Pulse heating calorimeter

Independent of all the efforts to implement ellipsometry to pulse-heating systems, a second branch of measurements has been jointly developed at NRLM-NMIJ [17] and NIST [101] to obtain the total hemispherical emittance. The principle of this technique is to interrupt the heating process of a pulse calorimeter and to create a short steady-state temperature condition by using the pyrometer as a feedback device. The energy input to maintain this steady-state equals the total radiative losses and thus can be related to the total hemispherical emittance, assuming that no convection occurs during the short steady-state time. 

Due to the lack of specific knowledge and details of each mentioned technique reported within this chapter, no general statement about the imcertainty can be given. Even for the puise calorimeter at TUG, an exact uncertainty evaluation takes certain experimental parameter into account that may change for different materials. Therefore the uncertainties reported here for iridium can be seen as typical uncertainties for the ps-pulse heating experiment at TUG but may change for other specimen materials. 

Molybdenum, SRM 781, has been certified for enthalpy and heat capacity from 273 to 2800 K. The Bunsen-type ice calorimeter and the high-temperature drop calorimeter were also used to measure these materials. In the temperature range above 1500 K, measurements were made with a high-speed pulse-heating technique [% This SRM is available in various lengths of 3- and 6-mm-diameter rods. 

Gmehling (1993) measured excess enthalpies of mixing of different liquids in a commercial isothermal flow calorimeter (Hart Scientific 7501) with an uncertainty of less than 1%. The flow mixing tube was thermostatized by constant Peltier cooling and controlled electrical pulse heating to keep the temperature constant. 

In the calculations proposed by Camia (44), a heat pulse is produced within the calorimeter cell, which is initially in thermal equilibrium. The heat pulse diffuses through the heat-conducting body toward the heat sink which is maintained at a constant temperature 03. 

Finally, experimental procedures differing from that described in the preceding examples could also be employed for studying catalytic reactions by means of heat-flow calorimetry. In order to assess, at least qualitatively, but rapidly, the decay of the activity of a catalyst in the course of its action, the reaction mixture could be, for instance, either diluted in a carrier gas and fed continuously to the catalyst placed in the calorimeter, or injected as successive slugs in the stream of carrier gas. Calorimetric and kinetic data could therefore be recorded simultaneously, at least in favorable cases, by using flow or pulse reactors equipped with heat-flow calorimeters in place of the usual furnaces. 

Figure 11.5 Typical curve for a continuous titration calorimetry study of an exothermic reaction, using the calorimeter of Figure 11.1 in the heat flow isothermal mode of measurement./ is the frequency of the constant energy pulses supplied to the heater C in Figure 11.1 b. Adapted from [196,197],...

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

The specific heat at constant pressure, Cpf of the HIP-treated sample with nominal composition LaVg 25 0,7504 was measured over the temperature range 4-400 K by the heat pulse method in a calorimeter that incorporates a feedback system to regulate the temperature of concentric radiation shields surrounding the sample (9). The Cp values are accurate to within 1%, as determined by calibration runs using a polycrystalline copper sample and a sapphire single crystal sample. 

Figure 2. Temperature-time curves for adiabatic type calorimeters (with a low time constant). Curves A and C show curves following the release of a short heat pulse in an ideal adiabatic calorimeter and a semiadiabatic calorimeter, respectively. Curves B and D show the curves from experiments where a constant thermal power was released between t, and t2 for an ideal adiabatic calorimeter and a semiadiabatic calorimeter, respectively. For the ideal adiabatic instrument the slope of the curve during the heating period is proportional to the thermal power, P.

Figure 4. Potential-time curves from experiments with a thermopile heat conduction calorimeter. A A short heat pulse released at time t,. B A constant thermal power released between time t, and t2. The steady-state potential value, USI is proportional to the released thermal power.

In this paper, the chemical adsorption of NH3, using pulses, has been studied by combining the results of calorimetric measurement of heat released (in a differential scanning calorimeter) with the measurement of desorbed amount of base (by FTIR analysis of desorbed gases). In this way, the differential adsorption heat, representative of the aridity strength distribution of the deactivated catalyst, is obtained and the restrictions inherent to other techniques, which are affected by the measurement of coke degradation products, are avoided. 

Pulse calorimeters pass electrical current through an electrically conducting sample to force a temperature increase, which is measured along with the voltage drop across the sample. If the heat loss from the sample is known (or estimated by calibration), the energy input divided by the temperature increase determines the true heat capacity, if the temperature change is small. Pulse calorimetry eliminates many of the drawbacks of drop calorimetry. It is fast, reproducible, and, with proper calibration, accurate. However, its use is limited to conductive materials. 

Two types of measurements are made with the adsorption calorimeter, also previously described (3). In the batch mode a dry-solid surface is covered with a solution. In the flow mode the enthalpy changes result from a solution flowing through a bed of adsorbent. The flow system uses an LKB 10200 Perpex pump (reference to specific trade names does not imply endorsement by the Department of Energy) with a flow rate of approximately 12 g h 1. Because the silicone tubing on the pump may adsorb surfactant, the pump is placed at the output of the flow system and draws the solution through the cell. An Altex six-way valve is at the input of the flow system, and any one of six solutions can be selected to flow through the cell. Minimum detectable heat pulse is 4.5 x 10 Cal for the batch and minimum power output is 2.4 x 10 ca sec for the flow mode. Measurements reported for the adsorption study were made at 25° and 30° C 0.05° C. 

The assembly fitted tightly into the cell and was pushed against a stainless steel spacer (0.04 cm. thick) that defined the position of the calorimeter relative to the front window. Spacers of lengths 0.2, 1.0, 1.5, 2.0, and 2.7 cm. were used. A 0.1 cm. hole was drilled through each calorimeter at about half radius to allow air interchange between the rear and front of the cell. This was found to be necessary as the thermal expansion because of the heating by the electron pulse of the air trapped in the front section was sufficient, in some cases, to displace the calorimeter assembly. 

Figure 4. The present evolution of Standard DSC towards a range of low- to high-speed calorimeters [32]. Commercial instruments like heat-flux and power-compensation Standard DSCs work typically at scan rates of 0.1 to 60 C/min High Performance DSC (HPer DSC), using a modified PerkinElmer power-compensation Pyris 1 or Diamond DSC, covers the range 0.1 to 500 C/min thin-film (chip) calorimeters have scan rates from 1000 to 1.2-10 C/min and rates as high as 6-10 C/min are attainable using the high-speed pulse-calorimeter (all numbers are approximate indications).

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