Sample thermocouple

A reliable determination of the adsorbent temperature is obviously a crucial requirement of the thermal desorption method. Thermocouples are mostly used to this end. With disks and foils, a thermocouple can be spot-welded onto the back or edge of the sample. Thermocouples can be attached also to ribbons as well as to the wall of the vessel containing an evaporated film. With powdered adsorbents, thermocouples are located in the layer of the sample. The adsorbents in the form of filaments and ribbons are frequently used simultaneously as resistance thermometers, switched... 

In the combination TG-DTA normally two crucibles of identical shape and size are used, one for the sample and the other one for the inert reference material. The crucibles may be e.g. cylindrical, conical, plate-type or any other form which favors a good contact between the sample, thermocouple and sample holder (Fig. 8 j). The thermocouples are placed either on the bottom of the crucible or in the center of a cylindrical crucible. The latter type allows the element to be brought protected or unprotected into the center of the substance. 

Voltages of the sample thermocouple corresponding to thermal arrests were converted to temperatures using N.B.S. Circular No. 561 (17). Table I hsts the temperatures of thermal arrests for the entire range of composition between 0 and 100 mole % UFg. The values listed in Table I are averages of several measurements, and the uncertainty values are standard deviations of the averages. The uncertainty values associated with the liquidus points obtained by the extrapolation procedure (i) have not been estimated. 

The sample was heated from room temperature to 500°C at heating rates ranging from 1 to 110°C/h. Previous experience indicates that the difference in temperature across the sample is less than 5°C for this size sample and heating rates less than 120°C/h. For the near-atmospheric pressure experiments, a thermocouple was used to measure the temperature near the center of the oil shale sample. For the high-pressure experiments, this sample thermocouple was omitted, and the sample temperature was estimated from the furnace temperature. The difference between the sample and furnace temperatures was determined in the near-atmospheric pressure experiments. 

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

The underlying heat flow signal is calibrated by the use of standards with known melting temperatures and enthalpies of fusion. A series of such samples is run over the operating temperature range of the instrument. The sample thermocouple has a nominally known relationship between its output and temperature. Any observed differences between measured (by the sample thermocouple) and expected melting... 

When accuracy greater than the tolerances in Table 16.9 is required, the wires must be calibrated. Normally, this requires comparison calibration of sample thermocouples taken from each spool to account for spool variability. Typically, two thermocouples—one fabricated from the beginning and the other from the end of the spool—are calibrated to determine an average calibration for the entire spool. If the deviation between the two calibrations is not within the required uncertainty, a third thermocouple fabricated from the center of the spool should be used. If the results are still unsatisfactory, then each thermocouple should be calibrated individually, or a different spool should be used. 

The calibration and use of base metal thermocouples at temperatures above about 300°C will produce inhomogeneities in the wires, which can change the calibration itself [43]. The usual practice to overcome this dilemma for application at high temperature is to calibrate sample thermocouples to obtain the calibration for the remainder of the spool of wire and discard the calibrated thermocouples. 

Curve (a) shows the differential temperature curve if the thermocouple is 0.06 cm from the center, instead of at the center, of the sample material. In curve (b), a rather extreme case is presented in which the sample thermocouple is 0.30 cm from the sample center. In both curves, the peak maxima temperatures are completely different from those of the symmetrically centered thermocouples. 

Figure 6. g. Sealed-tube DTA furnace and sample holder (62). A. insulated cover B, aluminum block C, glass capillary tube D, sample E, sample thermocouple F. aluminum heat transfer sleeve G. ceramic insulator tube H, reference chamber 1. transite platform J. terminal strip. 

Figure 6.28. Furnace and sample chamber. A. glass capillary tube for sample B. sample-holder plate C. sample heat transfer sleeve D. sample thermocouple E. furnace block G. reference capillary tube H. reference heat transfer sleeve J, reference thermocouple K, heater cartridge.

