TEMPERATURE MEASUREMENT IN THE PRESENCE OF AMBIENT RADIATION

Abstract: A method is disclosed for measuring the temperature of re­ mote hot samples (4) in the presence of ambient radiation (7). A portion of the surface of the sample (4) is for example treated (6), to obtain a spectral emissivity different from the untreated surface (5). The spectral radiances of the treated (6) and untreated (5) surface portions are measured and the tempera­ ture calculated from these values and the known emissivities.

Description: "TEMPERATURE MEASUREMENT IN THE PRESENCE OF AMBIENT RADIATION" The present invention relates to temperature measurements of remote hot samples. The temperature of hot samples can be inferred from its spectral radiance at one particular wavelength if the spectral emissivity is known or from a value of spectrally integrated radiance if a spectrally averaged effective emissivity is known. However, if the sample is exposed to intense ambient radiation the thermal radiance will be partially obscured by a portion of the ambient radiation reflected off the sample. Such a situation is encountered in the radiation section of fired furnaces. The present invention is concerned with separating the ambient radiation reflected off the sample from the thermal radiation emitted by the sample. It is then possible to obtain a true physical temperature of the sample. According to the invention there is provided a method for measuring the temperature of a remote sample in the presence of ambient radiation by observing its radiant emittance of about a given wavelength, characterised by (a) providing on a portion of said sample a surface area of spectral emissivity different from that of the adjacent surface areas of said sample at about said given wavelength; (b) measuring the apparent spectral radiance from the first-mentioned surface area with a radiometer at about said given wavelength; (c) measuring the apparent spectral radiance from an adjacent surface area of said sample with a radiometer at about said given wavelength; (d) obtaining the spectral emissivities of said first-mentioned and adjacent surface areas of said sample; and (e) computing said temperature of said sample from the said radiances and emissivities of said first-mentioned and adjacent surface areas. The invention will be more fully understood from the following description, given by way of example, which refers to the accompanying drawings in which the sole figure shows one preferred arrangement for practising the invention. For the purpose of measuring the temperature of a remote hot sample in the presence of intense ambient radiation, a portion of the sample surface is treated to change its emissivity for a selected wavelength or wavelength range. By a measurement of the spectral radiances from the treated and untreated (hot) surface areas one can determine the equivalent blackbody radiance for the sample temperature (and thus the temperature) provided that the spectral emissivities for the treated and untreated surface areas are known. A general requirement of this technique is that the surface temperature of the treated and untreated areas are at least approximately equal. Thermal conduction and convection tend to equalise the temperature over the surface, but a certain temperature gradient remains due to an emissivitydependent difference in the radiative heating rate. In order to minimise the temperature gradient it is preferable to select a surface treatment that renders the treated area about equal in total emissivity to that of the untreated surface, but different in spectral emissivity at about a selected wavelength. The surface treatment can be done by painting or coating with refractory materials, etching, sandblasting or any other treatment to modify the spectral emissivity. Alternatively, one could attach a separate body of different emissivity in close thermal contact with the sample. A small spot on the surface of the sample is painted with a refractory material that has a different spectral emissivity at a particular wavelength or wavelength range from the unpainted surface of the sample. As discussed above, one selects the paint such that its total emissivity is approximately equal to that of the unpainted surface, but different at the particular wavelength to provide for equal radiative heating at the painted and unpainted portions of the sample. Such paints include mixtures of TaC or NbB powder with A1203 or SiO2 powder. A paint formulated in this fashion exhibits a relatively low emissivity in the wavelength range of 1-3 m. Above and below this range the emissivity increases towards unity. The pyrometer measurement would be made in the 1 to 3 m range. The effective total emissivity can be adjusted by varying the mixture and the thickness of the painted layer. Using two relations R = ± RB(T) + (1- ± ) RA R' = E 'RB(T) + (1- f' ) RA where R and R' are the apparent spectral radiances at about a selected wavelength for the unpainted and painted portions of the sample; 02and i ' are the respective emissivities at about the selected wavelength; RB(T) is the spectral radiance of a black body at the temperature T; and RA is the ambient spectral radiance to which the sample area of interest is exposed. By combining the above equations one gets for the blackbody radiance at temperature T: EMI5.1 from which the temperature T is obtained via the Planck radiation formula. The spectral emissivities of the treated and untreated portions of the surface of the sample must be known at the particular wavelength or wavelength range. A standard technique for determining spectral emissivity involves heating of a representative sample to a precisely determined temperature and measurement of its spectral radiance. The ratio of that radiance to the blackbody spectral radiance at the above temperature is the spectral emissivity. For purposes of description, one way of putting the present invention into effect shall be illustrated and described with respect to a fired refinery furnace. The figure shows such a typical arrangement. To measure the temperature T of a metal tube 4 placed inside a fired furnace 10 where the tube 4 is exposed to ambient radiation 7 from the flames 9 and the furnace wall 8 one paints a small spot 6 on the tube 4 with a refractory material. The spectral emissivity ±', of the refractory paint is different from the spectral emissivity, ± , of the unpainted tube at about the wavelength of observation. To distinguish between the thermal emission from the tube and the ambient radiation reflected off the tube one aims a radiometer (pyrometer), 1 alternatingly at the painted spot 6 and an adjacent point 5 on the unpainted surface measuring the respective spectral radiances R' and R at about a selected wavelength. Knowing the values ± , f ', R, and R' one can calculate the blackbody radiance RB(T) for the sample temperature T from the above equation; and the temperature itself then from RB(T) using Planck's well known radiation formula.

Claims: C L A I M S: 1. A method for measuring the temperature of a remote sample in the presence of ambient radiation by observing its radiant emittance at about a given wavelength, characterised by: (a) providing on a portion of said sample a surface area of spectral emissivity different from that of the adjacent surface areas of said sample at about said given wavelength; (b) measuring the apparent spectral radiance from the first-mentioned surface area with a radiometer at about said given wavelength; (c) measuring the apparent spectral radiance from an adjacent surface area of said sample with a radiometer at about said given wavelength; (d) obtaining the spectral emissivities of said first-mentioned and adjacent surface areas of said sample; and (e) computing said temperature of said sample from the said radiances and emissivities of said first-mentioned and adjacent surface areas. 2. A method as claimed in claim 1, characterised in that the surface area of different spectral emissivity is provided on a portion of said sample by a treatment which consists of applying a refractory paint. 3. A method as claimed in either one of the preceding claims, characterised in that the surface area of different spectral emissivity is selected to have an effective total emissivity about equal to that of the unpainted material while exhibiting a different spectral emissivity from that of said unpainted material at said wavelength. 4. A method as claimed in claim 2 or 3 as appended to claim 2, characterised in that said refractory paint consists of a mixture of TaC and A1203. 5. A method as claimed in claim 2 or claim 3 or 4 as appended to claim 2, characterised in that said refractory paint consists of a mixture of Tac and ski02. 6. A method as claimed in claim 2 or claim 3 or 4 as appended to claim 2, characterised in that said refractory paint consists of a mixture of NbB and A1203. 7. A method as claimed in claim 2 or claim 3 or 4 as appended to claim 2 characterised in that said refractory paint consists of a mixture of NbB and Six2.

Inventors: STEIN, ALEXANDER
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1982
Main classification: G01J5
IPC codes: G01J5