Thermometers, calibration correction

Note 4 Whenever thermometers complying with ASTM requirements are not available, thermometers complying with the requirements for the Institute of Petroleum thermometer IP15F (or 15 C) PM-Low, or IP16F (or 16 C) PM-High, may be used, provided calibration corrections are used in temperature ranges where the IP requirements for scale accuracy are less stringent than those of ASTM... 

Thermometer Scale Correction. An NBS thermometer was used for the adiabatic jacket and a Beckmann differential thermometer for the calorimeter water. The Beckmann thermometer was calibrated against the NBS thermometer. Corrections were made to the initial and final temperatures recorded on the Beckmann thermometer. 

Since the mercury in the thread, as well as that in the bulb, is susceptible to thermal expansion, it is important in precise work to take account of the temperature of the thermometer stem. Most thermometer calibrations, especially those for enclosed-stem types, are for total immersion—it is assumed that the thread is at the same temperature as the bulb. Other thermometers are meant to be used with partial inunersion, often to a ring engraved on the stem, and the remainder of the stem is assumed to be at room temperature (say, 25°C). For precise work, stem corrections should be made if the stem temperatures differ significantly from those assumed in the calibration. The correction that should be added to the thermometer reading is given by the equation... 

Thermometer correction. The temperature which is read on the thermometric scale must be corrected because there are several errors in such determinations. One source of error arises from the construction and calibration of the thermometer. The bore of the capillary may not have the same diameter throughout further, the scale graduation and the calibration of low-priced thermometers are not very accurate. A second source of error is the method used in the common melting point apparatus. The common thermometer has been calibrated while totally immersed in a bath. In the melting-point apparatus described, only a part of the stem is immersed. The column of mercury above the oil bath has a lower temperature than that at which the thermometer was calibrated. Therefore either a thermometer calibrated by partial immersion should be used or a correction must be made for the unequal heating of the mercury in the stem of the thermometer. Although thermometers calibrated by partial immersion are available, the latter practice is the more common. 

Although most thermometers register correctly at 0° and 100°, it is well to make a calibration at these points. To determine the reading at 0°, a beaker of 100 cc. capacity is filled with finely chopped ice, and water is added until it reaches to within about 2 cm. of the surface of the ice. The thermometer is inserted into the ice and water until the zero point is just above the surface, so it can be seen. The reading on the thermometer is noted when the mercury no longer falls. Care should be taken to have the eye at the height of the top of the mercury column to avoid parallax. 

Your thermometer was not calibrated correctly but each degree separation was correct, and 100.°C separated the freezing point and the boiling point of water (for example, suppose the freezing point of water read 10.0°C and the boiling point registered a reading of 110.0°C). 

Constant errors arise when you use equipment that is incorrectly calibrated or that functions incorrectly the same way each time. The constant error is not a result of your care or lack of it, but rather a faulty instrument. For example, a thermometer might be calibrated to read 1 C too low at all temperatures. Unless you calibrate the thermometer and correct for this miscalibration, even temperatures measured with great care will be 1°C too low. Constant errors also arise when you use procedures that create bias in your measurements. For example, if the volume of a liquid is measured in one container and the mass of the liquid is measured in another container after a transfer, the mass, and thus the density calculated from it, will usually be too small because not all of the liquid will be transferred when it is poured from one container to another. Such a constant error can be eliminated by changing the procedure to eliminate the transfer so that volume and mass can be measured in the same container. 

The correct value for the emittance of the extrudate must be known and correctly introduced into the IR thermometer calibration. 

Correct the temperature readings from 8.1 in accordance with the thermometer calibration. Report the final corrected initial boiling point and temperature of each 5 % distilled or other specified amounts. 

If the range of three succesave observations of the aniline point temperature is not greater than 0.1 C (0.2 F) for light-colored samples or 0.2 C (0.4 F) for dark samples, report the average temperature of these observations, corrected for thermometer calibration errors, to the nearest 0.0S C (0.1 F) as the aniline point... 

Inaccuracy of Thermometer—This correction shall be obtained by calibration of the thermometer used in the test and applied to the observed thermometer reading. 

Thermometer Bore Correction— Apply the corrections for any variations in the bore of the thermometer as given by the calibration. 

The most frequently encountered source of error when using liquid-in-glass thermometers is the misuse or complete neglect of the emergent-stem correction. This correction derives from the use of the thermometer with a portion of the stem exposed to a different temperature from that of calibration. A common example is the use of partial immersion of a thermometer calibrated for total immersion. For detailed information on this correction, see the American Society of Mechanical Engineers Power Test Codes Temperature Measurement. 

The comparatively inexpensive long-scale thermometer, widely used by students, is usually calibrated for complete immersion of the mercury column in the vapour or liquid. As generally employed for boiling point or melting point determinations, the entire column is neither surrounded by the vapour nor completely immersed in the liquid. The part of the mercury column exposed to the cooler air of the laboratory is obviously not expanded as much as the bulk of the mercury and hence the reading will be lower than the true temperature. The error thus introduced is not appreciable up to about 100°, but it may amount to 3-5° at 200° and 6-10° at 250°. The error due to the column of mercury exposed above the heating bath can be corrected by adding a stem correction, calculated by the formula ... 

After the melting point has been determined, the thermometer reading is corrected by reference to the calibration chart of the thermometer. Methods for calibrating a thermometer are described in Section 11,9. 

Repeat the boiling point determination with the following pure liquids (a) carbon tetrachloride, A.R. (77°) (6) ethylene dibromide (132°) or chlorobenzene (132°) (c) aniline, A.R. (184-6°) and (d) nitrobenzene, A.R. (211°). An air condenser should be used for (c) and (d). Correct the observed boiling points for any appreciable deviation from the normal pressure of 760 mm. Compare the observed boiling points with the values given in parentheses and construct a calibration curve for the thermometer. Compare the latter with the curve obtained from melting point determinations (Section 111,1). 

In addition to the orthodox method, just described, for the determination of the boiling points of liquids, the student should determine the boiling points of small volumes (ca. 0 5 ml.) by Siwolobofifs method. Full details are given iri Section 11,12. Determine the boiling points of the pure liquids listed in the previous paragraph. Observe the atmospheric pressure and if this differs by more than 5 mm. from 760 mm., correct the boiling point with the aid of Table II,9,B. Compare the observed boiling points with the accepted values, and draw a calibration curve for the thermometer. 

Tn addition to the error due to the exposed stem, ordinary chemical thermometers of low cost are subject to errors due to irregularities in the bore and sometimes the scale graduations may not be very accurate. " It is therefore essential to check the thermometer at several temperatures against the melting points of pure solids or the boiling points of pure liquids as described below. The application of an exposed stem correction will of course be unnecessary if the thermometer is calibrated in this way. A calibration curve may then be drawn upon graph " paper from the data thus obtained. Temperatures at intervals of about 20° are marked as abscissae and the corrections to be added or subtracted as ordinates the points thus obtained are then connected by a smooth curve. The thermometer correction at any temperature may be read directly from the curve. 

A plan is draw up detailing how checks will be made to ensure that the critical limits are not exceeded. It shows how often the checks are made, how and by whom, so that it is clear who is responsible that it is done correctly. It also specifies regular inspections and calibrations of measuring equipment such as thermometers. 

Table 1 Corrected Melting Points of Compounds Suitable as Reference Materials in the Calibration of Thermometers...

Corrected melting points of compounds suitable as reference materials in the calibration of thermometers... 

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