Peltier sensors

Electric heaters are usually applied for calibration of calorimeters, especially for heat-flow instruments equipped with thermopiles or Peltier sensors. But often a chemical calibration is more appropriate and matching the experimental conditions more closely. For this end Wadso and coworkers recommended the hydrolysis of triacetin in imidazole/acetic acid buffer with stable, long-lasting heat production rates between 7 and 90 pW/mL at 37 °C [142,143]. Some other possible reactions were cited and discussed in connection with the most important types of calonmetric vessels, batch forms as well as flow-through containers. 

In a modern dew-point instrument, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared thermometer (Figure A2.2.2). At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears a beam of infrared light is directed onto the mirror and reflected back to a photodetector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation to reduce vapor equilibrium time and to control the boundary layer conductance of the mirror surface (Campbell and Lewis, 1998). Additionally, an infrared thermometer measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the water activity. The range of a commercially available dew-point meter is 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. The performance of the instrument should be routinely verified as described in the Support Protocol. 

Electrolytic type sensors Uxt thick film techniques, e.g. capacitor coated in gl bonded on to a ceramic disc mounted on a thermoelectric (Peltier effect) cooler. Control is by a platinum resistance thermometer which adjusts the temperature of the cooler to regain equilibrium after a change in capacitance due to moisture deposit. Range depends on technique. Capable of high precision. Limitations are similar to those for AIjO) sensor. Capable of being direct mounted. Relatively cheap. Suitable for on-line use. 

Because dark current limits the integration times obtainable at room temperature, Peltier cooling (to -150C) is used to reduce thermal population of the conduction band. In contrast to the silicon vidicon and the SIT [vide infra], where the presence of intense radiation may bloom out the entire sensor, blooming is greatly reduced with photodiode arrays even when intense lines saturate individual diodes. 

Development of simple, low-cost calorimeters for routine analysis, called thermal enzyme probes (TEP), has been attempted by several groups. These are fabricated by attaching the enzyme directly to a thermistor [4, 5]. However, in this configuration, most of the heat evolved in the enzymic reaction is lost to the surrounding, resulting in lower sensitivity. The concept of TEP was essentially designed for batch operation, in which the enzyme is attached to a thin aluminum foil placed on the surface of the Peltier element that acts as a temperature sensor [6]. 

The semi-conductor transducer (scintillation counter). Each X-ray photon increases the conductivity of the active zone (the junction) of a lithium-doped silicon diode (one electron for around 3.6 eV). The background noise is reduced if the sensor is maintained at low temperature (cooled by liquid nitrogen or a Peltier device). The entry surface is protected by a beryllium film of a few pm (transparent for Z > 11) (Figure 12.8). In one or other cases the impulse furnished by the detector allows to go back to the energy of the incident photon. 

The microreactors, which are operated in a continuous flow fashion, consist of a long, meander-shaped channel, the length of which can be designed to meet the required residence time to yield a fully developed reaction product. Heating or cooling of the reactor takes place by heaters or Peltier elements and can be controlled by temperature sensors (i.e. thermocouples or platinum elements). The chips comprised externally placed heaters and sensors, but could also be applied by sputtering afterwards, similar to the electrodes for EOF. 

The frequency and amplitude of an oscillating quartz crystal are influenced by even a minimal mass load. A sensitive dew sensor was developed by using a Peltier-cooled quartz plate [54]. This sensor is capable of measuring relative humidity with an accuracy better than l.St o of the measured relative humidity in automatic dew-point hygrometry. Since the frequency but not the amplitude of specially cut quartz crystals depends strongly on the temperature, the dew-point temperature can be determined simultaneously with the same sensor element. Compared with optical dew-point hydrometers, this quartz sensor combines dewpoint detection and temperature measurement in a single element. 

Figure 1. Refrigeration system to keep samples at-13 "C. A) external view with the temperature sensor B) samples container and Peltier system.

Biosensors based on the heat produced by enzyme/substrate reactions have traditionally used microcalorimeters (1), thermistors (2), and Peltier or other macro devices <3,A) The area has been reviewed by Guilbault (5). The size, response time, and thermal mass of these detectors suggests that thermally responsive microsensors need to be explored. The ideal sensor would be inexpensive, and require simple, low cost support electronics. A fiber optic based sensor (Part A), and a pyroelectric polymer film based sensor (Part B) are described below. 

Due to their compactness and standard fabrication technology, the temperature in thermal flow sensors is often measured by thermocouples, which rely on the thermoelectric effect. The thermoelectric effect describes the coupling between the electrical and thermal currents, especially the occurrence of an electrical voltage due to a temperature difference between two material contacts, known as the Seebeck effect. In reverse, an electrical current can produce a heat flux or a cooling of a material contact, known as the Peltier effect. A third effect, the Thomson effect, is also connected with thermoelectricity, where an electric current flowing in a temperature gradient can absorb or release heat from or to the ambient [10, 11]. The relation between the first two effects can be described by methods of irreversible thermodynamics and the linear transport theory of Onsager in vector form. 

There yawns a large gap between commercial (micro)calorimeters with maximum vessel volumes of 25, 30 or 100 mL and instruments of many litres for smaller domestic animals. The only exception known to the authors is the Seta-ram GF 108 1-L instrument used in the Leyden group [72]. A low-price solution for an intermediate size calorimeter was found in cooling/warming boxes sold as picnic equipment for less than US 200 [73], They are equipped with a Peltier battery as a heat pump between the inner volume of the box and the environment. In the same way the heat pump can work as a Seebeck heat flow sensor to determine heat production rates inside the box. The inner walls of the box may be additionally covered by copper foil of high thermal conductivity to facilitate heat flow to the sensor. 

Figure 13. Schematic sketch of a carousel calorimeter for small insects with 1 LKB air bath thermostat, 9 flight carousel, 12 proximity sensor, 13 microphone, 14 light guides, 15 Peltier heat flux sensors and 17 insect [112].

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