Thermometer, accuracy pressure

Accuracies achievable for measurements in the ocean are 0.002 mK for T, 0.05 % for P and 0.002 for S. Temperature and pressure sensors are calibrated in the laboratory. In situ comparison of the laboratory calibrations of these sensors only makes sense if additional sensors with digital output can be attached to the CTD. Die accuracy and stability of reversing thermometers and pressure sensors sometimes attached to rosette bottles is not suffi-... 

The most interesting liquids for low-temperature thermometry are 3He and 4He, especially for the calibration of resistance thermometers in the range from 0.5 to 4.2 K. Vapour pressure of H2 is also interesting to realize vapour pressure-fixed points included in ITS-90. The measure of He vapour pressure has been carried out with great accuracy [42,43] to establish the ITS-90 (see Section 8.3). There are several experimental precautions to be observed in order to obtain reliable measurements [2],... 

The boiling point temperature was maintained within 0.02°C of the selected temperature, and measured by using a mercury-in-glass thermometer. The equilibrium pressure was measured by means of a mercury-in-glass manometer, and was readable within an accuracy of 0.1 mm. 

Two main principles of temperature measurement use thermocouples and the so-called resistance thermometer. In chemical plants both methods were applied because they are easy to fit and to maintain.The accuracy of the measurement is influenced by, for example, radiation, which must be taken into account. Thermocouples can be inserted into the pressure system using special sealing techniques, or they may be mounted within a protective tube which is introduced into the pressurized volume. Thermocouple-wires are usually protected with an isulating input in closed-end capillaries with outer diameters of at least 0.5 mm. Thermocouples are technically well tested for pressures up to 6 kbar and temperatures to approx. 800°C. Above these ranges the exact measurement is negatively influenced by several parameters, and the deviations must be taken into account. The accuracy of the temperature measurement devices is normally better than 1 °C. 

Both methods cover a wide range (thermocouples, 250°C-1000°C and resistance thermometers, 230°C-2800°C, with an accuracy of better than 0.2%). The response-time of such devices is short, and the small dimensions are favourable for use under high pressure. 

For certain liquids, tile temperature of a boiling solution of the unknown may be compared with that of boiling water at the same pressure, For a given solution, the boiling-point elevation may be calibrated in terms of specific gravity at standard temperature. Usually two resistance thermometers are used. The system finds use in the control of evaporators to determine the endpoint of evaporation, Good accuracy is achieved in the determination of one dissolved component, or of mixtures of fixed composition. 

A knowledge of the variation of the vapour pressure in the neighbourhood of 100° is frequently of value in checking the accuracy of thermometers by immersion in steam at atmospheric pressure. 

The boiling point is one of the most important physical constants of a liquid. It is also easily determined with sufficient accuracy for most purposes if a reasonable quantity of a pure substance is available, for then it is only necessar to carefully distill the sample, noting both the vapor temperature and the barometric pressure. A particularly convenient apparatus for distillation of small quantities is shown in Fig. 1-10. The shape of the flask here is a desirable one because it confines the liquid to a smaller area than does a round-bottom flask. Thermometers with standard taper joints (1-in. immersion) are very convenient and reduce the possibility of contamination, and in vacuum distillation, of leakage. They are also calibrated for partial immersion, thus making emergent stem corrections (page 83) unnecessary. They... 

On account of the mechanical construction of pressure thermometers, both as regards the mechanism for indicating and recording and the necessity for using comparatively large and heavy bulbs, the accuracy that can be secured with this type... 

Variable area flow meter, 0-60 Nm /h, accuracy 1.6% of full scale Variable area flow meter, 0-5 Nm /h, accuracy 1.6% of full scale Resistance thermometer, 0-100 °C, accuracy 0.5% of full scale Pressure range 0-500 kPa, accuracy 0.5% of full scale Testo Hygrotest 600/650 humidity temperature accuracy of up to... 

The value of 1/a for nitrogen at 1 atm is 272. Experiments with other gases indicate that the ice point, 0°C, is equivalent to a value of near 273 K. The Kelvin scale is now defined with high accuracy such that the triple point of water (where ice, water and water vapour are all in equilibrium, at 0.01°C) has the temperature 273.1600° on the Kelvin scale. The triple point is more accurately defined than the ice point. On this basis 0°C is 273.15 K. Measurement on this ideal gas scale is best conducted with a constant pressure helium thermometer, although there are small deviations from the absolute scale. A comparison of the four temperature scales discussed above is given in Table 1.1. 

