Sample temperature deviations

Figure 4.12 Sample temperature deviation is displayed using a program which...

Schematic illustration of the measured sample temperature as a function of time for a polymer subjected to a linear heating ramp, and the corresponding DTA curve. In the region of the phase transitions the programmed and measured sample temperatures deviate significantly... 

The sample is enclosed in a heavy walled bomb with an internal volume of approximately 1 to 1 ml. Although similar to a differential thermal analysis (DTA) test, the samples used are much larger, and the conditions of confinement allow the liquid to remain in contact with any decomposition products that form as vapors. Heat is applied so that the bath temperature increases at a constant rate and the temperature of both the heating bath and the sample are recorded continuously. When the temperature of the sample exceeds that of the bath, an exothermic reaction must be occurring in the sample, and this process is frequently accompanied by a detonation. (The bomb is equipped with a blow out disc to avoid any major damage to the equipment). In the more usual case the discrepancy between sample temperature and bath temperature increases with temperature, and the point at which this deviation is 5°F./min. is called the self-heating temperature. Typical values for some liquid materials of interest in the propellant field are listed in Table V. 

Most problems with solid fat content determination arise from inconsistencies in sample preparation. As noted above, the solids content is dependent on the temperature history of the sample, so deviations from the tempering protocols can lead to inaccuracies. The time required to measure a sample is not expected to lead to extensive crystallization or melting, but care should be taken to start the measurement as soon as possible after the oil is removed from the tempering block. Several instruments can be fitted with a thermostatic control for the sample in the coil, and this should be used if it is reliable. It is recommended that the NMR tubes be capped, particularly for the long tempering times required for stabilizing fats. 

Suppose nine repeated calculations of a certain SOFC resulted in mean maximum solid temperature of 1167°C with a sample standard deviation of 12°C (see Table A.2 for an example of such a calculation), whereas a set of five repeated experiments for a similar cell resulted in mean max solid temperature of 1140°C with a sample standard deviation of 15°C. Calculate the 95% confidence interval for the modeling error assuming that the numerical errors in the calculations are negligible. 

The sample and reference chambers are heated equally into a temperature regime in which a transformation takes place within the sample. As the sample temperature infinitesimally deviates from the reference temperature, the device detects it... 

In a DTA scan at a constant heating rate where a solid sample fuses, the reference material increases in temperature at the designated heating rate, while the sample temperature remains at the melting temperature until the transformation is complete. Thus, when AT versus reference temperature is plotted, a linear deviation from the baseline value is seen on the leading... 

Under slower heating rates in heat-flux DSC, the deviation of sample temperature from the setpoint during a self-feeding reaction may be maintained adequately small so as to be neglected. If the furnace feedback control is set to act based on the temperature of the sample (that is the sample temperature thermocouple is the control thermocouple), then the control system may be able to allow the transforming sample to heat itself at a constant rate, and the heat input from the furnace will retreat as needed. 

If the total heat capacities of the empty sample and reference containers were perfectly matched, then the empty sample container versus empty reference container trace should appear as a flat baseline of zero value during the entire scan. This is rarely the case (due to asymmetries in instrument construction) and is actually not necessary for this calculation. The temperature deviation from this line for the other traces is a result of the extra thermal mass on the sample side, causing it to lag the reference in temperature during the heating ramp. Defining cp as the specific heat (J/g-K), the sample specific heat is determined by the ratio ... 

The intensity at peak maximum for the faster heating rates is greater than that for the slower heating rates, since for DTA the reference temperature increase is more rapid, while at the same time the sample strives to remain at the melting temperature. For faster heating rates in power-compensated DSC, the sample chamber temperature deviates more quickly from the rising setpoint, so the device compensates with more heat dissipation per unit time to the sample side. 

A melting endotherm is characterized by a linear deviation from the baseline as the sample temperature remains at the melting temperature, while the reference increases in temperature... 

