Thermopile

Thermopiles are considered temperature sensors and are fabricated incorporating a number of thermocouples. Each thermocouple is formed by a couple of different materials (Metall-Metal2, Metal-Semiconductor, Semiconductor-Semiconductor) and responds to a temperature difference localized between the two junctions (cold junction and warm junction ), see fig. 11. One of the two junctions can be considered the reference one. 

During operation the voltage developed at the thermopile output is proportional to the thermoelectric power of each of the two different materials and to the temperature difference between the warm and cold junction (Seebeck effect). 

When constituted of metals, thermopiles exhibit a very low noise, in particular only thermal noise if the voltage amplifier used for signal amplification has a very high input impedance. 

Thermopiles can be deposited through thermal evaporation or even sputtering on either hard or soft substrates. 

A thermopile can also be used as a chemical sensor if one of the two materials is a catalytic metal for a given volatile compound. In this case it is necessary to keep the warm and cold junctions at constant temperature. During absorption of the volatile compound on behalf of the catalytic material the thermoelectric power may change, giving rise to an output voltage which can be related to the concentration of the volatile compound. A typical example is the thermopile as hydrogen sensor, where one of the two materials is palladium, a standard hydrogen catalyzer. 

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Thermooxidatiion Thermopave Thermophilic bacteria Thermophotometry Thermopiles Thermoplastic... 

Fig. 3. Three-Stage cascaded thermopile. The relative sizes of the stages must be adjusted to obtain the maximum AT.

Total Radiation Pyrometers In total radiation pyrometers, the thermal radiation is detec ted over a large range of wavelengths from the objec t at high temperature. The detector is normally a thermopile, which is built by connec ting several thermocouples in series to increase the temperature measurement range. The pyrometer is calibrated for black bodies, so the indicated temperature Tp should be converted for non-black body temperature. 

The heated-thermocouple anemometer measures gas velocity from the cooling effect of the gas stream flowing across the hot junctions of a thermopile supplied with constant electrical power input. Alternate junctions are maintained at ambient temperature, thus compensatiug for the effect of ambient temperature. For details see Bunker, Proc. Instrum. Soc. Am., 9, pap. 54-43-2 (1954). 

Thermo-plastizitat, /. thermoplasticity, -reg-ler, m. thermo-regulator, heat regulator, -saule,/. thermopile. -Strom, m. thermoelectric current. 

Another application of the Seebeck effect is to be found ill detectors of small quantities of heat radiation. These sensitive detectors comprise a thermopile, a pile of thermocoup)les (small pieces of two different metals connected in V form and put into series). Half of the junctions of the thermopile are shielded within the detector, whereas the other half are exposed to... 

Steam, gas, petrol, and hot-air engines are heat engines a thermopile coupled with an electromotor also constitutes a heat engine. An electromotor is not a heat engine, since its effect is produced at the cost of electrical energy, which may, it is true, ultimately be derived from a heat engine coupled with a dynamo, but may equally well arise from chemical action in voltaic cells absorbing practically no heat from their environment (e.g., the Daniell cell). 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

E. H. Putley, The Pyroelectric Detector Norman B. Stevens, Radiation Thermopiles... 

One of the difficulties inherent in adiabatic calorimetry is that it will, almost inevitably, result in low results, because of heat losses. This is not an intrinsic deficiency of the OSU calorimeter unit, since it can easily be modified to incorporate other methods of heat measurement. However, the traditional detection device used (and recommended in the standards) is the use of a thermopile. 

More details on appliance markets are given in Chapter 2, together with some data on the sensors in question. There has been some recent research into the markets of modern micromechanical sensors in household appliances, documenting the market potential for various types of sensors in this area, including those for pressure, acceleration and tilt, thermopiles, flow and gas sensors [3]. Examples of future developments will also be given. 

We expect these micromachined sensors to become more and more important in the household industry, in many domestic applications of silicon pressure sensors, acceleration sensors, tilt sensors, infrared detectors and thermopiles, flow meters, as well as gas sensors and liquid constituent sensors. 

The first clinical IRET used thermopile sensors to achieve non-contact temperature measurement in the ear. In 1991 a tympanic thermometer for home use was first introduced to the consumer market (Thermoscan HM 1). It utilized a pyroelectric sensor which requires the use of a suitable mechanical shutter or chopper mechanism, since it is only sensitive to temperature changes [3]. The main advantage of the pyroelectric sensor unit was its lower cost. However, prices for thermo-... 

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). 

A thermopile sensor generates an output voltage that depends on the temperature difference between its hot and cold contacts. For infrared temperature measurement, the hot contacts are normally thermally insulated and placed on a thin membrane, whereas the cold contacts are thermally connected to the metal housing. Infrared radiation, which is absorbed by the hot contacts of the thermopile, causes a temperature difference between hot and cold contacts. The resulting output voltage is a measure for the temperature difference between radiation source and cold contacts of the thermopile sensor. It is therefore necessary to measure also the temperature of the cold contacts by an additional ambient temperature sensor in order to determine the temperature of the radiation source. 

Using Planck s law, the output voltage of the thermopile sensor can be written as... 

The thermopile output signal is converted to a digital value Zv. To take into account the temperature coefficient of the thermopile, the value U/S is written as ... 

For the calibration of most infrared ear thermometers the sensitivities S0 and R0 and the temperature coefficients Sj and a for both sensors have to be determined. Typically a two-step calibration is performed. In the first step the ambient sensor is calibrated by immersing it into two different temperature controlled baths. In the second step the thermopile sensor is calibrated by measuring the output signal while placing it before two different blackbody radiation sources. 

For the IRT 3000 a simultaneous calibration concept [6], that is shown schematically in Fig. 3.48, has been developed. At two different ambient temperatures two measurements at different blackbody temperatures are performed with completely assembled thermometers connected to external computers. This results in four independent sets of output values of both ambient and thermopile sensors (see Eq. 6 and 7) ... 

C02-IR detection systems for example contain a light source emitting infrared light ideally at wavelengths around 4.2 pm (often a simple light bulb), an absorption path (the so called cuvette), a spectral filter and a detector (thermopile). The filter is normally integrated into the detector housing. 

Thermopiles have been used for non-contacting temperature measurement in hairdryers, to prevent damage to the hair and to speed up the drying process. 

Thermopiles are also used in new ear thermometers or in forehead thermometers to measure the infrared radiation emitted from the skin. This allows quick and reliable temperature measurement and is easy and comfortable to use. 

A new application of thermopile sensors can be found in toasters, ensuring a more reliable control of the browning process, which makes toasting a healthier process. 

This can be realized with modem sensors such as infrared thermopiles, thermopile arrays, microspectrometers and color sensors, several types of humidity sensors, artificial noses and multi-gas sensors. 

Some of these functions could be monitored with improved sensors, instruments and microsystems, like microspectrometers and color sensors, thermopiles, artificial noses, etc. Also some dosing and mixing functions (e. g. of herbs and spices) could be controlled by microfluidic systems. 

Sensor System Thermopile sensor with patented metal shielding ... 

Heat fluxes in fire conditions have commonly been measured by steady state (fast time response) devices namely a Schmidt-Boelter heat flux meter or a Gordon heat flux meter. The former uses a thermopile over a thin film of known conductivity, with a controlled back-face temperature the latter uses a suspended foil with a fixed edge temperature. The temperature difference between the center of the foil and its edge is directly proportional to an imposed uniform heat flux. Because the Gordon meter does not have a uniform temperature over its surface, convective heat flux may not be accurately measured. 

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