Abstract:
A probe for use within a high voltage and high current electrical device is disclosed. The probe comprises an optical fiber, a substrate having a slot, and a photoluminescent material. The fiber has a first and second end and is configured to convey an activation light from the first to second end. A portion of the fiber is within the slot such that the slot receives the second end of the fiber. Emission of the photoluminescent material, as a function of temperature, is known. The photoluminescent material is disposed within at least a portion of the slot that faces the second end of the fiber so that they are in optical communication with each other. A change in intensity of a luminescent light emitted back into the fiber by the photoluminescent material when the activation light is conveyed by the fiber onto the photoluminescent material provides an indication of the integrity of the electrical device.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to temperature measurement probes, and more particularly to fiber optic measurement probes capable of measuring temperature in harsh environments such as those is found within utility transformers. Some embodiments of the present invention are directed to measuring the winding hot spot temperature of transformers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Sealed electrical devices, such as transformers, are used in several industries including the utility industry. A transformer winding is surrounded by a paper material and sealed in a container filled with oil. During operation, the transformer generates heat that can degrade performance and decrease device lifetime. Because the container is sealed, access to the transformer is limited and it is not easy to remove the transformer for service and inspection due to environmental concerns. Therefore, once the transformer is sealed within the container, it must operate within specified tolerances.  
         [0003]     A variety of transformer control systems are available to monitor sealed transformers and other electrical devices during operation. Such devices range from simple analog gauges to complex transformer monitoring systems that provide monitoring, control and communication functions all in one device. For example simulated Winding Hot Spot (WHS) as well as actual WHS temperatures of transformers provide information regarding safe transformer loading levels. There are three main methods for identifying the winding hot spot of a transformer: (i) simulated WHS temperature (gauge); (ii) calculation (electronic temperature monitoring); and (iii) direct measurement (fiber optic sensors).  
         [0004]     Conventional winding temperature indicators use a capillary thermometer to measure top oil temperature, and have a small heater in them to simulate the temperature rise of the winding hot spot over the top oil temperature (“the gradient”). Current from one of the bushing CTs is passed through the heater, raising the measured temperature. The wattage output of the heater is calibrated using a resistor or other calibrating device. The capillary thermometer provides a typical accuracy of 2-3° C. and is known to deteriorate with time. Errors of 5-10° C. on site are not uncommon. To remain accurate, the system requires regular calibration and servicing. Transformer manufacturers are responsible for calibrating the heater to read correctly at full load. If the calculated gradient is accurate, the tuned system will provide good readings at full load under steady state conditions. One of the most common complaints with traditional simulated winding hot spot gauge systems is the tendency of the gauge to stick. This problem has been noted on both new and old transformers and is a cause for concern, especially when the gauge is used for cooling control where a stuck gauge can cause excessive transformer aging or transformer failure. In addition, WHS analog gauges typically do not provide temperature information in an electronic format that can be transmitted back through their Supervisory Control And Data Acquisition (SCADA) system.  
         [0005]     The use of electronic temperature monitors (ETMs) has become the standard for many utilities, providing the needed temperature information to their SCADA systems. The most basic ETM systems operate exactly the same as a simulated WHS gauge, except that the additional temperature rise of winding hot spot over top oil is added digitally in the built-in computer, instead of thermally using a heater. Hence, they calculate the WHS instead of simulating it. More advanced systems incorporate more information, providing more precise hot spot calculations and providing many other diagnostic and communication functions.  
         [0006]     Measurement devices based upon fiber optic temperature measurement provide the ability to directly measure the winding hot spot temperature. It is not simulated, not calculated, it is the actual temperature. The main reason that many utilities have resisted the use of fiber optics is probe breakage. When fiber optic temperature measurement was first introduced to the transformer industry, the fibers being used were quite fragile and required a relatively large bend radius. The technology has progressed since then. While the probes available today are more rugged, more improvement is needed in the art. Moreover, the probe tips of such known sensors remain fragile and require careful placement inside the transformer to ensure that the tip does not get crushed in the transformer manufacturing process.  
         [0007]     By monitoring the temperature of such transformer hot spots, it is possible to determine whether the transformer is operating at peak efficiency and whether the electrical load on the transformer can or should be adjusted. For example, if a utility company decides to overload a transformer for a short period of time, winding hot spot temperature measurement accuracy is important. Fiber optic temperature probes using photoluminescent materials whose emission predictably varies with temperature have been used successfully to measure temperatures within transformers. Light to and from the photoluminescent material is coupled through the optical fiber to a controller/signal conditioner. The controller/signal conditioner processes the signal from the photoluminescent material and produces a temperature report. While known probes are functional, improvement is needed. Probes for detecting not only probe temperature, but also indicating material or device failure within the electrical device are needed in the art. Moreover, probes and probe tips that are less fragile are needed.  
