Twin calorimeters

Calvet-type calorimeter, twin arrangement temperature range up to 1500 °C noise 330 pW (RMS) resolution 8 pW alumina crucible, volume 5.7 ml. 

The heat capacity of a gas at constant pressure is nonually detenuined in a flow calorimeter. The temperature rise is detenuined for a known power supplied to a gas flowing at a known rate. For gases at pressures greater than about 5 MPa Magee et al [13] have recently described a twin-bomb adiabatic calorimeter to measure Cy. 

In this microcalorimeter, the heat sink is not a massive metal block but is divided into several parts which are mobile with respect to each other. Each thermoelectric element (E) and a cell guide (D) are affixed to a fluxmeter holder (C). The holder (C) is mobile with respect to a massive arm (B) which, in turn, rotates around a vertical axle (A). All parts of the heat sink are made of brass. Surfaces in contact are lubricated by silicone grease. Four thermoelectric elements (E) are mounted in this fashion. They enclose two parallelepipedic calorimetric cells, which can be made of glass (cells for the spectrography of liquids are particularly convenient) or of metal (in this case, the electrical insulation is provided by a very thin sheet of mica). The thermoelectric elements surrounding both cells are connected differentially, the Petit microcalorimeter being thus a twin differential calorimeter. 

The Petit-Eyraud apparatus is a differential calorimeter but it is not a twin calorimeter. The reference cell serves also as a heat sink of limited heat capacity, since it collects, at least transiently, the heat flowing along... 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

It is clear that the calibration of a calorimeter and the preliminary experiments which have been described are operations of paramount importance. In the case of the apparatus that we use, they have shown that corrections are often not necessary, and that the area of the thermogram is in many cases directly proportional to the amount of heat evolved during a adsorption phenomenon, provided that the gas pressure is maintained below 2 Torr. It may not be so with all adsorption calorimeters, especially if a large sensitivity is needed or if the symmetry of the twin system is not perfect. However, the calibration tests and preliminary experiments which have been described can be used to determine eventually the necessary corrections. Moreover, it should not be forgotten that the... 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

High-temperature solution calorimeters [34-36] are in general of the twin heat flux type. They are applicable from around 900 K to around 1500 K and a... 

For the more vigorous reactions, a twin-cell calorimeter was devised (188). It consisted of two nickel cylinders connected by a stainless steel needle valve and tubing and held rigidly to a metal top-plate. The cylinders and connections were immersed in a wide-necked Dewar vessel containing carbon tetrachloride which would react mildly with any BrF3 that escaped. Bromine trifluoride contained in one cylinder was transferred to the solid contained in the other cylinder by opening the valve and applying controlled suction. All measurements were made externally on probes in the carbon tetrachloride. 

Figure 9.2 A schematic diagram of a Calvet s calorimeter, adapted from [157], Only one of the twin calorimetric units (A), with its thermopiles (T) is shown. These units fit into high-conductivity metal blocks (B). Care cones for equipartition of thermal fluctuations D is a thick metal cylinder surrounded by a series of canisters (E). H is an electric heater, and the outer cylinder (I) is a thermal insulator.

All modern heat flow calorimeters have twin cells thus, they operate in the differential mode. As mentioned earlier, this means that the thermopiles from the sample and the reference cell are connected in opposition, so that the measured output is the difference between the respective thermoelectric forces. Because the differential voltage is the only quantity to be measured, the auxiliary electronics of a heat flux instrument are fairly simple, as shown in the block diagram of figure 9.3. The main device is a nanovoltmeter interfaced to a computer for instrument control and data acquisition and handling. The remaining electronics of a microcalorimeter (not shown in figure 9.3) are related to the very accurate temperature control of the thermostat and, in some cases, with the... 

The radiation source for the twin calorimeter of figure 10.2 is a 100 W tungsten lamp. The wavelength is selected by a monochromator, and the light is split in two parts and led into the radiation-absorbing cells of each unit by three light cables. With a 2 mm slit, the band pass is about 13 nm, and for radiation with A = 436 nm the power delivered to each cell is about 60 p,W. The reference cells are simply steel rods and receive no light. 

