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Temperature Measurements
Published in Francis S. Tse, Ivan E. Morse, Measurement and Instrumentation in Engineering, 2018
An optical chopper (see Fig. 3-54) can be used (1) to convert the dc input to a detector to ac, (2) to allow radiation of different wavelengths to reach the detector alternately, or (3) to compare the radiation from a target with that of a reference. For a dc-to-ac conversion, the chopper simply interrupts the radiation from the target to the detector. A circuit with chopped radiation to a thermistor detector is shown in Fig. 9-25a. This is the ballast circuit for strain gages (see Probs. 1-3 to 1-5). The ac output can then amplified with an ac amplifier. This circuit is less susceptible to the slowly varying ambient temperature. Another scheme uses two detectors in a bridge circuit, as shown in Fig. 9-25b (see Fig. 3-33). The detector D1 is exposed to the chopped radiation, but D2 is shielded from radiation and it is at the controlled temperature of the housing.
Radiometric and Photometric Measurements
Published in Lazo M. Manojlović, Fiber-Optic-Based Sensing Systems, 2022
The measurement of low radiant fluxes emanating from faint radiation sources can be a problem if the measurement has been performed with the sensing setups shown earlier. Due to the very low light levels, the influences of other parasitic effects can interfere and thus hide the useful signal. For example, relative high levels of background light, voltage offset of the signal amplifier, and its thermal and time drifts as well as high levels of flicker noise of the amplifier can significantly deteriorate the measurement accuracy and precision. However, there is a solution in moving the useful signal toward the higher frequencies and perform the phase-sensitive detection with the help of lock-in amplifier. The typical measurement setup is presented in Figure 2.13. Between the source and the main detector D1, an optical chopper has been installed. Optical chopper is a device that periodically interrupts a light beam. There are several different types of optical choppers. One of them is presented in Figure 2.13 and consists of an opaque rotating wheel with symmetrically positioned transparent parts (holes) that is driven by a motor M. The signal from the detector is amplified by the signal amplifier A1 in which output has been brought to the input port of the lock-in amplifier LA. To perform a phase-sensitive detection, there is a need for the corresponding reference signal that must be brought to the reference input of the lock-in amplifier. The reference signal is obtained with the help of auxiliary light source LS and detector D2 that are separated by the same optical chopper wheel. The reference signal is obtained at the output of the second signal amplifier A2. The lock-in amplifier output signal will be proportional to the detector captured radiant flux but most of the disruptive influences will be significantly suppressed.
Weathering induced morphological modification on the thermal diffusivity of natural pyrrhotite: a thermal lens study
Published in Philosophical Magazine, 2021
M. S. Swapna, V. Gokul, Vimal Raj, R. Manu Raj, S. N. Kumar, S. Sankararaman
For unveiling the modifications in the thermal behaviour, nanofluids of the sample are prepared in the base fluid ethylene glycol, and the thermal diffusivity (α) of the nanofluid is determined using the TL setup shown in Figure 1. The experimental setup is standardised by finding the thermal diffusivity of the base fluid, ethylene glycol. The reason for selecting ethylene glycol as the base fluid is that its viscosity is greater than that of water and we get a stable nanofluid. The nanofluids are prepared by dispersing 0.5 g of the samples (P1 and P2) in one litre of ethylene glycol and ultra-sonicated for 15 min. The TL setup uses an 80 mW He-Cd laser at 442 nm, and a convex lens of focal length 20 cm to focus the laser beam on to the sample in a quartz cuvette. The optical chopper (SRS 540), placed between the convex lens and cuvette, modulates the laser beam at 1 Hz. When the beam of spot radius 0.11 ± 0.01 mm, is focussed on to the nanofluid in the cuvette, a thermal lens is formed within the medium which diverges the beam. The intensity at the centre of the emergent beam, given by Equation (1), is detected by a photodiode and fed to a 500 MHz digital storage oscilloscope (DSO). The change of intensity of the emergent beam during the ON state of the chopper gives the TL signal. The TL signal is then curve fitted using Equation (1) to get the characteristic time constant (tc), from which the α is calculated using Equation (2) [27,28] where I(0) – the initial intensity, I(t) – intensity at time t, and – a parameter related to photothermal energy.