Calorimeters accuracy

Reactive System Screening Tool (RSST) The RSST is a calorimeter that quickly and safely determines reactive chemical hazards. It approaches the ease of use of the DSC with the accuracy of the VSP. The apparatus measures sample temperature and pressure within a sample containment vessel. Tne RSST determines the potential for runaway reactions and measures the rate of temperature and pressure rise (for gassy reactions) to allow determinations of the energy and gas release rates. This information can be combined with simplified methods to assess reac tor safety system relief vent reqiiire-ments. It is especially useful when there is a need to screen a large number of different chemicals and processes. 

Extrapolations are always subject to error, but fortunately the contribution to the entropy resulting from the extrapolation is a small part of the total. In glucose, for example, S g = 219.2 0.4 J-K -moF1, but the entropy contribution at 10 K obtained from the Debye extrapolation is only 0.28 J-K 1-mol 1. Well-designed cryogenic calorimeters are able to produce Cp measurements of high accuracy hence, the Third Law entropy obtained from the Cp measurements can also be of high accuracy. 

No theory can possibly take into account the arrangement of a real heat-flow calorimeter in all its details. Theoretical models of heat-flow calorimeters, which are necessarily simplified versions of the actual instruments, will therefore be used in the following calculations. It must be remarked that because of the limitations of the theory, no absolute measurements can be made with a heat-flow calorimeter, nor with any calorimeter. It is possible, however, to compare successive measurements with precision. A calorimetric study necessarily involves the calibration of the calorimeter and, upon this operation, depends the accuracy of the whole series of measurements. 

Proper calibration of the DSC instruments is crucial. The basis of the enthalpy calibration is generally the enthalpy of fusion of a standard material [21,22], but electrical calibration is an alternative. A resistor is placed in or attached to the calorimeter cell and heat peaks are produced by electrical means just before and after a comparable effect caused by the sample. The different heat transfer conditions during calibration and measurement put limits on the improvement. DSCs are usually limited to temperatures from liquid nitrogen to 873 K, but recent instrumentation with maximum temperatures close to 1800 K is now commercially available. The accuracy of these instruments depends heavily on the instrumentation, on the calibration procedures, on the type of measurements to be performed, on the temperature regime and on the... 

Fluorine flame calorimetry is a logical extension of oxygen flame calorimetry in which a gas is burned in excess of gaseous oxidant (214). The decision does not reach that of the oxygen flame calorimeter in which, for example, Affj(H20) was determined with a standard deviation of 0.01%. Combustions of H2, NH3 (8), and fluorinated hydrocarbons are typical applications, but the uncertain nonideality corrections of HF(g) prevent full realization of the inherent accuracy. 

Various substances and reactions have been used to test the accuracy of reaction-solution calorimeters [3 9,40]. The solution of tris(hydroxymethyl)amin-omethane (THAM orTRIS) in 0.1 mol dm-3 HCl(aq), which was first proposed by Wadso and Irving [133] in 1964 and recommended by the Standards Committee of the U.S. Calorimetry Conference in 1966 [134,135], is perhaps the most widely used method. Several problems encountered in the use of the THAM+HC1 (aq) reaction to assess the accuracy of reaction-solution calorimeters have been... 

The historical development of titration calorimetry has been addressed by Grime [197]. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia lfom a plot of the observed temperature change against the volume of ammonia added [208]. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol (chapter 8) or heat-flow instruments (chapter 9) [195,198],... 

The accuracy of a titration calorimeter is normally assessed using the reactions of NaOH(aq) with HCl(aq) or HCICUjaq) [209,210], The dissolution of crystalline tris(hydroxymethyl)aminomethane (THAM) in HCl(aq) has also been employed when the apparatus is equipped with a system for the introduction of solid samples (e.g., an ampule breaking device) [210]. As mentioned in chapter 8, the latter method is commonly recommended for testing conventional reaction-solution calorimeters [39,40]. 

