Temperature calibration wire method

A second method of temperature calibration [2] involves suspension of a platinum wire over a boatshaped crucible by spot welding (Figure 4.11 A). To this wire a hook, made of a material of well-characterized melting point, is attached which supports a platinum weight. When the melting point of the hook is reached the weight falls, causing an action-reaction response in the microbalance which produces a blip in the baseline and not a mass loss. 

Wire method This method of temperattue calibration uses wires made from high purity metals of known melting points such as In, Sn, Pb, Zn, Al, Ag, Au, Ni and Pd. These wires are used to attach a mass to the sample pan, which will detach immediately when the wire melts. These substances cover the broad temperature range (430-1800 K) of measurement encountered in the present work. The observed melting points were used in systems data acquisition program for subsequent temperatme corrections. 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

The fusible line method can also be used as a calibration temperature. A fusible wire with known melting temperature connects the microbalance suspension wire and inert mass pieces. The fusible wire should locate the position of sample in the furnace. Melting of the wire at its melting temperature causes mass loss, which is recorded in the TG curve. 

Electrical Resistivity. The electrical resistivity of these materials was measured from room temperature to 15 K at 5 K intervals on bars, 6-10 mm long and 1 mm square, using a DC four-probe method. The electrical contacts were made with fine platinum wire and silver paste. A constant current of <10 mA was used. The voltage difference was measured to 0.1 fiV. The temperature of the specimen was maintained by a closed-cycle cryogenic system and measured using a calibrated silicon diode. 

There was nothing to be altered in the experimental arrangement, for the method already described still gave, as was to be expected, any desired precision at these temperatures. The calibration of the platinum wire used occasioned some difficulties, but eventually we were able to fix with certainty a number of temperatures and to relate them by an equation, such that the very variable temperature coefficient for the region 20° to 40° abs which was what was really wanted, could be derived with sufficient accuracy (compare below). 

Measurement of Temperature.—Under favourable conditions the accuracy of the measurements is practically dependent only on the accuracy with which the platinum wire used has been calibrated. It is not sufficient to calibrate a sample of the wire in question once for all, but some control determinations at least must be made for every calorimeter used, for the temperature coefficient of the wire is slightly influenced by the manner in which it is embedded. The following methods were tried —... 

Calibration of the couple used was done by two different methods. A fairly long piece of homogeneous fine constantan wire was selected by the usual methods, and the thermal E.M.F, measured as accurately as possible, when one junction was in melting ice and the other in a series of baths at different low temperatures. These temperatures were determined by means of the Stock s thermometer calibrated by H. v. Siemens (91). For the lowest temperatures hydrogen was used, boiling either under the ordinary pressure or under an accurately measured reduced pressure. The values could be fairly well reproduced by the following interpolation formula,... 

Testing can be used to validate new experimental techniques. For example, the optical techniques for measuring temperature discussed in Chapter 5 have been tested against more traditional and well-established techniques such as bare wire thermocouples and suction pyrometers. Some of these optical techniques (e.g., tunable diode lasers discussed in Chapter 14) used more traditional methods such as bare wire thermocouples for calibration. In either case, testing is required to validate new experimental techniques. In many cases, a well known and fully tested configuration with sufficient measurements using proven techniques will be used to demonstrate the validity of a new measurement technique. 

The temperature attained by the wire-screen heating elements was related to the time of current flow by a calibration method. Times required to melt pure metal powders of like particle size and amounts as the coal were determined by trial and error. A number of calibration points were thus established, and the temperature at the end of the current pulse was proportional to the time of current flow in the region to 1450 °C. The heating rate was therefore a constant 8250°C/sec. 

Calorimeters measure the heat produced by a sample of nuclear material, whereby the heat originates primarily from the a-decay of the isotopes making up the nuclear material. Calorimeters have traditionally been fabricated using a sensor of nickel wire wound around a measurement chamber (Bracken et al. 2002). The nickel wire provides a temperature-sensitive resistance leading to highly accurate and precise electrical measurements of the power produced by a sample. Calorimeters are calibrated with Pu heat somces or plutonium samples with known mass and isotopic composition. Calorimetric assay is the most precise and accmate NDA measmement method for plutonium products (>100 grams). 

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