Figure 11.24. Apparatus used by Chiu (102) for parallel TG—DTG-DTA and ETA measurements. A. balance housing B, balance beam sheath C, beam stop D, quartz beam E, sample container F. thermocouple block G, sample measuring thermocouple H. ceramic tubing I, platinum jacket J. reference quartz tube K. sample quartz tube L, outer platinum electrode M, center platinum electrode N, cold beam member O. P. platinum lead wires Q, sample thermocouple junction R, reference thermocouple junction S. spacer T, ceramic insulation U, V. sample thermocouple wires W. platinum grounding wire.

The ARC calorimeter jacket and sample system are shown in Figure 11.49 (168). A spherical bomb is mounted inside a nickel-plated copper jacket with a swagelok fitting to a 0.0625 in. tee, on which is attached a pressure transducer and a sample thermocouple. The jacket is composed of three zones, top, side, and base, which are individually heated and controlled by the Nisil/Nicrosil type N thermocouples. The thermocouples are cemented on the inside surface of the jacket at a point one quarter the distance between the two cartridge heaters. The point is halfway between the hottesl and coldest spots of the jacket. The same type of thermocouple is clamped directly on the outside surface of the spherical sample bomb. All the thermocouples are referenced to the ice point that is designed to be stable to within 0.01°C. Adiabatic conditions are achieved by maintaining the bomb and jacket temperatures exactly equal. The sample holder has a capacity of 1-10 g of sample. Pressure in the system is monitored with a Serotec 0-2500 psi TJE pressure transducer pressure is limited in the vessel to 2500 psi. The maximum temperature of the system is 500°C. 

The digitized output voltage , from the sample thermocouple is the input to a computer. The computer controls the current input to the furnace in such a way that the sample temperature increases linearly and at a predetermined rate. The sample thermocouple signal is also converted to temperature l. which... 

One of the major considerations in DTA is obtaining valid readings of the actual temperature of the sample and reference materials conveniently and repro-ducibly. As in TG, thermal equilibrium is of utmost importance. There is always a definite temperature difference between the outer and inner portions of the sample indeed, reactions often occur at the surface of the sample while the interior is still unreacted. This effect is minimized by using as small a sample as possible with uniform particle-size and packing. Depending upon the instrument used, the thermocouple may be imbedded in the sample, or at the other extreme, may simply be in direct contact with the sample holder. In any case, the thermocouple must be precisely positioned for every experiment. To obtain the best results, the reference and sample thermocouples should be matched in temperature response and the geometric arrangement of the sample and reference thermocouple should be perfectly symmetrical within the oven. 

In this case the sample thermocouple is placed above the sample crucible. There are two purge gas systems. One flow passes over the... 

As the sample went through the Curie temperature region, the output from the sample thermocouple was transcribed onto the chart at regular intervals and at any unique points. 

In principle, any DSC can be modulated. As the details of construction of the DSC equipment vary, it may be advantageous to modulate in different fashions. One can modulate the block, reference, or sample temperatures, as well as the temperature difference (proportional to the heat-flow rate). For example, the temperature modulation of fee Mettler-Toledo ADSC is controlled by fee block-temperature thermocouple (see Fig. 4.57), while fee modulation of feeMDSC of TA Instruments in Fig. 4.85 is controlled by fee sample thermocouple. Of special interest, perhaps, would be a modulated dual cell as shown in Fig. 4.56. 

Figure 6. Schematic diagram of a classical DTA apparatus a) Reference thermocouple b) Sample thermocouple c) Healing block d) AT amplifier e) Recorder or computer, logging Ts, AT, and time f) Furnace g) Temperature programmer, which may be linked to the computer h) Gas inlet 7s, sample temperature 7r, reference temperature AT=7 s-7 r...

Since the sample thermocouple is not in direct contact with the sample, it measures the oven s atmospheric temperature near the sample. The actual sample temperature is a function of its thermal conductivity, mass shape and dimensions, the mass and composition of the fixture in contact with the sample, the uniformity of the oven thermal environment, and the heating rate. As noted above, the result is that a sample can have a thermal gradient within it. Even if the oven temperature were completely uniform, the sample temperature could lag behind the oven temperature because of heat transfer limitations. 

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