Introduction. Historically, low-pressure, constant-volume gas thermometers were the only primary thermometers that had the accuracy required for determining the temperatures of defining fixed points. Recent advances in thermometry have resulted in the development of several types of primary thermometers capable of accurate thermodynamic temperature measurements [5-6]. A list of present-day primary thermometers capable of thermodynamic temperature measurement includes ... 

Accuracy. Mercury-in-glass thermometers are relatively inexpensive and can be obtained in a wide variety of accuracy and temperature ranges. For example, between 0 and 100°C, thermometers with a 0.1°C graduation interval are readily available. Factors that affect the accuracy of the thermometer reading include changes in volume of the glass bulb under thermal stress, pressure effects, and response lag. With proper calibration by NIST [9,10] or traceable to NIST, an accuracy of from 0.01 to 0.03°C can be achieved. Table 16.5 summarizes... 

It should be possible to measure the column load at any moment. The measurements should still be reliable at loads above 500 nil/li, when the liquid fs a rule no longer flows in drops but as a continuous stream. A particularly important point is the accuracy of the temperature readings to ensure this, the head should be so constructed that liquid from the condenser cannot reach the thermometer bulb and that the pressure at the point of temperature measurement is the same as that at which the pressure is read. A flowmeter may be used to check the rate of the cooling water (section 8.6), as excessive sub-cooling leads to a false reflux. 

Ensure all flow and differential pressure indicators etre zeroed and calibrated. All control room pressure indicators need to be checked with a calibrated gage. All dial thermometers and outside pressure gages should be removed and checked for accuracy. The unsatisfactory ones should either be replaced or removed prior to the test and read with calibrated thermocouples, thermometers, or gages during the test. These calibrated devices should adequately fit in the existing fittings. 

Liquid-liquid Equilibrium Lines of the Amine-Water Systems Saturated with NaCl. The liquid-liquid equilibria experiments were carried out in a jacketed 125 ml glass flask. A magnetic stirrer in combination with a stir bar provided agitation and a Lauda RK-8-KP thermostat was used to control temperature. The temperature of the vessel content was measured with an ASL precision thermometer (0.01 °C accuracy, 0.002 °C repeatability) and a small hole in the vessel cover ensured atmospheric pressure. Figure 3 shows a schematic drawing of the set-up. 

The pressure can be measured by a Bourdon gauge or a U-shaped manometer, as shown in Figure 5.4. In the case of a Bourdon gauge, the volume is constant and the pressure increases. In the case of a U-tube manometer, the volume increases with pressure and therefore, may not strictly be considered a constant-volume gas thermometer. Although the simplicity of the apparatus is attractive, it is extremely difficult to achieve a high degree of accuracy. 

The shown calorimeter has an accuracy of +0.1% and a temperature range of 170 to 600 K. A sample of 100-300 g is placed in two sets of silver trays, one outside and one inside a cylindrical heater. In the middle of the sample, the tip of the platinum resistance thermometer can be seen. Sample trays, thermometer, and heater are enclosed in a rounded steel shell, which for ease of temperature equilibration is filled with helium of less than one pascal pressure. The shell is covered with a thin silver sheet on the outside, gold-plated to reduce radiation losses. The calorimeter is then hung in the middle of the adiabatic jacket, drawn in heavy black. This adiabatic jacket is heated by electrical heaters and cooled by a cold gas flow, as indicated by the dials of the instmments pictured on the left at the bottom. The whole assembly, calorimeter and adiabatic jacket, is placed in a sufficient vacuum to avoid convection. 

The principles of measurements with a gas thermometer are simple, but in practice great care is needed to obtain adequate precision and accuracy. Corrections or precautions are required for such sources of error as thermal expansion of the gas hulh, dead volume between the bulb and pressure measurement system, adsorption of the thermometric gas on interior surfaces, and desorption of surface contaminants. 

Very few modem calorimeters employ mercury-in-glass thermometers. The limit of accuracy of the most accurate instrument of this type, the Beckmann thermometer, is about 0.001 K it is easily broken, and subject to errors caused by exposed stem, pressure, sticking of the mercury column, and drift in calibration. 

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