Samples should be transported to the testing facility within 24h and testing should commence as soon as possible after collection. Samples must be kept in the dark during transport and the sample temperature should not deviate markedly ( 2°C) from that at the time of collection. The temperature history of the samples from collection to arrival at the testing facility should be recorded, ideally using disposable temperature recorders, where appropriate. 

Sample mass, volume and form are important for recording accurate and reproducible TG curves. Reliable TG curves rely on minimization of deviation between sample temperature and programmed temperature. Deviation typically results from either endothermic or exothermic reactions in the sample and subsequent heat transfer between heat source and sample. Sample mass is the most important parameter affecting TG curves. In general, small sample mass is better than large mass for minimizing temperature deviation. TG sample mass is usually about several milligrams. The low limit of sample mass depends on the resolution limit of the microbalance. 

Data from ref. 61 l4N spectra at 4-3345785 MHz ( 0-5 Hz), sample temperature 30 2°C concentric spherical sample containers are used in order to eliminate bulk susceptibility effects on shifts and signal shape external standards are used, CH3NO, (neat liquid) and, for signals within 15 ppm of that of neat CH,N02, tetranitromethane (neat liquid) reported shifts represent values obtained from iterative fitting of theoretical and experimental lineshapes using a differential saturation method, ref. 63, reported errors are standard deviations for the fitting of at least 200 data points, and represent 68% confidence limits for shifts which are recalculated from values referred to C(N02)4, the error of the shift of C(N02)4 relative to 0H,NO2 is included. 

Sometimes a repetition of a measurement yields a value that differs greatly from the other members of the sample (a discordant value). For example, say that we repeated the measurement of the melting temperature of Ca(N03)2 4H2O in the previous example one more time and obtained a value of 39.75 °C. If we include this eleventh data point, we get a sample mean of 42.43 °C and a sample standard deviation of 0.89 °C. Using the table of Student s t values, we obtain a value for of 0.60 °C at the 95% confidence level. Some people think that the only honest thing to do is to report the melting temperature as 42.4 0.6 °C. 

The extent to which endothermic or exothermic reactions of the sample will cause sample temperature to deviate from a linear temperature change (the larger the sample mass is, the greater the deviation is). 

In this method (96), the kinetic constants are calculated from the TG curve by a differential method. It takes into account also the thermal effects of reactions which result in a deviation of the sample temperature from the programmed values of the linear heating. Starting with the differential equation for the thermal decomposition of a solid,... 

Thus, the intensity of the back-diffracted electron beam depends exponentially on the temperature of the surface. Expressed another way, the logarithm of the beam intensity varies linearly (and inversely) with temperature. If the sample temperature is taken high enough, there will be a sudden, strong downward deviation (a knee ) in the plot of the logarithm of the diffracted beam intensity. For the experiment given in the problem, this transition occurred at about 600 K. 

In DTA, reactions are observed by measuring the deviation of the sample temperature from that of the reference material. This deviation causes thermal fluxes (Eqn. 

Kris and Seslik ( 4) have pointed nut that heh a and 7 may vary with position in the sample itnd that heal and mass intnspuri crfcL is arc scldotn inLcn into consideration, c cn in DTA bere the tea) temperature deviation from the ptogiammcd temperature is recorded IS. ... 

ED intensity data in [1] were reanalyzed using recalculated scattering amplitudes and vibrational spectroscopic data. The sample temperature was 1300(50) K. Slight distortion from Td symmetry, with a maximum deviation of 0.15 A in intemuclear distances and 5° in bond angles, could not be excluded. 

In enthalpimetric flow injection analysis, the sample pulses employed are too small to give a thermal steady state, but result in a temperature peak proportional to the enthalpy change and thereby to the substrate concentration. The peak height, the area under the peak, and the ascending slope of the peak are all linearly related to the substrate concentration. Sample introduction of a sufficient duration (several minutes) leads to a thermal steady state resulting in a constant temperature deviation, which is also proportional to the substrate concentration. 

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