentration of dissolved oxygen prepared both by saturation of water with oxygen from the air under constant temperature conditions and prescribed pressure (rarefaction) in the gas phase, and by mean of nitrogen-oxygen certified gas mixtures. The basis of the method for preparing solutions with a prescribed oxygen concentration dissolved in distilled water within the closed volume of the working chamber assumes a mathematical dependence of the Henry–Dalton rule specifying a linear change in equilibrium concentration of oxygen dissolved in water, obtained by changing the absolute air pressure within the working chamber and receiver, and the dependence of oxygen solubility in distilled water on temperature [6]. The value of concentration C at pressure p, the relative volume concentration of oxygen X (%) in a certified gas mixture and water temperature T is calculated by the equation C = ApX/p0x0, (1)
All-Russia Research Institute of Physicotechnical and Radio Engineering Measurements (VNIIFTRI), Moscow, Russia; e-mail:
[email protected]. Translated from Izmeritel’naya Tekhnika, No. 6, pp. 62–63, June, 2009. Original article submitted March 30, 2009. 676
0543-1972/09/5206-0676 ©2009 Springer Science+Business Media, Inc.
220 V
220 V
220 V
Fig. 1. HPD structural layout: 1) compressor unit; 2) receiver-aerator; 3) control unit; 4) vacuum gauge; 5) manometer; 6–8) certified gas mixtures; 9) argon cylinder; 10) control panel; 11) magnetic stirrer; 12) working chamber; 13) bubbler; 14) thermostat; 15) sealing ring; 16) adapter; 17) oxygen sensor; 18) thermometer.
C, mg/dm–3
X, mg/dm–3
p, kPa
a
b
Fig. 2. Dependence of concentration C on atmospheric air pressure in the working chamber (a) and on the volume content of oxygen X in the certified gas mixture (b).
where A is the solubility (equilibrium concentration) of oxygen in water (mg/liter or %) at normal pressure and temperature T, °C; p is current pressure (kPa); p0 is normal pressure, equal to 1.3 kPa; and X0 is relative oxygen concentration in a standard atmosphere, equal to 20.94% [6]. The HPD structural layout is provided in Fig. 1. It follows from relationship (1) that a change in the equilibrium concentration of oxygen dissolved in water at constant temperature is directly proportional to a change in absolute pressure in the working chamber and receiver. Consequently, the linearity of the analyzer transformation characteristic, subject to certification, may be estimated from the linearity of the change in absolute pressure in the working chamber and receiver, specified by readings of the standard absolute pressure meters (barometer, manometer, vacuum gauge). 677
TABLE 1. Estimate of Error and Uncertainty of Measurements Measurement range, µg/dm3
Error
MSD
Type A uncertainty
Type B uncertainty
Total uncertainty
Expanded uncertainty (k = 2)
10–100
±1
0.5
0.5
0.6
0.8
1.6
100–1000
±5
2
2
3
3,5
7,0
1000–10000
±15
6
6
9
11
22
10000–30000
±60
20
20
30
40
80
Since the ratio of absolute pressure to normal atmospheric pressure is used for the characteristic of the change in dissolved oxygen concentration within the closed working chamber, it is necessary to toughen the requirement for cleaning air from absorbing impurities and moisture, i.e., it is possible to prepare a verification solution with almost any accessible open source of atmospheric air. This gives rise to a reduction in difficulties of the certification method. In view of the fact that during analyzer calibration by two reference points within a selected measurement range the same atmospheric air is used, the linearity of transformation and precision of analyzer verification is only specified by the class of accuracy for standard absolute pressure measurement facilities. The linearity of the dependence of concentration C on atmospheric air pressure within the working chamber of the device is shown in Fig. 2a, and on the volume content of oxygen X in a certified gas mixture [2] in Fig. 2b. The device provides reproduction of a concentration unit for oxygen dissolved in water from 10 µg/liter to 30 mg/liter within the temperature range 15–50°C. The results of estimates for errors and uncertainty of HPD measurements are provided in Table 1. Measurement of oxygen within the “microgram” concentration range (less than 10 µg/dm3) give rise to particular complexity. The specific nature of measurement of oxygen in this range imposes additional, quite stringent requirements for metrological and operational characteristics for the amperometric sensors themselves, as also for the HPD calibration and certification [7]. Currently there is no settled method for reproducing the concentration unit for oxygen dissolved in water within this range, although a number of producers in prospect point to a range starting from 0.1 µg/dm3. Electrochemical analyzers are also used extensively for analytical monitoring of other electrochemically active gases (H2, N2O, CO, CO2, etc.) in resolving various scientific and applied problems. Therefore, one of the next stages should be expansion of the range of verification instruments for monitoring other gases.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
678
R 50.2.045−2005, Analyzers for oxygen dissolved in water: Certification procedure. TU 6-16-2956−01, Certified Gas Mixtures – Standard Composition Samples: Technical Conditions. A. F. Albantov et al., Atomenergoanalitika–2005, Sosnovyi Bor (2005), p. 20. L. M. Zhivilova and Yu. A. Slobodskaya, Novoe v Rossiiskoi Energetike, No. 8, 34 (2008). A. Sh. Groisman and N. E. Khomutov, Uspekhi Khimii, 59, 1217 (1990). ISO 5814:1990(E), Water Quality: Determination of Dissolved Oxygen: Electrochemical Probe Method. A. F. Albantov et al., Practical and Procedural Aspects of Metrological Provision for Electrochemical Measurements [in Russian], Mendeleevo (2001), p. 25.