Temperature measurement radiation error

The temperature of the gas leaving the sulfur burner is a good indication of SO2 concentration, even though the thermocouples employed for temperature measurement (qv) frequently read somewhat lower than the tme temperatures, because of radiation and convection errors. A temperature of 970°C corresponds to about 10.0 vol % SO2, 1050°C to 11.0 vol % SO2, and 1130°C to 12.0 vol % SO2. Other temperatures and concentrations are in similar proportion. 

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. 

The value of k is determined experimentally by gas temperature measurement. The measurement error of a simple pyrometer can be 250 to 300 K, due to re-radiation to water-cooled surroundings, and the values given below are based on measurement by a Land multi-shielded high-velocity suction pyrometer. Typical values for normal excess air at or near full boiler load are ... 

The guarded hot plate is a standard instrument for measuring the relative thermal resistance of textiles as heat flows from a heated plate in contact with the textile and dissipates into still air at a lower ambient temperature via radiation, conduction, and convection. By design, it minimizes errors due to edge heat losses and validates the total quantity of heat flowing through the specimens. Convection and surface radiation can be controlled by use of a hood (2j+). Simpler devices such as the Reeves warmth tester and a chamois-covered copper cylinder also measure thermal... 

From this energy balance we see that the temperature indicated by the thermometer is not the true gas temperature but some radiation-convection equilibrium temperature. Very large errors can result in temperature measurements if this energy balance is not properly taken into account. Radiation shields are frequently employed to alleviate this difficulty. 

Traditional bucket samples generally are inadequate for the calibration of in situ temperature because preferential near-surface absorption of solar radiation occurs in the upper few meters of the water column 12). Hence, uncertainty in the depth of the bucket sample can introduce significant errors in the in situ temperature measurement. If the flow rate to the ship s laboratory is sufficiently high (>15-20 L/min), then discrete temperature measurements can be made with a laboratory-grade thermometer inserted in the flow prior to the debubbler. The combination of discrete temperature measurements with a precise thermal-bath calibration of the in situ temperature sensor prior to use at sea produces high-quality underway temperature data to within 0.03 °C. 

At present, no calibration procedure for black and white standard thermometer is available that includes all stress factors (air temperature, air velocity, and humidity). Today, calibration traceability is guaranteed by a contact thermometric procedure. It would be preferable to measure the temperature at the surface of the coated sensor because this is the temperature of interest. A contactless surface temperature measurement requires knowing the emission ratio of the material and a minimization of the reflected and scattered radiation. For a minor error contact surface temperature measurement, a known method is the multiprobe measurement with extrapolation to the surface temperature. 

Qualitative and Quantitative Analysis of the DRIFT Spectra. DRIFT spectra are usually presented in Kubelka-Munk units. DRIFT spectra with small baseline errors can be obtained when measurements are made at ambient temperature. However, if measurements are performed at higher temperatures, IR radiation emitted from the heated sample can affect the collected spectra, especially if MCT detectors are employed. This is even more pronounced when the refractivity of the sample changes with time. The baseline artifacts are added to the collected spectra. 

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.)... 

The standard instrument for temperature measurement in the coal-fired power plant industry is the High Velocity Thermocouple (HVT) probe (Babcock Wilcox Company 1972, Mullikin and Osborn 1941). This device is a water-cooled tube, up to approximately 20 feet in length that is inserted into the combustion gases. Combustion gases are drawn into the tube and pass over a thermocouple located near the inlet to the tube. One of the objectives of locating the thermocouple in the tube is to reduce errors due to radiation losses from the thermocouple to the cooler zone within the combustion environment, such as the walls of the boiler. 

Stack gas temperature measurements are taken to provide a basis for furnace continuous emission monitoring (CEM) and also to setup safety interlocks for emergency shutdown. Hence, accurate measurement of stack gas temperature is crucial to the operation of the furnace. The measurement location must be chosen in such a way that the bulk of the flue gases are sampled and the radiation from refractory tiles is minimal. A suction pyrometer is preferred to a thermocouple for stack gas measurement. Although not recommended, if an unshielded thermocouple is used, it must be corrected for radiative losses from the bead otherwise, the measurement would result in errors up to the order of 200°F to 400°F. 

If the chamber has built-in radiation sensors to measure radiation intensity then these sensors must also be calibrated against a qualified standard. As with the temperature and humidity probes, the radiation sensors should agree within a specified tolerance with instrument error taken into consideration. 

Flue gas temperature measurement errors can cause difficulties in heat recovery systems. If a thermocouple can see cold recuperator tubes (i.e., if the T-sensor can radiate heat to cold recuperator tubes), it may read 100°F to 250°F (55°C to 139°C) lower than it actually is, so it will not be able to protect the recuperator tubes. The corrosion reaction rate of steel doubles with every 16°F to 18°F of temperature rise, so an error of 100°F in the flue gas temperature can reduce tube life to about one-third of its intended life. 

Transfer calibration - A technique for correcting a temperature measurement or a thermogram for various errors by placing a radiation reference standard adjacent to the target. 

Figure 4. Quantum yield of Norrish type II reaction as a function of temperature for 0.5 mm PS-PVK films irradiated with monochromatic radiation of 313 mu wavelength as measured by GPC. Error bars represent errors ( 20%) in reproducibility of results.

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