PYROMETERPYROMETER High temperature measuring

instrument………….

Introduction

A Pyrometer, or radiation thermometer, is a non-contact instrument that detects an object's surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object.

Pyrometer is any nonconducting device that intercepts and measures thermal radiation. This measure is often used to determine temperature, often of the object’s surface.

Typical Broadband Pyrometer

The results of temperature measurements obtained by means of thermovision camera have been compared with the results from two-wavelength pyrometer. A scheme of the pyrometer is shown in Fig. 1. It is a device adapted for very fast measurements. During the experiment the frequency of sampling was two orders of magnitude higher than frequency of a thermovision camera and amounted about 5 kHz. The number of the obtained data allows their additional processing, e.g., averaging with no loss of information on fast changes [2]. Optical working bands were chosen in respect with the predicted range of the measured temperatures, i.e., 50 °C ¸ 150°C. Separation of determined radiation bands is ensured due to application of the following interference filters, Filter I - 5.46 m m and Filter II - 4.5 mm. Moreover, the detectors with spectral characteristics ensuring maximal signal were used. The Detector I is of PDI type [3] and it is optimised for a wavelength of 5.5 mm. The Detector II is a PbSe photoreceiver. For the required signal-to-noise ratio, the interference filters of relatively wide band were applied, e.g., for Filter II of l = 5460 nm this band has the Half Width of 500 nm . A measuring area of the pyrometer was 1´1 mm2.

Two-wavelength fast pyrometer

Common pyrometers include……

Optical Pyrometer (a.k.a. Brightness Pyrometer or Disappearing Filament Pyrometer)-Designed for thermal radiation in the visible spectrum.-Utilizes a visual comparison between a calibrated light source and the targeted surface. When the filament and the target have the same temperature, their thermal radiation intensity will match causing the filament to disappear as it blends into the targeted surface in the background.-When the filament disappears, the current passing through the filament can be converted into a temperature reading.  

Infrared Pyrometer-Designed for thermal radiation in the infrared region (0.75 ~ 1000 µm; 30 µin ~ 0.04 in) usually 2 ~ 14 µm (80 ~ 550 µin)-Constructed from pyroelectric materials, e.g., triglisine sulfate (TGS), lithium tantalate (LiTaO3), or polyvinylidene fluoride (PVDF).-Similar to the charge generated by stressed piezoelectric materials, a pyroelectric charge dissipates in time. Hence, a rotating shutter is required to interrupt the incoming radiation to obtain a stable output.

Optical pyrometer

Optical pyrometers are quite effective and accurate for applications that allow manual operation. Automated processes that require fast data acquisition rates are better suited to equipment utilizing infrared technology. But, for the application that requires only periodic monitoring, the optical pyrometer is an efficient and effective solution.

Disappearing-Filament Pyrometer

This type of instrument uses telescope sighting optics for small target sighting (figure 3). Temperatures are determined by adjusting a precision rheostat that changes the internal calibrated lamp’s intensity until a color blend is made between the apex of the pyrometer lamp and the target. The current to the lamp then is output via analog or digital signal to a temperature display. One of the benefits associated with this type of instrument is that the target need not fill the entire field of view. This is especially effective when measuring targets such as 0.0005" dia. wire. Temperature ranges can be measured between approximately 1,300 to 5,800°F (700 to 3,200°C), and with appropriate filters, the disappearing-filament pyrometer temperature ranges can be ex-tended to approximately 18,000°F (10,000°C). Electronic units allow users to insert an emissivity value to display an emissivity-corrected temperature.

Disappearing-filament pyrometer

Wedge-Type Optical Pyrometer

A wedge-type optical pyrometer’s optical lenses and prisms provide a clear, enlarged view of the target (figure 2). The instrument incorporates a rotating optical photoscreenic wedge that functions as a neutral density filter. Brightness from a hot target, sighted through the optical system, is varied by rotating the internal wedge until the target’s intensity matches that of the internal calibrated lamp. The temperature scale around the outside of the optical pyrometer is directly coupled to the instrument’s optical wedge. Typical temperature ranges for optical pyrometers are approximately 1,300 to 5,800°F (700 to 3,200°C).

Optical wedge-type pyrometer

Principle of operation

A pyrometer has an optical system and detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (Temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object.

This output is used to infer the objects temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouple and Resistance temperature detector (RTDs).

Pros and Cons

  •Pros: -Non-contact measurement  -Fast response time  -Good stability  

•Cons: -Expensive  -Accuracy maybe affected by suspended dust, smoke,

Pyrometers are essentially photodetectors which are capable of absorbing energy, or measuring the EM wave intensity, at a particular wavelength or within a certain range of wavelengths.

Applications

Pyrometer are suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched.Smelter IndustryTemperature is a fundamental parameter in metallurgical furnace operations. Reliable and continuous measurement of the melt temperature is essential for effective control of the operation. Smelting rates can be maximized, slag can be produced at the optimum temperature, fuel consumption is minimized and refractory life may also be lengthened. Thermocouples used to be the traditional device, but they are unsuitable for continuous measurement because they rapidly dissolve.Over-the-bath PyrometerContinuous pyrometric measurement from above the bath surface is still employed, but is known to give poor results because of emissivity variations, interference by gases and particulate matter in the intervening atmosphere, and dust accumulation on the optics.Tuyère PyrometerThe Tuyère Pyrometer is an optical instrument for temperature measurement through the tuyeres which are normally used for feeding air or reactants into the bath of the furnace.

