Electrical heating rate with calorimeters

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]. 

Electrical heating circuit for solution calorimeter. The standard resistor should be a wire-wound resistor with a low temperature coefficient and rated for 2 W. If a heating coil of higher resistance (say, 60 2 is to be used, the current can be reduced to 0.5 A and a 1-H standard resistor can be used. 

The glass wall which surrounds the calorimeter block is naturally made as thin as possible, and it is desirable that there should be good thermal contact between them. Otherwise irregularities in the rate of change of temperature may easily be observed. The block was therefore cemented with Woods metal into the vacuum vessel. For this purpose some of the alloy was put into the vessel the block, electrically heated from the inside, was then introduced and pushed down a suitable depth into the Woods metal. 

Classical adiabatic calorimeters, with volumes of the order of tens of cm, are a vanishing breed. Although perhaps inevitable, this is unfortunate because the technique requires only electrical measurements and calibration using known material standards is not necessary, in principle. Many current calibrants have been characterised in this type of calorimeter. To ensure that data are always taken in near-equilibrium conditions, and apply to a definite temperature, measurements should be made under isothermal conditions, or with the lowest heating rate possible. 

In calorimeters of a flowing type, a liquid evaporates from a separate vessel of a calorimeter. The vapors are directed into the second calorimeter where the thermal capacity of gas is measured. The design of such calorimeters ensures a precise measurement of the stream rate. Heaters and electrical controls permit control of heat flow with high precision and highly sensitive thermocouples measure temperature of gas. There are no excessive... 

A unique solution to fast DTA is the foil calorimeter, shown schematically in Fig. A. 10.4. A copper-foil is folded in such a way that two sheets of the sample (also very thin, so that the mass remains small) can be placed between them. The copper foil is used as the carrier of electrical current for fast heating. Between the inner portion of the stack of copper foil and sample, a thin copper-constantan thermocouple is placed. Only three folds of the stack are shown. In reality, many more folds make up the stack so that there are no heat losses from the interior and measurements can be made under adiabatic conditions. Heating rates of up to 30,000 K min" (500 K s" ) have been accomplished. Measured is temperature, time, andthe-rate-of-change of temperature for a given heat input. With such fast heating rates it becomes possible to study unstable compounds by measuring faster than the decomposition kinetics of the compound. This super-fast calorimeter has seen httle apphcation, likely because it requires a new calorimeter for each sample. 

This calorimeter (Picker, Jolicoeur, and Desnoyers, 1969) represents a twin instrument with countercurrent auxiliary circulation (Figure 7.24). All liquids are brought to a constant temperature at the inlet. The reactants are mixed with one another before entering the first flow tube, and the heat of reaction is transferred in a heat exchanger from the reaction product to the auxiliary liquid, which flows in the opposite direction. In the second flow tube - also connected by means of a second heat exchanger with a counterflowing auxiliary liquid - a nonreacting reference liquid (e.g., the reaction product) flows. The temperature difference between the two countercurrents is measured it is proportional to the heat flow rate of the reaction. The calorimeter is equipped with electric calibration heaters. 

When an electric current was passed through a heating coil of resistance 16.49 ohms, the fall of potential across the coil was 3.954 volts. Neglecting loss of heat by radiation, etc., determine the rate of increase of temperature, in deg. sec.", of a calorimeter- with a heat capacity of 125.4 (defined) cal. deg.". ... 

ASTM E906 [99] defines the OSU calorimeter. The apparatus consists of an insulated box containing a vertical specimen, a parallel electric radiant heater, and a pilot ignition device. Air at a controlled rate flows through the box, and the inlet and outlet temperatures arc recorded. ASTM E906 also records the temperature of the box wall to compensate for the nonadiabatic characteristics of the apparatus. The box is calibrated using a preset gas flame. The vertical specimen size is 150 mm square, and the incident heat flux has a maximum value of 100 kW, m. Tests with a horizontal specimen, 110 x 150mm, which involve the use of aluminum foil to reflect heat onto the specimen, are apparently... 

Two liquids capable of reacting with one another and possessing the same known temperature Ti (at position flow into a reaction tube (Figure 1.5). There they react. At the measuring position for T2 (i.e., at position xf), where the reaction is assumed to be already completed, the liquid flows out of the tube. The calorimeter operates continuously. With the establishment of a thermal steady state between the liquid-containing reaction tube and the surroundings, a constant temperature difference AT = T X2f — T(%i) = AT(xi, xf) is established that is proportional to the heat of reaction. The proportionality factor has to be determined by proper calibration. This can be done in a subsequent experiment in which the collected reaction product flows with the same flow rate and temperature around an electric heater inside the reaction tube. 

Figure 7.20 shows an ideal flow calorimeter. A fluid flows at a constant rate v in a tube. Its temperature at the point Xi is Ti = T(xi) = Tp, usually the temperature of the surroundings. If a heat quantity is released in the fluid (by electric means or in the course of a reaction), the result is a rise in the fluid s temperature, which is measured at the point X2- In the presence of ideal adiabatic conditions, the function r2(f) reflects exactly the heat production 0(t) at the heat source but with a time lag At = Axjv (Ax is the distance between the end of the heat source and the point X2). Because the tube has a certain heat capacity of its own, ideal adiabatic conditions cannot be expected. Part of the heat to be measured passes to the tube, the surroundings, and the temperature... 

Lai et al. (1997) fabricated a chip calorimeter with a 100 nm thick Si-N membrane with two Ni thin-film stripes (30 nm thick and 0.4 mm wide) as the differential heater pair, one serves as the sample heater and the other as the reference heater. With two synchronized electrical current pulses, the two heaters can be heated up to 300 °C at a rate of about 30000Ks. The heater stripes also function as temperature sensors. The apparent heat capacity of the chip is about 6 x 10 J at 300 K and the lowest detectable heat is given as 0.2 nj. 

To complete the setup, the sealed bomb vessel is immersed in a known mass of water in the calorimeter. A precision thermometer and a stirrer are also immersed in the water. With the stirrer turned on, the temperature is monitored until it is found to change at a slow, practically-constant rate. This drift is due to heat transfer through the jacket, mechanical stirring work, and the electrical work needed to measure the temperature. A particular time is chosen as the initial time t. The measured temperature at this time is Fi, assumed to be practically uniform throughout the system. 

---