Self-heating, thermometers

Temperature variations in the instrument are a source of error and electrical power dissipation is limited to avoid the effects of self-heating. This is achieved by means of the four lead system shown in Fig 6.25ft. This minimises any effects of variations in temperature on the resistance RCL of the connections between the RTD and the bridge and is used normally with digital thermometers and data acquisition systems where the sensor non-linearity is corrected within the computer software. 

Most low-temperature engineering temperature measurements are made with metallic resistance thermometers, nonmetallic resistance thermometers, or thermocouples. In the selection of a thermometer for a specific application one must consider such factors as absolute accuracy, reproducibility, sensitivity, heat capacity, self-heating, heat conduction, stability, simplicity and convenience of operation, ruggedness, and cost. Other characteristics may be of importance in certain applications. 

The second type of test we run on the Sikarex is an adiabatic test. In this test the jacket temperature is controlled by the sample temperature. When the sample thermometer detects an increase in the sample s temperature, the jacket temperature is increased an equal amount. In other words, the sample is being held under adiabatic conditions. The test is run by step-heating the sample into the exotherm detection range found in the previous Sikarex test by means of a heating coil attached to the sample tube. External heating is then stopped, and the sample is allowed to self-heat. The adiabatic temperature rise of both the jacket and sample are recorded. 

Temperature is undeniably the most important property for all calorimetric measurements, because it is the common denominator. Two different techniques for temperature measurements are used for pulse calorimetry contact thermometry (e.g. thermocouples) and radiation thermometry or pyrometry. Because pulse calorimetry is often used to handle and measure liquid materials, non-contact radiation thermometry is far more common in pulse-heating than contact thermometry. Other reasons for non-contact temperature measurement methods include the fast heating rates and temperature gradients (inertia of the thermocouples), difficulties mounting the contact thermometers (good thermal contact needed), and stray pick-up in the thermocouple signal because the sample is electrically self-heated. 

Nuclear orientation thermometry depends on the anisotropic emission of y-rays from polarized radioactive nuclei. The extent of the anisotropy is detemined by the degree of polarization which, in turn, varies inversely with the absolute temperature. Since the nuclear hyperfine splittings are usually known to high precision, the thermometer is in principle absolute and does not require calibration. The useful temperature range is relatively narrow, however, being limited by radioactive self-heating at the lower end and insensitivity at the upper end. 

RTDs exhibit a highly linear and stable resistance versus temperature relationship. However, resistance thermometers all suffer from a self-heating effect that must be allowed for, and must be kept below 20 mW, where I is defined as the electrical current and R is the resistance. 

A solution of sodium ethoxide is prepared under nitrogen from 70 g. (3.04 gram atoms) of sodium and 2-1. of absolute ethanol (Note 1) in a 3-1. three-necked flask which is equipped with mechanical stirrer, efficient reflux condenser, dropping funnel, and a thermometer which dips below the level of the liquid in the flask. Chloropicrin (100 g., 0.61 mole) (Note 2) is placed in the dropping funnel, and the stirred solution is heated to 58-60° with a water bath. The chloropicrin is added at a rate of 30-35 drops per minute until the reaction becomes self-sustaining (about 20 minutes), at which point the water bath is removed and the balance of the chloropicrin is added at a rate sufficient to maintain the temperature at 58-60° (Note 3). When the addition, which requires nearly 2 hours, is complete, the stirrer is stopped and the mixture is allowed to stand overnight. 

From container 18 dimethyldichlorosilane is sent into batch box 1 placed over apparatus 2, where a 2% solution of the initiator in dimethyldichlorosilane is prepared. Apparatus 2 is an enameled flask with an agitator and a filling hatch. While the agitator operates, one sends there dimethyldichlorosilane and adds the initiator solution in the amount necessary to form a 2% solution. After 30 minutes of agitation at 20 °C the mixture of dimethyldichlorosilane and initiator is poured into intermediate container 17, from where it is periodically pumped into pressure batch box 4. After that, the mixture self-flows through rotameter 6 into chlorinator 5. The chlorinator is a steel cylindrical apparatus with a heating jacket and a thermometer pocket the lower part of the apparatus contains a distribution device which feeds chlorine. The temperature in the chlorinator is maintained within 65-70 °C and regulated with vapour sent into the jacket of the apparatus and with the speed at which chlorine is fed. 

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