Pyrometers accuracy

Several types of secondary pyrometer are available. In addition to those that measure by varying lamp current, some pyrometers maintain the lamp at constant current but interpose a wedge of graduated neutral density, whose position is a measure of temperature. Also, automatic pyrometers are available in which the eye is replaced by a detector and the measuring element is operated by a servo. In general, the accuracy of the automatic pyrometer is somewhat less than that achieved manually by a skilled operator. 

Accuracy of Pyrometers Most of the temperature estimation methods for pyrometers assume that the objec t is either a grey body or has known emissivity values. The emissivity of the nonblack body depends on the internal state or the surface geometry of the objects. Also, the medium through which the therm radiation passes is not always transparent. These inherent uncertainties of the emissivity values make the accurate estimation of the temperature of the target objects difficult. Proper selection of the pyrometer and accurate emissivity values can provide a high level of accuracy. 

Special problems arise in measuring local temperature within spray flames. Liquid and solid particles cause deposits and blockage of orifices in instruments. High-temperature conditions, with particles having high emissivity, result in complex radiative heat transfer which affects the accuracy of temperature measurement. In industrial furnaces and gas turbine combustion chambers, suction pyrometers have been used for... 

Resistance of Indicating Instruments.— When operated at the highest safe working temperatures most base-metal couples develop a maximum electromotive force of less than 50 to 70 millivolts and the LeChatelier couple an electromotive force of about 16 millivolts. In order to measure such small electromotive forces accurately a very sensitive indicator or millivoltmeter is required. On the other hand the instrument must be robust and able to withstand rough handling to which it is necessarily more or less subjected. These conditions of mechanical robustness and of high accuracy as a pyrometer indicator are difficult to satisfy. 

Similar to the change in volume the porosities of a material fired to different temperatures may be plotted against the temperature. The curves thus obtained are equally instructive and valuable, for the comparison of the pyro-technical properties of refractories. The lower curve of Fig. 3 gives the results obtained with a fireclay material. Here again the rate of porosity decrease, the temperature at which the structure has become dense, shown by the approach to zero porosity, and the evidence or overfiring offer data of practical importance. The accuracy of the results obtained depends of course upon the accuracy of the temperature measurements. Too much attention cannot be given to the calibration of the pyrometers. 

A common mefhod used to generate fhe sound waves is using blasfs of air fhrough a nozzle. One source can be used wifh mulfiple receivers. In some cases, Pifof tubes and thermocouples are used in conjunction with the acoustical pyrometry system to improve accuracy. The practical frequency range for fhese pyrometers is between 500 and 2000 Hz. A study funded by fhe Electric Power Research Institute (EPRI) showed acoustic pyrometry is easy to use, accurate, nonintrusive, and real-time, which makes it useful for monitoring and control [93]. It can be used as a diagnostic tool to identify burner and furnace problems. 

Prior to performing additional teshng in a coal-fired power plant, verification of fhe insfrumenf accuracy fhrough additional testing in gas-fired furnaces with path lengths of no more that approximately 10 feet is recommended. Independent temperature measurements along the beam path may be made by both suchon pyrometers and diminishing diameter thermocouple arrays to ensure an accurate measurement of fhe actual furnace temperature. 

The critical volume and temperature are later obtained from the expansion data and the pyrometer measurement, respectively. Although this method is not of high accuracy, it is one of few techniques permitting access to critical parameters and results obtained are in reasonable agreement with theoretically estimated values [2]. 

A useful tool is a pyrometer with a surface contact probe and a melt probe (needle probe). The contact probe can be used to check for heater burnout, barrel temperatures, die temperatures, and temperature distribution and variation. The melt probe can be used to check melt thermocouple accuracy and to measure the actual melt temperature as it exits the die. The melt temperature at the die exit can be higher than the melt probe temperature at the end of the extruder barrel. 

Secondary standards are liquid-in-glass thermometers and base-metal thermocouples. They are calibrated by comparing them with primary-standard platinum-resistance thermometers or standard platinum-rhodium versus platinum thermocouples at temperatures generated in comparators. These secondary standards are used in turn for the calibration of other devices, such as liquid-in-glass thermometers, bimetallic thermometers, filled-system thermometers, and base-metal thermocouples, in which the highest degree of accuracy is not required. Optical pyrometers as secondary standards are compared with primary-standard optical pyrometers, and they are then used for calibration of r ular test pyrometers. 

A high-frequency induction device, which utilizes a power of 30 kW, was employed to determine the oxidation behavior of ZrBj-SiC-ZrC ceramic. In the analysis, specimens that measured 3 x 4 x 36 mm were employed. Each specimen was fixed in porous zirconia, and the temperature of the specimen center was measmed using a multi-wavelength pyrometer with a measurement range of 1000-2500 °C. Isothermal oxidation of the specimens was performed in static air at a constant temperature of 1600 15 °C. The mass of the specimen was determined using an electronic balance the accuracy of the measurement was 0.1 mg. The weight change/unit area (w) of the specimen was calculated as... 

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