Thermometer, accuracy mercurial

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... 

Thermometers—Calibrated mercury-in-glass thermometers of suitable range and graduated to 0.1 T (0.0S C). They shall conform to the requirements of Specification E 1. Calibrated ASTM kinematic viscosity thermometers are satisfactory. Other thermometric devices ate permissible provided their accuracy, precision, and sensitivity are equ or better than ASTM kinematic viscosity thermometers. 

Liquid-in-glass thermometers measure the thermal expansion of a liquid, which is placed in a solid container, on a length scale. The mercury thermometer is one example of liquid thermometers. Alcohol is also used with this type of instrument. The temperature range is -80 to a-330 °C depending on the liquid. The quality, stability, and accuracy vary considerably. The advantages are a simple construction and low price. A disadvantage is that they are not compatible for connection to monitoring systems. 

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. 

The calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

Temperature can be measured with mercury-filled thermometers or by thermistors. Resolution and accuracy of+0.1°C are desirable. Thermometers should be calibrated by comparison with standard thermometers, or by immersing in fresh water and ice mixtures (0° C) and in fresh boiling water (the local boiling point is altitude dependent). 

In the experiments on heats of combustion, we make use of approaches 2 (optionally), 3, and 4. In the experiment on heats of ionic reaction, we make use of 1, 3, and 4. In all cases the small temperature changes can be measured with adequate precision with a relatively inexpensive mercury thermometer. Alternatively the measurements can be made using a sensitive thermistor (see Chapter XVII), which can be monitored repetitively by a computer. Calibration of the thermistor for improved linearity and accuracy is needed in this case, but this procedure itself can serve as a convenient introduction to interfacing a computer to a measurement device. 

Temperature. Water temperature is an important parameter in calculations of oxygen solubility, calcium carbonate saturation and stabiUty, and various forms of alkalinity, as well as in determining basic hydrobiological characteristics. The temperature should be taken in situ for accuracy, and a standard mercury thermometer with readings to the nearest 0.1°C should be used. It should be calibrated against a precision thermometer certified by the National Bureau of Standards. A thermistor is preferable when attempting to measure temperature at different depths and for automated monitoring and surveillance, and should be similady calibrated (see TemperaTUREMEASUREMENT). 

These thermometers contain either mercury, alcohol, etc., as liquids. The thermal expansion of these liquids is greater than the glass, so the height of liquid in the glass capillary rises as the temperature increases. A major problem is that the glass can be easily broken. Furthermore, mercury causes toxicity problems if the thermometer breaks. Visual observations are usually required to read the thermometers. Often these instruments are restricted to temperatures from about 0°C to 400°C. Their advantages are low costs, long life if properly protected, and reasonable accuracy. They still are widely used in experimental setups and for various home uses. 

If somewhat better accuracy and/or a sensor smaller (with a shorter response time) than the mercury-in-glass thermometer is required, then an industrial-grade PRT may be used. Normally, one can expect such PRTs to have an uncertainty of 0.02°C, plus whatever statistical uncertainty is present, if the PRTs are calibrated periodically (Mangum and Evans, 1982 Connolly, 1982).Some industrial PRTs are capable of giving better results than this and some much worse (Mangum and Evans, 1982). A few can be stable and reproducible to 0.005°C, but that represents a small percentage of PRTs and they must be specially selected. 

Mercury-in-glass thermometers 1. Stable 2. Cheap 3. Good accuracy 1. Slow time response 2. Automation and remote sensing impractical... 

TABLE 16.5 Expected Accuracies of Various Mercury-in-Glass Thermometers... 

A number of errors in the mercury-in-glass thermometer need to be considered. Most are connected with irreversible or slowly reversing bulb contractions and are coupled with time and immersion effects. From the differential expansivity of mercury and glass, one can calculate that the volume within the capillary corresponding to one kelvin on the scale of the thermometer must communicate with 6,000 times this volume in the bulb of the thermometer to register the proper change of temperature. The most important limitation of the absolute accuracy of a thermometer, thus, resides in the precision to which the volume of the bulb can be maintained. 

Finally, every contact thermometer has a thermometer lag due to the time for heat conduction. It takes a certain time for the heat to flow into or out of the thermometer. For a typical laboratory thermometer with a bulb of size 4.9 x 25 mm, filled with mercury, this effect has a time constant of 1 - 2 s to reach half of the initial temperature difference. Thus, if one wants an accuracy of 0.001 times the initial temperature difference, one must wait for a period of 10 such half-times. 

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. 

The Thermometer.—In carrying out a fractional distillation one must be able, not only to rea a constant or nearly constant temperature with great accuracy, but also to take readings of rapidly rising temperatures. These requirements are best fulfilled by the ordinary mercurial thermometer, which is therefore, notwithstanding its m ny drawbacks, used in preference to the air or the platinum resistance thermometer. If accurate results are to be obtained the following points must be attended to. 

The measuring range with liquid-filled thermometers (mercury with a glass stem) is situated between —200 and +370°C, with an accuracy of +1% of full scale, usually less than +0.5°C [4]. 

The sources of error in the mercury-in-glass thermometer are usually coupled with time and inunersion effects. From the expansion coefficient of mercury one can calculate that for every kelvin one wants to see on the scale of the thermometer, one needs the expansion of the volume of about 6000 of these kelvins in the bulb (see one of the problems at the end of Chapter 1). The most important limitation of accuracy of the thermometer resides thus in the precision with which the volume of the bulb can be maintained. The most bothersome effect is the irreversible contraction of the glass after the bulb is blown. This process continues for many years and may cause an effect as large as one to two divisions of the scale. To eliminate the effect, all mercury-in-glass thermometers must be calibrated from time to time at one temperature. The correction is then to be added to all temperatures. It is best to keep a log to see how much the whole scale has to be shifted to higher temperature over the life time of the thermometer. 

The speed at which this will take place depends on the heat transfer coefficient from the environment to the glass. In air the temperature change is gradual, in boiling water the temperature rise is almost instantaneous. Calibration thermometers always use mercury as a liquid. The accuracy is approximately 0.1 °C. The thermometer should always be fully submerged, the scale of these thermometers is not linear. 

The static dielectric constant has been measured with a conventional dipolemeter DM 01 (WTW) working at 2MHz with a vertical cylindrical condenser especially designed to allow a great temperature stability (better than 0.03 C) and correct determination in situ of the temperature of the solution using either a thermocoax calibrated to 0.05 C or a conventional mercury thermometer. The estimated accuracy is 0.5%. 

---