Twin differential microcalorimeters have been described by Berghausen el al. (S), by Hackerman (8), and by Whalen and Johnson (9). Hacker-man employs thermistors, whereas the other two are based on thermocouples and in addition are run adiabatically. These calorimeters appear to have about 10 times the sensitivity of simpler designs, but for many purposes the large additional diflSculties in design, construction, and operation do not seem to be warranted. Berghausen and coworkers, however, have shown that they can estimate slow heat evolutions, after the first few minutes, due to surface reactions. 

Further developments in calorimetry include the invention of the twin- calorimeter" by Joule (1845) and its modification by Pfaundler (1869XRef 25,p 543) "phase- change calorimer (isothermal) of Bunsen(Ref 15,p 796 Ref 25,p 547) "labirinth flow calorimeter (Ref 25,p 549) "adiabatic calorimeter (nonisothermal), first used by Richards in 1905 (Ref 15,p 797) and modified by Yost, Osborne others (Ref 25,p 550)(See also Ref 3,p ll6)(Parr adiabatic calorimeter is described in Refs 16 29) "constant- temperature- enviroment calorimeter", first used by Nemst in 1907, was modified by Giauque in 1923(Ref 15>p 797)... 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

The device can be designed as a single or a twin calorimeter, also called differential calorimeters. The twin calorimeter technique eliminates the perturbations due to heat loss to the surroundings, for example. They measure the difference of heat release in two symmetrically constructed alorimetric cells. 

Figure 6. Schematic diagrams of sections through two types of twin calorimeters. A Twin heat conduction (micro)calorimeter (vessels not shown). B Semiadiabatic twin calorimeter, a, vessel holder b, thermopile c, heat sink d, vessel (stirrer, thermometer, heater not shown) e, air or vacuum.

Calorimeters of any type in twin arrangements can also be used as power compensation calorimeters an exothermic process in one of the vessels can be simulated by evolution of electrical energy in the other (a thermal balance ). 

In the twin calorimeter, first developed by Joule177 [72], the sample is placed in one calorimeter, while a reference compound of known thermal properties is placed in a second calorimeter matched as closely as possible to the sample calorimeter. This has been very useful in studying rapid reactions, or for measurements of very small heats or slow reactions. 

Most calorimeters described above rely on a measurement of temperature (heat-Flow Calorimeters). The Tian182-Calvet183 calorimeters (some with a twin calorimeter design) use a thermopile (instead of a thermocouple) to measure heat flow directly (Fig. 11.78). 

This work is a continuation of our earlier study [1] of the hydrogen interaction with intermetallic compound (IMC) AB2-type Tio.9Zro.1Mn . 3V0.5. The measurements were carried out in twin-cell differential heat-conducting Tian-Calvet type calorimeter connected with the apparatus for gas dose feeding, that permitted us to measure the dependencies of differential molar enthalpy of desorption (AHdes.) and equilibrium hydrogen pressure (P) on hydrogen concentration x (x=[H]/[AB2]) at different temperatures simultaneously. The measurements were carried out at 150°C, 170°C and 190°C and hydrogen pressure up to 60 atm. 

Hong and Kleppa (2), using a twin micro calorimeter, have determined the enthalpy of mixing for the reaction 3 NaP(t) +... 

In the following list, the Calvet-type twin micro-calorimeter, working up to temperature of 1200 K, is shortly described. The micro-calorimeter consists of the following main parts ... 

In order to provide good time and temperature stability of the calorimeter, the two thermopiles are connected in opposite direction, which eliminates most of the problems with external thermal disturbances. The computer, processing all the input temperature signals, controls the calorimeter. The isoperibolic Calvet s twin micro-calorimeter is schematically shown in Figure 4.3. 

Figure 4.3. Schematic representation of the isoperibohc Calvet s twin micro-calorimeter. A - thermocouple, B - calorimetric cell, C - thermal and electric shielding, D, F, H - three parts of the calorimetric block,...

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