Joule-effect calibration

Careful heat-flow calibrations have to be performed. Chemical calibrations present many disadvantages they rely on prior results, with no general agreement and no control of rate, and are generally available only at a single temperature. On the contrary, electrical calibrations (Joule effect) provide many advantages and they are easy to perform at any temperature [103],... 

The procedure may start with the reference experiment, which, in the case under analysis, involved a solution of ferrocene in cyclohexane (ferrocene is a nonphotoreactive substance that converts all the absorbed 366 nm radiation into heat). With the shutter closed, the calorimeter was calibrated using the Joule effect, as described in chapter 8, yielding the calibration constant s. The same solution was then irradiated for a given period of time t (typically, 2-3 min), by opening the shutter. The heat released during this period (g0, determined from the temperature against time plot and from the calibration constant (see chapter 8), leads to the radiant power (radiant energy per second) absorbed by the solution, P = /t. ... 

The calibration of the calorimetric unit P, leading to the calibration constant s (see chapter 9), can be made by the Joule effect, with a resistor inserted into the photochemical reactor cell. As justified shortly (equation 10.16), no calibration is required for the photoinert cell in unit R. 

A catalytic reaction must be performed in aqueous solution at industrial scale. The reaction is initiated by addition of catalyst at 40 °C. In order to evaluate the thermal risks, the reaction was performed at laboratory scale in a Dewar flask. The charge is 150 ml solution in a Dewar of 200 ml working volume. The volume and mass of catalyst can be ignored. For calibration of the Dewar by Joule effect, a heating resistor with a power of 40 W was used in 150ml water. The resistor was switched on for 15 minutes and the temperature increase was 40 K. During the reaction, the temperature increased from 40 to 90 °C within approximately 1.5 hours. The specific heat capacity of water is 4.2 kj kg K 1. 

The determination of the constants S and t constitutes the static and dynamic calibration of the calorimeter. The calibration of the calorimeter may be done by means of a known thermal effect produced during suitable conditions inside the cell, for example. Joule effect calibration (2, 4, 41). 

This is the shape of the input function that is applied when the calibration of the calorimeter consists in generation of a Joule effect that is constant in time for a defined duration. The exceptions to the rule are those instruments in the calibration of which the frequency characteristics are used. 

There are quite a few different methods for the calibration of DSC instruments, of which the most popular are (a) calibration by Joule-effect and (b) calibration by heats of fusion.Joule-effect calibration is relatively simple and straight-forward in that it consists of an electrical heater inserted into the sample and reference compartments. A pulse of predetermined duration and intensity is sent to the sample, and the dissipated power is then measured. The disadvantage ofthis method is that some heat flux can be dissipated in the heater wires, and, therefore, not truly measured. Furthermore, the electrical heater is not necessarily composed of the same material as the sample and reference holders. Still, the accuracy of this calibration technique is better that 0.2%. 

The calorimeter can be calibrated by the Joule effect. The resistor is placed in a cell containing mercury and a liquid representative of the solution, in amounts identical with those for the measurements (Figure 7.1b). Another possibility is to replace this absolute calibration by the measurement of a known enthalpy of reaction with BF3, which may be used as an indirect calibration. 

The calorimeter may be calibrated by the Joule effect (see experiment 7.1) or using a standard reaction, chosen among those suggested by an lUPAC technical report [13]. 

However another calibration technique is available when the Calvet type DSC is used. The principle is to apply a known amount of power in a dedicated calibration vessel. To reach this target, a resistance is embedded in the crucible. A known current I is delivered and the corresponding tension U measured, providing the power P = UI that is applied. The corresponding Joule effect provides a DSC exothermic deviation in microvolt (Fig. 2.8). Such an electrical calibration is very interesting as it can apply at any temperature, even at constant temperature. This will be more detailed in paragraph 5 for the calorimetric techniques. 

Fig. 2.8 Principle of the DSC electrical calibration using the Joule effect...

As seen before in the calibration paragraph, the calibration using the Joule effect technique allows converting any calorimetric signal in mW without the need of standard reference materials. That means that in relation (2.10) all the parameters (sample mass, calorimetric signals, heating rate) are accurately known to determine the specific heat capacity of the sample Cp(s) (expressed in J.g . " C ) at a given temperature. The variation of Cp(s) versus temperature can be determined. 

The dissipation of a known amount of electrical energy inside the calorimetric cell by means of a calibration coil (i.e., the Joule effect) is used to relate the area of the thermal peaks recorded to the enthalpy effects which this represents. The difficulty with this type of calibration in the titration calorimetry systems is related to the fact that the mass of solution in the calorimetric cell is constantly increased by successive injections, thereby changing the calorific capacity of the cell. Therefore, thermal calibration should be regularly repeated after each series of injections in the same calorimetric run. 

A very precise determination of the power absorbed in a system is described by Margulis, who compared the ultrasonic conditions to Joule s effect from a calibrated thermistor.35 Other physical measurements of power (pp. 13 and 14), less commonly applied, include the use of microphones, acoustic balances, and the erosion of metallic foils by cavitation. ... 

The use of the Joule heat effect from a resistance element on passage of electric charge is the preferable method for an absolute calorimetric calibration. It certainly requires a special setup of the measuring head enabling... 

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