Thermopiles reduced

The sensor unit of an IRET usually consists of an infrared sensor, in most cases a thermopile sensor in a TO-5 or TO-46 housing, a gold plated barrel, which reflects the infrared radiation from the ear to the sensor and reduces the sensitivity of the sensor to ambient temperature changes (see Fig. 3.43). 

If silicon technology is involved all thermal sensors suffer from the high thermal conductivity of silicon, which dramatically decrease their sensitivity [12]. However, by use of micromachining and integrated silicon technology a powerful thermal biosensor can be realized. Using a thermopile integrated on a thin micromachined silicon membrane reduces thermal loss due to the substrate and so excellent performance can be accomplished [13]. 

A typical optical system is shown in Fig. 26. A lens of short focal length (7-10 cm) projects a nearly parallel beam of radiation from the source A through a filter F, to remove unwanted radiation. The stop Sj prevents unfiltered radiation from reaching the RV. It is sometimes useful to converge the beam slightly with a second lens (focal length 40 cm) such that the beam reaches its smallest diameter in the centre of the RV. The latter may be divided into two compartments, one of which contains a compound used for actinometry, or alternatively the beam is focused by the lens L3 onto a photocell or thermopile P. The intensity of the beam is suitably reduced by the density filter F2. To provide maximum possible intensity... 

The integrated thermopile (1.6x10 mm) was manufactured by the following method. A quartz chip (25.2 x 14.8 x 0.6 mm) was used as a substrate instead of a silicon wafer, in order to reduce the heat conductivity of the chip. A 0.5 jim thick layer of polysilicon was deposited using LPCVD (low-pressure chemical vapour deposition) onto the quartz substrate. The layer was boron-doped using... 

The exponential decay of the power with time may be integrated to measure the heat produced. Cooling in the thermopiles may be induced by passing a current in the reverse direction allowing a more efficient means of temperature control, reducing experimental time. Such principles are used in Calvet or thermoelectric heat pump calorimeters. 

Davy had shown that the current in a wire is proportional to the cross-section and inversely proportional to the length, also that it depends on the nature of the material of the wire. Ohm at first used galvanic cells of inconstant electromotive force and the results were incorrect. Following a suggestion by Poggendorfi, who took a great interest in his work. Ohm then used a thermopile and established his law. He uses the name electroscopic force for electromotive force and does not use the name resistance but speaks of reduced lengths of conductors. 

Thermocouples and thermopiles have been used as transduction devices for chemical sensors. These are usually based on a measurement of the heat of reaction and have been coupled to specific enzyme reactions for selectivity. Sensitivity and selectivity of these devices has yet to reach truly satisfactory levels, but there may be specific applications in which they are practical. They do offer the advantage of reduced susceptibility to fouling, compared to electrochemical detection methods. 

Simple thermocouples are not sensitive enough to detect variations in enzymatic reaction enthalpies. Thermistors and thermopiles can, however, detect such small variations in temperature. Thermistors are mixtures of metallic oxides and polycrystalline semiconductors. The high resistivity values of these materials gives a rapid response time owing to their small size and reduced calorific capacity. TTie resistance R of a thermistor is a function of temperature, T, according to the following expression ... 

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