Temperature sensor calibration

The most common reference points for temperature sensor calibration are the freezing and boiling points of water. The freezing point is a function of the purity of the water-ice system. When both pure ice and pure liquid water are present, the ice is melting and the temperature of a well-stirred mixture is, by definition, 0°C. It is the most accurate calibration point for that reason. If it is possible to use such a quantity in a calibration, then the calibration is true and without question. 

For thermal characterization and temperature sensor calibration a microhotplate was fabricated, which is identical to that on the monoHthic sensor chips, but does not include any electronics. The functional elements of this microhotplate are connected to bonding pads and not wired up to any circuitry, so that the direct access to the hotplate components without electronics interference is ensured. The assessment of characteristic microhotplate properties, such as the thermal resistance of the microhotplate and its thermal time constant, were carried out with these discrete microhotplates. 

An example of a simulated overnight experiment on Mars is shown in Fig. 3.22 for eight temperature intervals using the Compositional Calibration Target (CCT made out of magnetite rock slab) on the rover as the target. In the case of contact-ring temperature sensor failure, the internal temperature sensor would be used (software selectable). 

Routine calibration of an NO sensor is essential in order to ensure accurate experimental results. One of three calibration techniques is generally used, depending on the sensor type, and will be described in the following section. Each of these methods has already been the subject of several reviews [23, 72-74] and will therefore only be summarized here. NO sensors are typically sensitive to temperature. Therefore, calibration is usually best performed at the temperature at which the measurements will be made. 

Fig. 6.14 (a) OFRR vapor sensor responses to DNT vapor samples extracted with various sampling time at room temperature, (b) Calibration curve of DNT mass extracted by on SPME fiber under various extraction times at room temperature... 

The accuracy and precision of melting point determinations depend on several factors. Calibration of the temperature sensor is the first and... 

This latter point begs the question Who can verify the accuracy of a reference point if its value may vary In other words, who is the ultimate source of calibration materials, such as a thermometer In the U.S., it is the National Institute of Standards and Technology (NIST). This is the same organization that we cited as the source of accurate standardization materials in Chapters 3 and 4. Experiments 16 and 17 in this chapter are exercises in the calibration of a temperature sensor and how such a calibrated sensor can be used. 

Comment in your notebook on how well the temperature sensor is calibrated. 

Figure 8.1 Scheme of a Dewar vessel isoperibol reaction-solution calorimeter. A ampule containing the sample B ampule breaking system C calorimeter head D temperature sensor E stirrer F electrical resistance G Dewar vessel H plunger of the ampule breaking system I, J inlets K plug connecting the calibration resistance to the calibration circuit. 

Ideally, the energy equivalents e and f should be measured over the same temperature range of the reaction ran, to avoid errors from their variation with temperature and to achieve maximum compensation for errors in the calibration of the temperature sensor [26,128,129], These errors are, however, frequently negligible in the temperature ranges involved, and the measurement of or f is normally performed outside the Jj -> Tf interval. This procedure saves time because there is no need to readjust the initial temperature of the calorimeter between the calibration and main experiment runs. It is therefore a common practice, even when an exothermic reaction is studied, to measure before the reaction and ef after the reaction and adjust the experimental conditions so that Jr is the midpoint between J] and Tf. In this case, the temperature of the thermostatic... 

A schematic view of the microhotplate functional elements is presented in Fig. 4.4. A resistive temperature sensor is embedded in the heated area of the microhotplate. The resistance is measured in a four-point measurement The calibration procedure of the temperature sensor will be explained in the next section (Sect. 4.1.4). The heating power dissipation is determined using also a four-point configuration. The external wiring of the heater typically adds another 5% to the heater resistance, which has to be eliminated for an accurate measurement of the dissipated power. A heating current, /heat, is applied, and the voltage drop, Vheat. across the heater is measured on chip. 

