Manometers calibration

Xl.6.4 Mercury Manometer calibrated in 10-mm divisions with a distinguishing mark at 152 mm (equivalent to 20.3 kPa). 

Calibration of Gauges Simple liquid-column manometers... 

The question is often asked. How often should calibration be carried out Is it sufficient to do it once, or should it be repeated The answer to this question depends on the instrument type. A very simple instrument that is robust and stable may require calibrating only once during its lifetime. Some fundamental meters do not need calibration at all. A Pitot-static tube or a liquid U-tube manometer are examples of such simple instruments. On the other hand, complicated instruments with many components or sensitive components may need calibration at short intervals. Also fouling and wearing are reasons not only for maintenance but also calibration. Thus the proper calibration interval depends on the instrument itself and its use. The manufacturers recommendations as well as past experience are often the only guidelines. 

From the calibration point of view, manometers can be divided into two groups. The first, fluid manometers, are fundamental instruments, where the indication of the measured quantity is based on a simple physical factor the hydrostatic pressure of a fluid column. In principle, such instruments do not require calibration. In practice they do, due to contamination of the manometer itself or the manometer fluid and different modifications from the basic principle, like the tilting of the manometer tube, which cause errors in the measurement result. The stability of high-quality fluid manometers is very good, and they tend to maintain their metrological properties for a long period. 

The second, mechanical and electrical manometers, require more frequent calibration. Changes in the elastic properties of the pressure transducer, wearing in mechanical parts, and electronic circuitry drift influence the properties of the instruments, giving rise to repeated calibration. 

The principle of calibration is to compare the measurement result of the manometer to be calibrated to that of the measurement result of the reference... 

One of the best and most convenient methods of measuring the flow in the terminal is to use the terminal characteristic pressure difference. This requires that the manufacturer of the terminal provide calibration curves, where the flow rate is expressed as the function of the characteristic pressure difference. Some devices have integrated pressure measurement tappings, and the user has only to attach a manometer to measure the pressure difference. 

Slant gauge An inclined calibrated manometer tube. 

The transfer flow for 2-stage dilution could be measured only after partially dismantling the olfactometer, which is a clear disincentive to regular calibration. WSL s portable olfactometer included an orifice plate and micro-manometer for measuring this flow, which could thus be checked and adjusted at any time. This investigation has therefore demonstrated the value of using in situ, non-interfering flow measurement devices that are accessible to the operator at all times. 

The diameter of the capillary tube must be known with accuracy and the cross-section must be truly circular. It is not as a rule easy to obtain tubing of uniform circular bore, but in default of time for the tedious process of calibrating tubing the difficulty may be overcome by the following method due to Ferguson Proc. Phys. Soc. XXXVI. 37, 1923) Lengths of capillary tube are examined under a micrometer until one is found whose end cross-section is circular. This tube is then used so that the measured end dips downwards into the liquid the upper end, instead of being open to the atmosphere is connected to a source of pressure and a manometer, and the meniscus is forced down until its lowest point is level with the end of the tube, so as to be observed at the only point where its curvature is accurately known. If then p be the pressure in dynes/cm. recorded by the manometer we have... 

If vapour pressure measurements are to be an essential part of the work to be undertaken, a cold cathode manometer is probably the best choice, despite the fact that it needs to be calibrated for each molecular species, and its use with mixtures of gases containing two or more species is correspondingly more difficult. If such mixtures are to be investigated, or if the chemicals concerned are corrosive, it is probably most efficient to use a mechanical gauge as a null-point instrument and to measure the pressure by means of a McLeod gauge. 

Many types of vacuum adsorption apparatus have been developed and no doubt every laboratory where serious adsorption measurements are made has equipment with certain unique features. The number of variations are limited only by the need and ingenuity of the users. However, all vacuum adsorption systems have certain essential features, including a vacuum pump, two gas supplies, a sample container, a calibrated volume, manometer and a coolant. 

The sample cell is not completely filled by the adsorbent it contains a void volume which is the volume not occupied by adsorbent, up to stopcock 4. If the calibrated volumes and manifold are filled with nitrogen at pressure Pj and stopcock 4 is opened the pressure will fall because of expansion and adsorption of nitrogen in the evacuated cell. After equilibrium is attained and the mercury in the manometer is returned to the fiducial mark the new pressure P is noted. The number of moles adsorbed is given by... 

Points on the desorption isotherm are obtained by first adsorbing at some relative pressure close to unity. Then, stopcock 4 is closed and the calibrated volumes and manifold are evacuated. Stopcock 4 is opened which permits desorption to occur. The equilibrium pressure is obtained from the manometer and the volumes desorbed are calculated using equation (14.8) except that P is now the pressure after desorption. 

Each orifice constructed in the above manner was calibrated with a rotameter and manometer. The set-up used for orifice calibration is shown in Figure 3. The uncorrected flow readings were obtained from the rotameter calibration curve. The corrected flows were then calculated using the following equation ... 

Stock solution. The concentration of the stock solution was determined by the sodium iodate-thiosulfate titration method. For each determination, a 100.0-ml solution was prepared and placed in a vessel connected to a manometer for measuring the pressure. The vessel was sealed after insertion of a measured piece of catalase-immobilized CoFoam. The reaction of catalase with peroxide produces O2, and an increase in pressure indicates a degradation of the peroxide. Thus, a change in pressure in the vessel is a measure of the reaction rate. Since it is sufficient to show differences in test samples, the ideal gas law was used to convert the pressure into mass. The barometer was calibrated with a gauge traceable to National Institutes of Standards and Technology (NIST) standards. 

Furnace temperatures were measured by two Pt—10% Rh thermocouples. One was mounted next to the suspended substrate samples and the other next to the vapor source. The rate of flow of the dry air through the furnace was determined by measuring the air pressure upstream from a capillary restriction in the air line. The pressure was measured by an oil manometer which had been calibrated against known flow rates of air through the capillary restriction. 

Water activity (aw) is the ratio of the partial vapor pressure of water above a solution to that of pure water at the same specific temperature. It plays an important role in evaluating the microbial, chemical, and physical stability of foods during storage and processing. The vapor pressure in the headspace of a food sample can be measured directly by a manometer. A manometer has one or two transparent tubes and two liquid surfaces where pressure applied to the surface of one tube causes an elevation of the liquid surface in the other tube. The amount of elevation is read from a scale that is usually calibrated to read directly in pressure units. Makower and Myers (1943) were the first to use this method to measure vapor pressure exerted by food. Later, the method was improved, in terms of design features of the apparatus, by various scientists (Taylor, 1961 Labuza et al., 1972 Lewicki, 1987). Trailer (1983), Lewicki (1989), and Zanoni et al. (1999) used a capacitance manometer instead of a U-tube manometer for the measurement of vapor pressure. Lewicki et al. (1978) showed that the precision and reproducibility of the method can be improved by the simultaneous measurement of the water vapor pressure and temperature of the food sample. The method is reviewed in detail by Rizvi (1995) and Rahman (1995). 

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