Sample temperature measured by two-wavelength pyrometer ( ° ) and thermovision

camera.(·)

Brightness (single color) pyrometerThese add up all the thermal radiation intensity in some range of wavelengths (the sensitive area of the instrument's detector, or bandwidth of the detector) and relate it to temperature through linearizing electronics or a look-up table. The instrument's sensitivity may vary across the bandwidth, making it difficult to use published values of emissivity, especially those of metals, which change with wavelength. For these instruments, the emissivity value is either entered by the operator or assumed to be unity. Many manufacturers supply emissivity information with their instruments. These abbreviated lists are hopelessly simplified. The reality is that emissivity is affected by many things, and even the most commonly-used refractory materials vary wildly. The most common brightness pyrometer is the vanishing filament pyrometer, which came to prominence in the early 1900s. Biggest problems: emissivity is often unknown or changing; anything that affects the radiated intensity (sight glass, dirt, smoke, steam, process or combustion gases) affects the temperature. Ratio (two color) pyrometerThese instruments, which date from about the middle of the last century, use two detectors to add up all the intensity in two wavebands and then relate the quotient of the two intensities to temperature, again by look-up table or linearizing electronics. In ideal cases (black or greybodies) the emissivity cancels out. In non-ideal cases, the operator is expected to enter the relative emissivity. Biggest problems: emissivity often changes with wavelength (some sight glasses mimic this problem as their transmission changes with wavelength); ratio instruments lack the precision of brightness pyrometers and are susceptible to noise (differences in intensity at the two wavelengths are relatively small). Relative emissivity is even less well-known than the absolute value of emissivity; errors can be extremely large if one color is affected more than the other by an environmental or material variation. Wavelength in pyrometryManufacturers of traditional pyrometers typically quote an operating wavelength (or wavelengths in the case of a two-color instrument) when they actually mean a finite range of wavelengths, or a bandwidth. A typical one or two-color pyrometer bandwidth ranges from 50 to 500 nanometers. The bandwidth for the SpectroPyrometer is 2 nanometers or less. This very narrow bandwidth for each wavelength of the SpectroPyrometer is an advantage: when the emissivity is changing with wavelength it is not averaged over a wide spectral area, so it's possible for the expert system to unravel the emissivity's behavior.

All infrared thermometers function utilizing the same basic design principles. Using various infrared filters, an optical lens system focuses energy to an infrared detector, which coverts the energy to an electrical signal. This electrical signal is compensated for emissivity, typically manually. Through linearization and amplification in the instrument’s processor, an analog signal (typically, 1 to 5 VDC or 4 to 20 mA) is output. Electronics can be incorporated to convert the analog output to digital signals that can be transmitted at high speeds, allowing for extremely fast data acquisition rates. Ambient temperature compensation electronics ensure that temperature variations inside the infrared thermometer do not impact its output. Some applications with extremely hot operating environments can require water-jacket cooling at the infrared sensor.

Infrared thermometers usually operate in either very broad or very narrow bandwidths (figure 5). Typically, broad-bandwidth infrared thermometers have a much wider temperature range than narrow-bandwidth units. The drawback to broad-bandwidth infrared thermometers is that emissivity influences them to a greater degree. Keep in mind that

The Technique of measuring high temperature is known as pyrometry and the instrument employed is called pyrometer. Pyrometer is specialized type of thermometer used to measure high temperatures in the production and heat treatment of metal and alloys. Ordinary temperatures can be measured by ordinary thermometer, instead pyrometer is employed for measuring higher temperature.Any metallic surface when heated emits radiation of different wavelengths which are not visible at low temperatures but at about 5400C radiations are in shorter wavelength and are visible to eye and from colour judgement is made as to probable temperature, the colour scale is roughly as follows.Dark red - 5400CRed - 7000CBright red - 3500COrange - 9000CYellow - 10100CWhite - 12050C and above

When a substance receives heat, change in pressure, electric resistance, radiation, thermoelectric e.m.f and or colour may takeplace. Any of these change can be used for measurement of temperature. Inorder to exercise provision control over the heat treatment and melting operation in the industry temperaturemeasuring device known as pyrometers are used. Also accurate measurement of temperature of Furnaces, molten metals and other heated materials

Thin Filament Pyrometry (TFP) is an optical method used to measure temperatures. It involves the placement of a thin filament in a hot gas stream. Radiative emissions from the filament can be correlated with filament temperature. Filaments are typically Silicon carbide (SiC) fibers with a diameter of 15 micrometres. Temperatures of about 800 - 2500 K can be measured.

TechniqueThe typical TFP apparatus consists of a flame or other hot gas stream, a filament, and a camera. AdvantagesTFP has several advantages, including the ability to simultaneously measure temperatures along a line and minimal intrusiveness. Most other forms of pyrometry are not capable of providing gas-phase temperatures.DrawbacksCalibration is required. Calibration typically is performed with a thermocouple. Both thermocouples and filaments require corrections in estimating gas temperatures from probe temperatures. Also, filaments are fragile and typically break after about an hour in a flame.ApplicationsThe primary application is to combustion and fire research.

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