Calibrations temperature

A standard procedure for the temperature calibration of differential thermal analysers and differential scanning calorimeters has been published as ASTM E 967 (1999). In the two point method two calibrants are chosen to bracket the temperature range of interest. It is assumed that the correct temperature T is related to the experimental temperature Texp by the relationship, 

In principle the method is simple enough - small quantities of the two calibrants in turn are allowed to equilibrate 30 °C below the temperature of the transition as indicated by a constant instrument signal. The calibrants are then heated through the transition and the extrapolated onset temperatures obtained from the endotherms. These temperatures correspond to Texpi and rexp2- The calibration will be affected by heating rate and possibly the nature of the atmosphere and its flow rate. The experimental conditions for the calibration should be chosen to match those for subsequent measurements. Calibration over more than two points may be carried out and the relationship between T and T xp determined statistically. The extent to which the instrument output can be corrected by the software will depend on the detailed design of the computer system. 

The process of temperature calibration and measurement has been considered in considerable detail in connection with a program of work by GEFTA (German Thermal Analysis Society). The authors raise the fundamental issue that whereas the temperatures of the fixed points are defined for the substances in phase equilibrium the experimental results are measured under dynamic conditions. The authors recommend a procedure based on extrapolation of results to zero heating rate. The details are contained in a series of publications. The authorshave also considered the problem of temperature calibration under conditions of decreasing temperatures. 

In any DSC experiment where the sample is heated, the sample and its surroundings are not in thermal equilibrium. The sample temperature will be slightly lower than that of the furnace temperature. The transfer of heat between the furnace, the sample, 

This temperature calibration does not take into account thermal lag (and therefore temperature gradients) within the sample, one of the arguments put forward to support the use of organic materials to temperature-calibrate a DSC for pharmaceutical measurements. However, the poor availability of pure and certified organic temperature standards of sufficient accuracy is a powerful argument against using such materials. 

FIGURE 2.1 The effect of heating rate on the onset of melting of indium. 

If measurements need to be made at a variety of heating rates during the day, recalibration each time can be a significant inconvenience. One approach to overcome this is to calibrate at 0°C/min. At first this may seem odd, but it does have some advantages. First, it means that experiments can be made at any heating rate, and then a correction may be applied to the data to take into account the heating rate used. This calibration methodology also benefits isothermal experiments (for example, in kinetic studies) as the isothermal temperatures are exactly calibrated as well. 

To calibrate at 0°C/min, construct a graph of the onset of melting temperature against heating rate and extrapolate back to the onset temperature for 0°C/min. This value is then entered into the temperature calibration software of the DSC. 

Use the same conditions as for the subsequent tests that you want to perform. Measure the onset values and enter them into the cahbration software. Sample weights are not so critical if temperature measurement only is being made and typically a few milligrams should be used. The onset value of indium melt from the heat of fusion test is usually employed to save repeating the process. 

An important prerequisite for the reproducibility of NMR experiments at elevated temperatures is the accurate determination of the temperature inside the sample volume of the probe. Often, there is a systematic error in the temperature displayed by the controller unit. Therefore, methods for the temperature calibration of MAS NMR probes under various working conditions, such as various heating rates, sample spinning frequencies, etc., are required for experiments at elevated temperatures. 

A simple method for determining the temperature inside the sample volume of an NMR probe is the quantitative evaluation of the absolute intensity using Curie s law (Eq. (25)). However, this approach is limited to systems that heat the sample volume only and not the radio frequency coil or other electronic parts of the probe. A heating of the radio frequency coil strongly influences the quality factor of the NMR probe and leads to an additional change of the signal intensity and, therefore, renders the quantification of intensity more complicated. 

A more suitable method to calibrate the temperature behavior of an NMR probe involves the characterization of temperature-dependent melting points and phase transitions. Table 1 is a list of materials that can be applied for the calibration of variable-temperature NMR probes in the temperature range of 279-791 K. 

Another way to calibrate temperatures in NMR spectroscopy consists of investigating materials that lead to signals with temperature-dependent chemical shifts (shift thermometers). For the development of shift thermometers, the temperature-dependent chemical shift is compared with the occurrence of melting and phase transitions, allowing a temperature calibration with high accuracy over a broad temperature range. 

On the basis of the calibration by melting points and phase transitions, a number of shift thermometers have been developed for solid-state NMR spectroscopy in various temperature ranges. Wehrle et al. 144), for example, used the line splitting in the N CP/MAS NMR spectrum of the organic dye molecule tetra-methyldibenzotetraaza annulene (TTAA) in the temperature range 123—405 K. A high-temperature shift thermometer for temperatures of up to 790 K was developed by van Moorsel et al. 40) on the basis of Sn MAS NMR spectroscopic 

INPUT Enter input data filename , filein  

INPUT Enter corrected data output filename , fileoutt OPEN filein FOR INPUT AS 1 OPEN fileout FOR OUTPUT AS 2 

PRINT Enter measured followed by literature calibration PRINT temperatures, separated by a comma. At least two PRINT data pairs must be entered. Terminate entries by PRINT pressing ENTER  

IF meas(kX) temp(jX) AND maas(kX) high THEN high aeas(kX) lowkX = k /. 

After a series of melting standards (see Table 3.1) have established a correlation between indicated temperature and correct temperature, this table may be entered into the program. The routine determines between which two calibration points a particular temperature in the experimental data set fall, and then it shifts the temperature to a corrected temperature in accordance with a calibration line between the points. For experimental data whose temperature values fall before the lowest calibration temperature or above the highest calibration temperature, extrapolation of the line defined by the two nearest calibration points is used. 

