Models thermal detectors

For a simple estimate of the sensitivity and its dependence on the detector parameters, such as the heat capacitance and thermal losses, we shall consider the following model [4.99]. Assume that the fraction p of the incident radiation power P is absorbed by a thermal detector with heat capacity H, which is connected to a heat sink at constant temperature (Fig. 4.73a). When G is the thermal conductivity of the link between the detector and the heat sink, the temperature T of the detector under illumination can be obtained from... 

Fig. 4.73a-c. Model of a thermal detector (a) schematic diagram (b) equivalent electrical circuit (c) frequency response AT Q)... 

Figure 4.79 Model of a thermal detector a schematic diagram b equivalent electrical circuit c frequency response...

We can predict the responsivity of most thermal detectors using a simple thermal model the detector is connected to a heat sink by some thermal impedance Z (or thermal conductance G, where G = 1/Z). Radiant power 4> arriving at the detector is conducted to the heat sink, causing the detector temperature to increase by an amount AT = ZO above the temperature of the heat sink ... 

To model diffraction intensities, detector effects and the background intensity from thermal diffuse scattering must be included. A general expression for the theoretical intensity considering all of these factors is... 

A simplistic model, the DETACT-QS model used to predict the thermal response of detectors and sprinklers, has been subjected to the ASTM El 355 guidelines as a test case Oanssens, 2002). Over five years of effort have been dedicated to this evaluation, showing the difficulty in performing a detailed, recognized validation process. 

As seen in Chapter 9.C.2, a very wide variety of organics are found in particles in ambient air and in laboratory model systems. The most common means of identification and measurement of these species is mass spectrometiy (MS), combined with either thermal separation or solvent extraction and gas chromatographic separation combined with mass spectrometry and/or flame ionization detection. For larger, low-volatility organics, high-performance liquid chromatography (HPLC) is used, combined with various detectors such as absorption, fluorescence, and mass spectrometry. For applications of HPLC to the separation, detection, and measurement of polycyclic aromatic hydrocarbons, see Wingen et al. (1998) and references therein. 

Gas chromatography was used to determine n-paraffin distribution in the oil and wax samples. An F and M Instrument Company Model 500 chromatograph was used with an uncompensated single column, a helium carrier gas flow rate of 25 ml/min., and a thermal conductivity detector. The column was 4.8 mm in diameter and 3.3 m in length, and was packed with 3% Dexil 300 on Chromo-sorb P. The block and injection port temperatures were maintained at 673 K. The column was temperature-programmed from 348 K to 673 K at a rate of 5.7 K per minute. Peak identification was aided by the use of internal standards of decane, dodecane, and hexadecane. The baseline was determined by heating without sample injection. Response values were not available for the various areas on the traces, so the analyses were reported as % by area. 

For determing the concentration of hydrogen sulfide in the product gas, a Gow-Mac thermal conductivity cell (Model 10-952) was used. The cell was equipped with four matched pairs of AuW filaments, especially used because of their resistance to corrosion from the hydrogen sulfide. Layers of styrofoam were used to insulate the cell from changes in ambient temperature. This detector was found to be very sensitive to changes in the flow-rate. 

A 280-nm conversion unit is available. A special thermal-isolation design results in a noise level of only 5 X 10 s AU, which makes this model the most sensitive of its type which is currently available. The detector provides a linear response over 4 orders of magnitude of... 

With these models, an order-of-magnitude estimate of the signal size is obtained. Both models require calculation of the magnetization of the nuclear spins and the noise due to the detector coil. The rms thermal noise voltage for the probe circuit resistance, R0, is (Fig. 5)... 

Procedure (See Chromatography, Appendix IIA.) Use a suitable gas chromatograph equipped with a thermal conductivity detector (F and M Model 810, or equivalent), containing a 0.61-m x 6.35-mm (od) stainless steel column (Perkin Elmer Instruments, or equivalent) packed with 20% Silicone SE-30, by weight, and 80% Diatoport S (60- to 80-mesh), or equivalent materials. Program the column temperature from 100° to 270°, heated at a rate of 15°/min. Set the injection port temperature to 300° the bridge current at 140 mA and the sensitivity, lx for the integrator (Infotronics CRS-100, or equivalent) and 2x for the recorder. Use helium as the carrier gas, with a flow rate of 100 mL/min. 

Gas chromatographic analyses were performed on a Hewlett Packard Model 5880 system equipped with a thermal conductivity detector with an injector temperature of 200°C, detector temperature of 250°C and an oven temperature of 35°C. All runs were isothermal. The columns used for the separation were 20 foot AgNOj-ethylene glycol columns. Absolute retention times were 1.15 min (trans 2-pentene), A.65 min (1-pentene), and 5.81 min (cis 2-pentene). Methods of automatic peak integration that were used during the analyses were peak height, peak area, area %, and internal standard experiments. 

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