The versatility of the DSC method and the high speed of the experiments have costs in terms of accuracy. For example, the best accuracy in the determination of heat capacities of solids by DSC is typically 1% [3,248-250], at least one order of magnitude worse than the accuracy of the corresponding measurements by adiabatic calorimetry [251]. This accuracy loss may, however, be acceptable for many purposes, because DSC experiments are much faster and require much smaller samples than adiabatic calorimetry experiments. In addition, they can be performed at temperatures significantly above ambient, which are outside the normal operating range of most adiabatic calorimeters. 

Microcalorimetry has gained importance as one of the most reliable method for the study of gas-solid interactions due to the development of commercial instrumentation able to measure small heat quantities and also the adsorbed amounts. There are basically three types of calorimeters sensitive enough (i.e., microcalorimeters) to measure differential heats of adsorption of simple gas molecules on powdered solids isoperibol calorimeters [131,132], constant temperature calorimeters [133], and heat-flow calorimeters [134,135]. During the early days of adsorption calorimetry, the most widely used calorimeters were of the isoperibol type [136-138] and their use in heterogeneous catalysis has been discussed in [134]. Many of these calorimeters consist of an inner vessel that is imperfectly insulated from its surroundings, the latter usually maintained at a constant temperature. These calorimeters usually do not have high resolution or accuracy. 

The accuracy with which the differential heats of adsorption could be measured is ca. 2%. Rapid collection of evolved heat is an important criterion and sometimes, the calorimeter response has to be corrected from the instrumental distortion due to thermal lags. The peak width at half maximum of the signal from the thermal fluxmeters allows comparing the various calorimeters responses [62]. 

Knowing the so-called " water equivalent factor of the calorimeter, it is possible to determine the heat of combustion at constant volume with an accuracy better than 1%... 

Because knowledge of the heats of combustion of organic compds provides important information for making physico-chemical calculations (such as heats of formation), attempts have been made to find empirical rules for calculating the heats of combustion of compds which have not been determined with accuracy in calorimeters... 

The adiabatic character of EB energy deposition is used in calorimetry, which is the primary absolute method of measuring the absorbed dose (energy per unit mass).1 It measures the amount of heat produced by the absorption of the ionizing radiation. An example is the water calorimeter developed by Risp National Laboratory in Denmark.2-3 This instrument is reported to be suitable for electrons from a linear accelerator with energies higher than 5 MeV and shows accuracy of 2%.4... 

The calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

The bomb calorimeter provides the most suitable and accurate apparatus for determination of the calorific values of solid and liquid fuels. Since the combustion takes place in a closed system, heat transfer from the calorimeter to the water is complete, and since the reaction is one between the fuel and gaseous oxygen, no corrections are necessary for the heat absorbed during the reduction of the oxidizing agent. In addition, the losses due to radiation can be reduced to comparatively small quantities, and more important, can be determined with a considerable degree of accuracy. Corrections due to the heat evolved in the formation of nitric and sulfuric acids under the conditions existing in the bomb can be determined accurately. 

A Styrofoam coffee cup serves as an inexpensive calorimeter for measurements that do not require high accuracy. One gram of KCI(.v) was added to 25.0 ml of water in such a cup at 24.33°C. It dissolved completely and quickly with gentle stirring. The minimum temperature reached was 22.12°C. Estimate the AH° of the heat of solution of KC1 in kJ/mol. You may assume the solution has the same heat capacity as water and that the heat capacity of the cup and thermometer need not be considered. 

Gaseous fuel, including GH2, is a costly commodity that is being consumed with much more care and efficiency than in the past. For closed-loop control applications, the fast calorimeters (speed of response of less than a minute) are recommended, as shown in Table 3.11. The table also lists calorimeters that are specifically designed for custody transfer applications and offer improved accuracy (at the expense of response time). 

The type of calorimeter and the method of calculating the heats of adsorption from the experimental data were essentially the same as described in previous papers (I, 4, 10). Two calorimeters of the same design were used, one employing a filler made of copper as described by Dry and Beebe (4) and the other a filler of aluminum. (A drawing and brief description of this calorimeter will be supplied on request addressed to the authors at Amherst College.) In one run for nitrogen adsorption on the bare surface we employed a liquid nitrogen trap to prevent contamination of the sample in the calorimeter by condensed mercury. Data from all runs on the various calorimeters and samples checked within the accuracy of the experiments. 

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