Temperature measurement conduction error

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

With contact temperature measurement, placing the measurement probe in contact with the object of measurement (duct, surface, etc.) produces an additional route for heat conduction to or from the object. This perturbation error changes the initial temperature field in the vicinity of the contact point and creates measurement errors. 

The fouling of the probe when inserted into a duct or pipe acts as an isolating layer and increases the measurement error. To avoid this conduction error, the probe should be a poor heat conductor. In measuring surface temperatures, the probe should not have an insulating effect, as this will change the temperature in the measuring point. 

Hydrated Zeolites. The zeolitic pellets are hydrated by equilibration at atmospheric moisture content. The cell is immersed in liquid air, and a minimum equilibrium temperature of — 120°C was obtained. At that temperature the conductivity and capacity of the samples are measured over the frequency range 200-107 Hz. After eliminating the cooling liquid, the temperature rises slowly (0.5°C/min). Measurements are performed continuously in the same frequency range during the. temperature rise up to room temperature. The results are near-equilibrium values, and the errors are assumed to be the same over the complete temperature range. The same procedure was applied by Mamy for dielectric measurements on montmorillonite 11). 

We close our discussion of thermal conductivity with the observation that experimental determinations of this property at elevated temperatures have large error bars. Indeed, the two measurements of which we are aware differ by roughly 50%. (The experimental line in Fig. 8 is the recommended linear... 

The electrical conductance of a given water solution increases with temperature. Field probes of electrical conductance are therefore temperature compensated. Conductivity values obtained in the field should be plotted against the corresponding TDI concentrations measured in the laboratory. A good linear correlation confirms the high quality of the data. Outstanding values should be suspected as erroneous and should be discussed with the laboratory staff for possible detection of errors or repetition of measurements. Conductivity measurements are of special use in the following cases ... 

To test this theory, the room temperature conductivity of "Nafion" perfluorinated resins was measured as a function of electrolyte uptake by a standard a.c. technique for liquid electrolytes (15). The data obey the percolation prediction very well. Figure 9 is a log-log plot of the measured conductivity against the excell volume fraction of electrolyte (c-c ). The principal experimental uncertainty was in the determination of c as shown by the horizontal error bars. The dashed line is a non-linear least square law to the data points. The best fit value for the threshold c is 10% which is less than the ideal value of 15% for a completely random system. This observation is consistent with a bimodal cluster distribution required by the cluster-network model. In accord with the theoretical prediction, the critical exponent n as determined from the slope of... 

Air leaves a heat exchanger at about 300°C and 1.5 atm, and the temperature is measured using a thermocouple inside a in.-diameter thermowell mounted normal to the air flow, If the gas velocity is 25 ft/s and the pipe wall temperature is 270 C, what error in temperature measurement does radiation cause Ignore conduction along the axis of the thermowell.)... 

A.ll experiments were conducted at atmospheric pressure in a quartz-glass flow tube reactor (2.-5 cm diameter. 20 cm length). The reaction gases were premixed and flowed perpendicular to the catalytic foil in a stagnation point flow configuration (inset fig. 1).. All experiments were conducted at total gas flow rates between 1 slpm and 6 slpm. which did not influence the results within experimental error. The high-purity platinum foils were resistively heated and the foil temperature was determined by a chromel/alumel thermocouple spotwelded to the back of the foil. Temperature measurements were reproducible within 10 K on the same foil and within 30 K in independent runs with different foils. 

Standard temperature measurement in heat transfer experiments is still done using thermocouples. Thermocouple wires have diameters down to 12.7 (im. For shielded thermocouples, the smallest diameters available are in the region of 100 pm. The drawbacks are conduction losses through the thermocouple wire and flow disturbance. These errors are obviously more pronounced in microfluidic flows. 

In the measurement of barrel temperature, a temperature sensor is pressed into a well in the extruder barrel the sensor is generally spring-loaded see Fig. 4.13. Most temperature sensors are constructed with a metallic sheath to obtain sufficient mechanical strength. As a result, significant thermal conduction errors can occur. 

The device used to measure the temperature profiles is represented in Fig. 4-24. The thermocouples were iron-constantan, enclosed first in hypodermic tubing and then in a stainless steel sheath. Teflon tubing was used as an insulation between the tubing and the sheath. This construction insured against heat-conduction errors (Fig. 4-25). 

Figure 14.11 shows the results when sensor errors were introduced to the observer. It can be seen that the NO and NH3 sensor errors did not introduce significant influences to the estimations. However, a small offset of the temperature measurement caused an obvious difference between the real value and the estimated one. The same result (high sensitivity to temperature error) can also be obtained from the analysis of the observer sensitivities as conducted in [36]. 

Conduction error. Ck>nduction error, or immersion error, is caused by temperature gradients between the sensing element and the measuring junction. This error can be minimized by high heat convection between fluid and sensor and low heat conduction between sensor and measuring junction. In the thermocouple this would mean a small diameter, low conductivity, and long immersion length of the wires. 

The density determination may be carried out at the temperature of the laboratory. The liquid should stand for at least one hour and a thermometer placed either in the liquid (if practicable) or in its immediate vicinity. It is usually better to conduct the measurement at a temperature of 20° or 25° throughout this volume a standard temperature of 20° will be adopted. To determine the density of a liquid at 20°, a clean, corked test-tube containing about 5 ml. of toe liquid is immersed for about three-quarters of its length in a water thermostat at 20° for about 2 hours. An empty test-tube and a shallow beaker (e.g., a Baco beaker) are also supported in the thermostat so that only the rims protrude above the surface of the water the pycnometer is supported by its capillary arms on the rim of the test-tube, and the small crucible is placed in the beaker, which is covered with a clock glass. When the liquid has acquired the temperature of the thermostat, the small crucible is removed, charged with the liquid, the pycnometer rapidly filled and adjusted to the mark. With practice, the whole operation can be completed in about half a minute. The error introduced if the temperature of the laboratory differs by as much as 10° from that of the thermostat does not exceed 1 mg. if the temperature of the laboratory is adjusted so that it does not differ by more than 1-2° from 20°, the error is negligible. The weight of the empty pycnometer and also filled with distilled (preferably conductivity) water at 20° should also be determined. The density of the liquid can then be computed. 

Some physical properties, such as heat capacity and thermal conductivity, are difficult to measure accurately at higher temperatures and error as great as 20% are common. For critical appHcations, consult the heat-transfer fluid manufacturer concerning methods that were employed for these measurements. 

The low conductivity of high-purity water makes it difficult to study electrode processes potentiostatically, since too high an electrical resistance in the circuit can affect the proper functioning of a potentiostat, and it can also introduce large iR errors. The increase in conductivity of water with temperature has been measured and /7 -corrected polarisation data have been obtained in hot water that originally had very low conductivity at room temperature. Other results in high-temperature water are all for tests where the conductivity was deliberately increased through the addition of electrolytes. 

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