Time constant thermal detector

Because the response time of the detector depends on the thermal time constant of the detector element / electrode assembly, coupled with the electrical time constant of the device capacitance and load resistor - the response versus modulation frequency (f shows a typical l//m form. 

The TCD measures changes In the thermal conductivity of the carrier gas, perturbed by the eluting analyte. Thermal conductivity detectors which rely on diffusion by the analyte to the hot filament generally have the lowest limit of detection (due to the very low noise of the detector), yet have very long time constants. Flow-through thermal conductivity detectors can retain peak symmetry without excessively large volumetric time constants. 

The performance of the radiation detectors depends on their intrinsic properties, temperature and external conditions of use. They can be compared by using a factor of merit D, known as the detectivity, equal to the inverse of the NEP for a detector with unit area used with an electrical band-width A/ of 1 Hz and expressed in cm Hz1/2 W-1. When a value of D is indicated for a thermal detector, it is considered to be independent of the radiation frequency and the time modulation frequency is assumed to be adapted to the intrinsic time constant t, of the detector. For a photoconductive detector, D peaks at a radiation frequency very close to the band gap for an intrinsic detector or to the ionization energy of the relevant centre for an extrinsic detector and decreases steadily at lower energies. 

A thermal detector has a heat capacity H = 10 J/K and a thermal conductivity to a heat sink of G 10 W/K. What is the temperature increase AT for 10 W incident cw radiation if the efficiency P = 0.8 If the radiation is switched on at a time r = 0, how long does it take before the detector reaches a temperature increase AT(t) = 0.9 A Too What is the time constant of the detector and at which modulation frequency co of the incident radiation has the response decreased to 0.5 of its dc value ... 

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