The microhotplate with the transistor heater was electrothermally characterized similarly to the procedures presented in Sect. 4.1.3. Special care was taken to exclude wiring series resistances by integration of on-chip pads that allow for accurate determination of Fsg and sd- With the two on-chip temperature sensors in the center (Tm) and close to the transistor (Tt) the temperature homogeneity across the heated area was assessed as well. Both sensors were calibrated prior to thermal characterization. The relative temperature difference (Tj - Tm)/Tm was taken as a measure for the temperature homogeneity of the membrane. The measured thermal characteristics of a coated and an uncoated membrane are summarized in Table 4.6. The experimental values have been used for simulations according to Eq. (4.10). 

Two identical polysilicon temperature sensors with a nominal resistance value of 10 kQ are located in the membrane center. One resistor is connected to the temperature controller, the other sensor is totally decoupled from the circuitry. This second temperature sensor can be directly accessed via bond pads in a four-point configuration. It enables an accurate calibration and a verification of the temperature controller... 

Moreover, the indicated temperature does not even correspond to the temperature of a certain section of the capillary. The temperature is never measured inside the capillary, but at best right at the outside. In many cases, there is even a considerable distance between temperature sensor and capillary. Furthermore, even the sensor does not give a true temperature but it has to be calibrated, which may be done in different ways. 

Dew-point measurement is a primary method based on fundamental thermodynamics principles and as such does not require calibration. However, the instrument performance needs to be verified using salt standards and distilled water before sampling (see Support Protocol). To obtain accurate and reproducible water activity results with a dew-point instrument, temperature, sensor cleanliness, and sample preparation must be considered. Equipment should be used and maintained in accordance with the manufacturer s instruction manual and with good laboratory practice. If there are any concerns, the manufacturer of the instrument should be consulted. Guidelines common to dew-point instruments for proper water activity determinations are described in this protocol. The manufacturer s instructions should be referred to for specifics. 

In order to use the sensor on an industrial scale, an appropriate housing is needed in which the required electrodes and temperature sensor are positioned in the scientifically and technically most considered and logical way. Additional requirements imply that the system should be robust and offer good protection against blows and/or other possible causes of defects. The system should be easy to handle, electrodes and other components should be straightforward to replace, the calibration of the electrodes should be accomplishable in a quick and particularly simple way, and the system must... 

Uncertainties in calibrating different temperature sensors at various temperatures. (From NBS Technical Note No. 262.)... 

Commercially available carbon resistors have been used as temperature sensors in the cryogenic temperature area near absolute zero, from about -253°C to -272°C (-424°F downward to below -458°F). One major benefit of the carbon resistor at low temperature is its lower susceptibility to adverse effects caused by a magnetic field and stray radio interference. They do require individual calibration to keep the measurement error under 1%. Carbon resistors may be incorporated into resistor networks to improve linearity. These sensors exhibit a large increase in resistance below -253°C ( 424°F). Reproducibility on the order of 0.2% is obtainable when calibrated individually. Small size, low cost, and general availability make their use attractive in cryogenic work. 

Temperatures can be measured with thermocouple (T/C) or resistance temperature detector (RTD). RTD provides for stability its variation in temperature is both repeatable and predictable. T/Cs tend to have shorter response time, while RTDs have less drift and are easier to calibrate. RTD provides for stability its variation in temperature is both repeatable and predictable. RTD contains a temperature sensor made from a material such as high purity platinum wire resistance of the wire changes rapidly with temperatures. These sensors are about 60 times more sensitive than thermocouples. 

The difference between the actual temperature of a sample and the temperature indicated on TG recording paper is highly dependent on the geometrical relationship of the sample, furnace, and temperature sensor. If the sensor is not directly attached to the sample holder, the temperature indicated may be profoundly influenced by ambient conditions and heating rate in such instances, a temperature calibration is usually required. 

Many other temperature sensors are in use. One which is convenient and reproducible is a bifilar, annealed platinum resistance thermometer. Here the resistance R and absolute temperature T are related by the expression R-Ro(l+aT+ bT2 + cT3), where R0 a, b, c, are constants which must be established by calibration. Quite commonly two-junction thermocouples are used as thermometers these produce an emf E... 

Pownceby M. 1. and O Neill H. St. C. (1994) Thermodynamic data from redox reactions at high temperatures IV. Calibration of the Re-Re02 oxygen buffer from EMF and NiO -I- Ni — Pd redox sensor measurements. Contrib. Mineral. Petrol. 118, 130-137. 

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