Response calorimeter

A calorimeter Is a device used to measure heat flows that accompany chemical processes. The basic features of a calorimeter include an Insulated container and a thermometer that monitors the temperature of the calorimeter. A block diagram of a calorimeter appears in Figure 6-15. In a calorimetry experiment, a chemical reaction takes place within the calorimeter, resulting in a heat flow between the chemicals and the calorimeter. The temperature of the calorimeter rises or falls in response to this heat flow. 

The proportionality constant g includes such parameters as the number of thermoelectric couples in the pile, their thermoelectric power, and the gain of the amplification device. It is supposed, moreover, that the response of the recording line is considerably faster than the thermal lag in the calorimeter. The Tian equation may also be written therefore ... 

The basic principle of heat-flow calorimetry is certainly to be found in the linear equations of Onsager which relate the temperature or potential gradients across the thermoelements to the resulting flux of heat or electricity (16). Experimental verifications have been made (89-41) and they have shown that the Calvet microcalorimeter, for instance, behaves, within 0.2%, as a linear system at 25°C (41)-A. heat-flow calorimeter may be therefore considered as a transducer which produces the linear transformation of any function of time f(t), the input, i.e., the thermal phenomenon under investigation]] into another function of time ig(t), the response, i.e., the thermogram]. The problem is evidently to define the corresponding linear operator. 

The calorimeter response to a unit impulse must therefore be determined. This may be achieved by solving the Fourier equation [Eq. (23)] for a theoretical model of a heat-flow calorimeter and for this particular heat evolution. 

The calorimeter response (the emf-time curve or the thermogram) is, of course, proportional at any time to the temperature difference which exists between two definite values of the space variable ri and r2 where the active and reference junctions of the thermoelement are located ... 

The combination of Eqs. (28) and (22) gives the Laplace transform of the impulse response H(p) which allows us to solve Eq. (21). By the inverse transformation, the relation which gives the output of the linear system g(t) (the thermogram) to any input/(0 (the thermal phenomenon under investigation) is obtained. This general equation for the heat transfer in a heat-flow calorimeter may be written (40, 46) ... 

The development of the theory of heat-flow calorimetry (Section VI) has demonstrated that the response of a calorimeter of this type is, because of the thermal inertia of the instrument, a distorted representation of the time-dependence of the evolution of heat produced, in the calorimeter cell, by the phenomenon under investigation. This is evidently the basic feature of heat-flow calorimetry. It is therefore particularly important to profit from this characteristic and to correct the calorimetric data in order to gain information on the thermokinetics of the process taking place in a heat-flow calorimeter. 

From Tian s equation [Eq. (30)3, it appears that in order to transform the calorimeter response g(t) into a curve proportional to the thermal input f(t), it is sufficient to add, algebraically, to the ordinate of each point on the thermogram g(t), a correction term which is the product of the calorimeter time constant n, by the slope of the tangent to the thermogram at this particular point. This may be achieved manually by the geometrical construction presented on Fig. 10. 

The problem is apparently simple and may be expressed in the following way knowing g(t), the thermogram, and h(t), the calorimeter response to a unit impulse, solve Eqs. (20) or (35) and determine/(<), the thermokinetics of the phenomenon taking place in the calorimeter. However, the digital information which is used in the computer does not allow the continuous integration of Eq. (35). Both functions g(t) and h(t) are indeed stored and manipulated as series of discrete steps (samples). For a computer s convenience, Eq. (35) must therefore be written... 

In order to get an overall idea of the effects of the above-mentioned phenomena on the response of a non-ideal calorimeter, let us now considerer a simplified model of one of the calorimeter of COURICINO (Fig. 15.7) experiment (see Section 16.6). 

Using the model of Fig. 15.8, we have simulated an event leading to an energy adsorption AE. To evaluate the corresponding temperature increase AT, at different heat sink operating temperatures, a T3 dependence of the absorber heat capacity was supposed. To obtain the calorimeter response (temperature change on the 7) thermal node) for a simulated event, a SPICE program was used. 

Techniques are available to quantify the generation of smoke, toxic and corrosive fire products using the NBS Smoke Chamber (15), pyrolysis-gas chromatography/mass spectrometry (PY-GC-MS) (J 6), FMRC Flammability Apparatus (2,3,5,17,18), OSU Heat Release Rate Apparatus (13) and the NIST Cone Calorimeter (JJO. Techniques are also available to assess generation of 1) toxic compounds in terms of animal response (19), and 2) corrosive compounds in terms of metal corrosion (J 7). In the study, FMRC techniques and AMTL PY-GC-MS techniques were used. 

Table 5 Ignition-time measured in the ASTM E 1354 cone calorimeter and thermal response parameter values derived from the data... 

The accuracy with which the differential heats of adsorption could be measured is ca. 2%. Rapid collection of evolved heat is an important criterion and sometimes, the calorimeter response has to be corrected from the instrumental distortion due to thermal lags. The peak width at half maximum of the signal from the thermal fluxmeters allows comparing the various calorimeters responses [62]. 

Heat is the most common product of biological reaction. Heat measurement can avoid the color and turbidity interferences that are the concerns in photometry. Measurements by a calorimeter are cumbersome, but thermistors are simple to use. However, selectivity and drift need to be overcome in biosensor development. Changes in the density and surface properties of the molecules during biological reactions can be detected by the surface acoustic wave propagation or piezoelectric crystal distortion. Both techniques operate over a wide temperature range. Piezoelectric technique provides fast response and stable output. However, mass loading in liquid is a limitation of this method. 

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. 

The energy involved in the folding and association of copolymer chains in solutions can be measured by a micro-calorimeter (MicroCal Inc). We used US-DSC at an external pressure of 180 kPa. The cell volume is only 0.157 mL. The heating rate can be varied and the instrument response time is normally a few seconds. All the DSC data should be corrected for instrument response time and can be analyzed using the software in the calorimeter. Note that the concentration used in DSC is normally not lower than 10-3 g/mL, much higher than that used in LLS (10 6-10 3 g/mL). 

Gaseous fuel, including GH2, is a costly commodity that is being consumed with much more care and efficiency than in the past. For closed-loop control applications, the fast calorimeters (speed of response of less than a minute) are recommended, as shown in Table 3.11. The table also lists calorimeters that are specifically designed for custody transfer applications and offer improved accuracy (at the expense of response time). 

The external heat flux (q"x) from the cone heater does not exclusively determine the heat flux important for samples pyrolysis in the cone calorimeter, since the reradiation from the hot sample surface (q"eTad), the loss by thermal conductivity into the specimen and the surroundings ( I SS), and the heat flux from the flame (q Lmt) are also of the same order of magnitude.82 85 Thus, the heat flux effective with respect to pyrolysis during a cone calorimeter run (qeii) is the result of the external heat flux and the material s response (qeB = q L + < L - gCad - qLs). 

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