Calibration heat flow calorimeter

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. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

A heat flow calorimeter and the drop calorimetric method were used by Connor, Skinner, and Virmani to investigate the thermal decomposition of Cr(CO)6 at 514 K (the calibration was made with iodine as described above) [164], The only peak observed corresponded to an endothermic process ... 

A marginal but very important application of the drop calorimetric method is that it also allows enthalpies of vaporization or sublimation [162,169] to be determined with very small samples. The procedure is similar to that described for the calibration with iodine—which indeed is a sublimation experiment. Other methods to determine vaporization or sublimation enthalpies using heat flow calorimeters have been described [170-172], Although they may provide more accurate data, the drop method is often preferred due to the simplicity of the experimental procedure and to the inexpensive additional hardware required. The drop method can also be used to measure heat capacities of solids or liquids above ambient temperature [1,173],... 

The home-made heat-flow calorimeter used consisted of a high vacuum line for adsorption measurements applying the volumetric method. This equipment comprised of a Pyrex glass, vacuum system including a sample holder, a dead volume, a dose volume, a U-tube manometer, and a thermostat (Figure 6.3). In the sample holder, the adsorbent (thermostated with 0.1% of temperature fluctuation) is in contact with a chromel-alumel thermocouple included in an amplifier circuit (amplification factor 10), and connected with an x-y plotter [3,31,34,49], The calibration of the calorimeter, that is, the determination of the constant, k, was performed using the data reported in the literature for the adsorption of NH3 at 300 K in a Na-X zeolite [51]. 

In a heat flow calorimeter, a feedback controller is used to maintain a constant desired reactor temperature by adjusting the jacket temperature. From (1), with a constant calibration probe heat flow, at steady state (dT/dt = 0), the overall heat transfer coefficient can be found from... 

A heat flow calorimeter is calibrated by an electric heater of measured power (W = amps X volts). 

Last but not least, it should be reemphasized that the fundamental equation (7.5) for heat flow calorimeters is valid only for steady-state conditions. The constmction of the heat conducting pathway and the support of the sample containers has to be done in such a manner that the disturbances of the steady-state heat flow during a reaction are as small as possible. Nevertheless, in principle, every deviation from steady state influences the factor fC in Eq. (7.5) thus, the calibration of the calorimeter should be done with the aid of a thermal event as equal as possible to the event to be investigated (for details, see Hohne, 1983). 

TTie calibration of flow calorimeters represents a particular problem. A separate calibration has to be made for each medium because the calibration factor depends on the specific heat capacity and the flow rate. In principle, a flow calorimeter can only be calibrated for dilute solutions because otherwise the calibration factor depends on the concentrations and their changes during the reaction, too. The same applies to changes of specific heat capacities and other reaction parameters. This aspect will be considered in greater detail below. 

For viscous reaction mixtures, where we have a particular interest in ) nowing h, the physical properties required to determine Y are not easily accessible. It is much simpler to calculate Y from the overall heat transfer coefficient U in the heat flow calorimeter, which is obtained by the calibration procedure ... 

Experiments were performed in tlie SIMULAR calorimeter using the power compensation method of calorimetry (note that it can also be used in the heat flow mode). In this case, the jacket temperature was held at conditions, which always maintain a temperature difference ( 20°C) below the reactor solution. A calibration heater was used to... 

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... 

The RC1 Reaction Calorimeter is marketed by Mettler-Toledo. The heat-flow calorimetric principle used by the RC1 relies on continuous measurement of the temperature difference between the reactor contents and the heat transfer fluid in the reactor jacket. The heat transfer coefficient is obtained through calibration, using known energy input to the reactor contents. The heat trans-... 

The heat flux and energy calibrations are usually performed using electrically generated heat or reference substances with well-established heat capacities (in the case of k ) or enthalpies of phase transition (in the case of kg). Because kd, and kg are complex and generally unknown functions of various parameters, such as the heating rate, the calibration experiment should be as similar as possible to the main experiment. Very detailed recommendations for a correct calibration of differential scanning calorimeters in terms of heat flow and energy have been published in the literature [254,258-260,269]. 

E. Gmelin, S. M. Sarge. Temperature, Heat and Heat Flow Rate Calibration of Differential Scanning Calorimeters. Thermochim. Acta 2000, 347, 9-13. 

ASTM E 968-83, Standard Practice for Heat Flow Calibration of Differential Scanning Calorimeters, 1983. 

A typical reaction calorimeter consists of a jacketed reactor, addition device, temperature transducer(s) and calibration heaters. There are a number of devices within Dow ranging from the commercially available Mettler RC-1 (1-2 L volume) to smaller, in-house reactors (10-50 ml). While each of these devices has their unique attributes (e.g., in-situ spectrometry, quick turn-around, ability to reflux, etc.), all of the calorimeters will produce a signal of heat flow vs. time. The heat flow is usually produced in response to the addition of a reagent or an increase in temperature. Volume of gas or pressure generated may also be measured. 

Sarge, S.M. Hohne, G.W.H. Cammenga, H.K. Eysel, W. Gmehn, E. Temperature, heat and heat flow rate calibration of scanning calorimeters in the cooling mode. Thermochim. Acta 2000, 361, 1-20. 

The basic operation of the gaseous flow calorimeters is essentially identical to that of the flow-through solution-phase calorimeters with an external gas/vapour source that is passed, through a single calorimetric cell, across the solid of interest and the resulting heat change measured. For these instruments, the detectors are thermistors in direct contact with the solid under study. The form of the returned data is volts as a function of time. The signal can be converted to J s via a calibration constant. 

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. 

E 968 (1999) Standard practice for heat flow calibration of differential scanning calorimeters E 1131 (1998) Test method for compositional analysis by thermo gravimetry... 

Mathematically this simation without a AT loop is expressed by the upper three boxed equations in Fig. 4.60, where dQs/dt and dQs/dt are the heat-flow rates into the reference and sample calorimeters, respectively. The measured and true temperatures are represented by T and T. For simplicity, one can assume that the proportionality constant K is the same for the sample and reference calorimeters. Differences are assessed by calibration. Both bottom equations are then equal to the power input from the average temperature amplifier. Wav (ill W or J s ). 

The use of the corrections B in Fig. A.l 1.2 needs two calibration runs of the DSC of Fig. 4.54. The heat capacities of the calorimeter platforms, C pi and C pi, and the resistances to the constantan body, R pi and R pi, must be evaluated as a function of temperature. First, the DSC is ran without the calorimeters, next a run is done with sapphire disks on the sample and reference platforms without calorimeter pans. From the empty run one sets a zero heat-flow rate for and This allows to calculate the temperature-dependent time constants of the DSC, written as = C piRspi and Tr = CrpiRrpi, and calculated from the equations in the lower part of Fig. A. 11.2. For the second run, the heat-flow rates are those into the sapphire disks, known to be mCpQ, as suggested in Figs. 4.54 and 4.70. The heat-capacity-correction terms are zero in this second calibration because no pans were used. From these four equations, all four platform constants can be evaluated and the DSC calibrated. 

---