Calibration gas mixtures

Traceability requirements in this area have so far mostly been fulfilled with calibration gas mixtures directly... 

NPL, in line with other national standards laboratories, retains its primary standard gas mixtures (PSMs) in-house. These primary standards are disseminated, however, through different types of calibration gas mixtures. These disseminated standards are known at NPL as Primary Reference Gas Mixtures , Secondary Gas Standards and Certified Gas Mixtures . An NPL leaflet has been prepared which explains the differences between these types of traceable standards and which also explains the relationship of these with the different types of standards produced by other national metrology institutes (NMIs). The main type of gas standards disseminated by NPL, are however, secondary gas standards and the procedures used for preparing and certifying these are outlined below. 

ISO 1998 ISO/CD 6142(1998) Gas analysis Preparation of calibration gas mixtures Gravimetric method. ISO, Geneva... 

The mode of calibration is determined by the design of the instrument. Most instruments contain a barometer or a transducer responsive to P(Amb) so that barometric pressure is always known to the microprocessor. With such instruments, only a keyboard entry of the fractional composition of O2 and CO2 in low and high calibrator gas mixtures needs to be made. Today, most analyzers auto-calibrate without the necessity for user input. The microprocessor will calculate the values for PO2 and PCO2 (according to Dalton s law) for gases saturated with water vapor at 37 °C. 

The uncertainty of commercial calibration gas mixtures is typically 1% of the amount fraction, and the contribution of this to the overall measurement uncertainty is much greater than the intrinsic error of the oxygen sensor for amount fractions of oxygen that are 0.1 ppm and above [2]. Consequently, test procedures that require calibrated gas mixtures will be limited by the uncertainty of those gas mixtures, and therefore carmot be used to test the accuracy of oxygen and other zirconia-based gas sensors, assuming the uncertainty of the sensor calibration is no worse than that of the test mixtures. At present, the uncertainty of commercial calibration gas mixtures is a limitation to verifying the accuracy of these gas sensors. 

Generating calibration gas mixtures with a pump is inexpensive and a universally employable solution in a situation that demands only a small number of components, but it can become quite costly if there is an increase in the number of components to be proportioned simultaneously. Pumps allow dilutions of up to a factor of 100 this limit is a result of the minimal conveying flow rate of the dosing pumps, approximately 10 cubic centimeters per minute (CCM). With other volumetric techniques, with a dosing volume of approximately 40 pL, significantly lower concentrations can be achieved. 

However, if highly flexible calibration gas mixtures consisting of liquid components with short response times need to be generated, vaporizers or saturators are suitable solutions. 

ISO, Gas Analysis - Determination of Composition of Calibration Gas Mixtures - Comparison methods , ISO/DIS 6J43/1 Geneva ISO, 1981. 

ISO, Gas Analysis — Preparation of Calibration Gas Mixtures — Dynamic Volumetric Methods — Part 1 Calibration Methods , ISO/DIS 6I45/I Geneva ISO, 1986. 

VDI, Measurement of Gases — Calibration Gas Mixtures - Dynamic Preparation by Gas-Mixing Pumps, VDI-Guideline 3490, sheet 6 Berlin Beuth, 1988. 

Cylinder Material. A major area of effort of the program has been to verify that the choice of cylinder material will not affect the stability of the calibration mixture. To test this, we filled duplicate sets of aluminum and stainless steel cylinders with the calibration gas mixture from the carbon steel source cyhnder. These five cylinders have been analyzed on a monthly basis for the past six months. As expected, no significant concentration changes in any of the 10 components are evident in any of the five cylinders at this point. 

In common with other analytical techniques, the best accmacy will be obtained by reference to calibrated gas mixtures whose composition is similar to that of the sample. This is straight forward if the sample composition remains fairly constant. If the gas composition varies more widely, one may need to calibrate by reference to a small number of selected gas mixtures. In a typical case, for a complete calibration, to provide accurate composition monitoring, it is necessary to use a minimum number N calibration gases where N is calculated as ... 

The reactor outlet was directly connected to a quadrupole mass spectrometer (Balzers QMS 200) and to a UV-analyzer (ABB LIMAS 1IHW) in parallel. NH3, NO, NO2, N2O, O2, and He were dosed from bottled calibrated gas mixtures by mass flow controllers, while water vapor was added by means of a saturator. The catalyst temperature was measured by a K-type thermocouple directly immersed in the catalytic bed. 

Generally, gas mixtures intended for the application as calibration gas mixtures have to fulfil higher demands on manufacturing and analysis as well as on the purity of the raw materials. 

There are two official German definitions for the term calibration gas mixtures ... 

Component Gaseous or vaporized component of a calibration gas mixture, known in quantity and quality, and directly used for examination of the calibration . 

For a comprehensive description of a gas mixture/calibration gas mixture, apart from the indication of the kind of balance gas and components, even the indication of the mole fraction resp. of the concentration is required. 

The mole fraction of a component is the ratio of the number of moles of the component to the sum of the number of moles of all components of the calibration gas mixture. 

Only consequent pretreatment of the containers [9.9] enables the production of stable calibration gas mixtures. Since calibration gas cylinders can be used for the most different compositions, both residues of the previous mixture and a possibly existing moistiue film on the inner surface have to be removed during the pretreatment, if possible quantitatively. For this purpose, the containers are being... 

Basically, calibration gas mixtures are produced by combining defined quantities of different components. The preparation methods are characterized by special features ... 

The basic principle is the blending of different volume or mass flows. The simplest mixing arrangement consists of one pressure regulator and one flow meter per type of gas. The low uncertainties of the content of components desired for the calibration gas mixtures cannot be achieved with this simple arrangement. If manufacturing tolerances of 2 to 3% rel. shall be obtained, the volume ratio control has to be taken over by the measured variable of the respective analytical device. Moreover, the filling into compressed gas cylinders requires a suitable compressor. 

Advantage of this method Economical large-scale production of calibration gas mixtures with constant composition is possible. 

Precision balance (Beam balance) With a resolution of 3 mg, for instance, these balances dispose of a capacity of 30 kg. They are also applied for the production of calibration gas mixtures according to the method of re-weighing with an inaccuracy of up to 0.01% rel. (Fig. 9.1). 

In the production of these extremely precise calibration gas mixtures [9.11], apart from the inaccuracies of the balances even other influencing factors, such as gas impurities, filling errors or lift forces have to be considered. The balance itself only appears at the third level in the uncertainty hierarchy [9.12]. 

Mass of the component to be dosed The mass of the smallest component has to be significantly higher than the absolute uncertainty of the balance. With electronic balances of a capacity of 150 kg and a resolution of 0.1 g (s.a.), a production tolerance of + 1% rel. can only be achieved with the weighed gas sample being at least 10 g. In case this mass is not achieved owing to the desired content of the calibration gas mixture, premixtures that are also gravimetrically produced have to... 

Disadvantage of this method Increased manufacturing expenditure (time-, staff and plant-constuning). Meanwhile, these disadvantages have been reduced by automation to the extent that today, the gravimetric method is the preferred method for calibration gas mixture production. 

Gas mixtures/calibration gas mixtures are used in a number of applications in environmental protection, process optimization, protection of people and plants and in research. Table 9.5 shows an excerpt of the most important applications. 

Unsuitable fittings and pipes can reduce the quality of the pure gases and calibration gas mixtures considerably. The selection of the adequate gas supply system depends on the chemical, physical and physiological properties of the gases applied and the application-specific demands. 

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