---

From the ventilation point of view, the fixed points -38.83 °C (triple-point of mercury), 0.010 °C (triple-point of water), 29.76 °C (melting point of gallium), and 156.60 °C (freezing point of indium) are of relevance. The triple-point of water is relatively simple to achieve and maintain with a triple-point apparatus. Some freezing point cells are covered in standards. In practical temperature calibration of measuring instruments, the lTS-90 fixed points are not used directly. 

For isothermal measurements, it is advisable to use a furnace of low thermal capacity unless suitable arrangements can be made to transport the sample into a preheated zone. The Curie point method [132] of temperature calibration is ideally suited for microbalance studies with a small furnace. A unijunction transistor relaxation oscillator, with a thermistor as the resistive part with completion of the circuit through the balance suspension, has been suggested for temperature measurements within the limited range 298—433 K [133]. 

The spectra have been reduced with the GIRAFFE BLDRS pipeline developed at the Geneva Observatory. EW have been measured with DAOSPEC [2], based on a linelist produced with the Vienna Atomic Line Database (VALD) [3]. Preliminary estimates of the stellar parameters Te//, log g, vt and [M/H] have been obtained from the WFI photometry published by [4] and the color-temperature calibration by [5]. MARCS model stellar atmospheres [6] have been... 

The calibration of DTA systems is dependent on the use of appropriate reference materials, rather than on the application of electrical heating methods. The temperature calibration is normally accomplished with the thermogram being obtained at the heating rate normally used for analysis [16], and the temperatures... 

G. W. H. Holme, H. K. Cammenga, W. Eysel, E. Gmelin, W. Hemminger. The Temperature Calibration of Differential Scanning Calorimeters. Thermochim. Acta 1990, 160, 1-12. 

H. K. Cammenga, K. Gehrich, S. M. Sarge. 4,4 -Azoxyanisolefor Temperature Calibration of Differential Scanning Calorimeters in the Cooling Mode—Yes or No . Thermochim. Acta 2006, 446, 36—40. 

Analysis Temperature Calibration Temperature Weight-Average Molecular Weight... 

Swanson DK, Prewitt CT (1986) A new radiative single crystal diffractometer microfumace incorporating MgO as a high temperature cement and internal temperature calibrant. J Appl Crystallogr 19 1-6... 

FarrugiaLJ, Macchi P, Sironi A (2003) Reversible displacive phase transition in P fi(en)3] (N03 )2 a potential temperature calibrant for area-detector diffractometers. J Appl Crystallogr 36 141-145... 

Optimization of drying, ash and atomization temperatures calibration and determination of Cu. 

ASTM E 967-92, Standard Practice for Temperature Calibration of Differential Scanning Calorimeters and Differential Thermal Analyzers, 1992. 

A. W. Nicholls and R. J. Mortishire-Smith, Temperature calibration of a high-resolution magic-angle spinning NMR probe for analysis of tissue samples, Magn. Reson. Chem., 2001, 39, 773-776. 

The Evans method gives excellent results provided adequate care is taken. A most important requirement is that the solution temperature is measured reliably. One effective means of accomplishing this for H NMR is to insert into the NMR tube a capillary or additional coaxial sample of an NMR temperature calibrant solvent, usually methanol (158) or ethylene glycol (88). In this way the temperature measurement is made simultaneously with the susceptibility measurement. A second important factor is the variation of the solvent density with temperature (126). Because the density difference between the solvent and solution depends linearly on the concentration of the solute, it is only... 

Accuracy of thermocouples should be 0.5°C. Temperature accuracy is especially important in steam sterilization validation because an error of just 0.1 °C in temperature measured by a faulty thermocouple will produce a 2.3% error in the calculated F0 value. Thermocouple accuracy is determined using National Bureau of Standards (NBS) traceable constant temperature calibration instruments such as those shown in Figure 6. Thermocouples should be calibrated before and after a validation experiment at two temperatures 0°C and 125°C. The newer temperature-recording devices are capable of automatically correcting temperature or slight errors in the thermocouple calibration. Any thermocouple that senses a temperature of more than 0.5°C away from the calibration temperature bath should be discarded. Stricter limits (i.e., <0.5°C) may be imposed according to the user s experience and expectations. Temperature recorders should be capable of printing temperature data in 0.1 °C increments. 

To calibrate the pixel sensitivities black body radiation is usually measured at different temperatures. Since a black body has an emissivity of 1 at every position, variations in detector pixel sensitivities are eliminated by a calibration function. As this IRT-method should be used here to quantify very small heat signals on combinatorial libraries with diverse materials, differences in emissivities have to be considered. Most materials are grey bodies with individual emissivities less than 1. Therefore, the calibration was not performed with a black body but with the library, as described before, a procedure that corrects not only for pixel sensitivity but also for emissivity differences across the library plate [5]. For additional temperature calibration, the IR-emission of the library is recorded at several temperatures in a narrow temperature window around the planned reaction temperature. By this procedure, emissivity changes, temperature dependence and individual sensitivities of the detector pixels can be calibrated in one step. After this... 

Another measurement principle is the DSC, after Boersma [8]. In this case, no compensation heating is used and a temperature difference is allowed between sample crucible and reference crucible (Figure 4.5). This temperature difference is recorded and plotted as a function of time or temperature. The instrument must be calibrated in order to identify the relation between heat release rate and temperature difference. Usually this calibration is by using the melting enthalpy of reference substances. This allows both a temperature calibration and a calorimetric calibration. In fact, the DSC after Boersma works following the isoperibolic operating mode (see Section 4.2.2). Nevertheless, the sample size is so small (3 to 20 mg) that it is close to ideal flux. 

---