       SUMMARY OF THE INVENTION  
       [0008]     A probe suitable for measuring temperature and/or indicating material or device failure is disclosed. The probe comprises an optical fiber, photoluminescent material and a probe holder made of materials suitable for use in devices conveying, converting or switching electrical power. The photoluminescent material is placed on or within a component or material that is typically maintained or replaced within the electrical device. The optical fiber is configured to transfer light between the photoluminescent material and its controller/signal conditioner. The photoluminescent material&#39;s optical emission varies predictably with temperature and when processed by the controller/signal conditioner yields a temperature report. As the material supporting the optical fiber or photoluminescent material degrades and changes the relative positions of the optical fiber and photoluminescent material, the intensity of light conveyed through the optical fiber will change. With an understanding of the relationship between maintenance requirements and relative light intensity, the device owner can monitor the condition of materials that eventually need maintenance. The optical fiber of the probe is surrounded over its entire length by several protective layers including a spirally-wound final jacket. The protective layers may be made permeable to oil, vapor and gases to facilitate complete penetration of high-dielectric strength transformer oil throughout.  
         [0009]     Another aspect of the present invention provides a method of sensing the temperature and condition of an electrical device such as a transformer. An optical fiber and photoluminescent material whose optical emission varies predictably with temperature are placed within the electrical device in optical communication with each other. The optical fiber is configured to transfer light between the photoluminescent material and its controller/signal conditioner. The controller/signal conditioner processes the photoluminescent material&#39;s emission to yield a temperature report. As the material supporting the optical fiber or photoluminescent material degrades and changes the relative positions of the optical fiber and photoluminescent material, the intensity of light conveyed through the optical fiber changes. With an understanding of the relationship between maintenance requirements and relative light intensity, the device owner can monitor the condition of materials within the electrical device that eventually need maintenance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein:  
         [0011]      FIG. 1  is an exploded view of a temperature measurement probe, in accordance with an embodiment of the present invention.  
         [0012]      FIG. 2  is plan view of the temperature measurement probe inserted at two locations of the transformer, in accordance with an embodiment of the present invention.  
         [0013]      FIG. 3  is a plan view of the optical fiber from the temperature measurement probe of  FIG. 1 , in accordance with an embodiment of the present invention.  
         [0014]      FIG. 4  is a cross-sectional view of the fiber tip shown in  FIG. 3 , in accordance with an embodiment of the present invention.  
         [0015]      FIG. 5  is a plan view of the fiber tip shown in  FIG. 4  in which the fiber tip is fixedly held with respect to a permeable spiral wrap in accordance with one embodiment of the present invention.  
         [0016]      FIG. 6  is a plan view of the fiber tip shown in  FIG. 4  in which the fiber tip is fixedly held with respect to a permeable spiral wrap in accordance with another embodiment of the present invention.  
         [0017]      FIG. 7  is a curve illustrating, as an example, the characteristics of a phosphorous material in accordance with an embodiment of the present invention.  
     
    
       [0018]     Like reference numerals refer to corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION  
       [0019]     Referring now to the drawings wherein the showings are for purposes of illustrating preferred embodiments of the present invention only, and not for purposes of limiting the same,  FIG. 1  is an exploded view of a fiber optic measurement probe  10  that is installed within a utility transformer. As seen in  FIG. 2 , transformer  12  has multiple windings  14  surrounded by insulating paper  16 . The hottest spot of each winding is known as the hot spot determined by the transformer design. In some embodiments of the present invention, the probes of the present invention are placed in the vicinity of such hot spots in order to detect transformer degradation and/or to monitor hot spot temperature. In some embodiments, transformer  12  is a large transformer (e.g., greater than 100 MVA). In some embodiments, transformer  12  is a mid-size transformer (e.g., greater than 25 MVA). Probe  10  is placed between the paper  16  of adjacent windings  14  as seen in section A, or placed within the paper  16  of a single winding  14  as seen in section B. Probe  10  is placed in a location that is most likely to yield an accurate hot spot temperature reading of transformer  12 .  
         [0020]     The construction of probe  10  illustrated in  FIG. 1  is designed to detect disintegration of paper  16  of transformer  12 . Over time, paper  16  of transformer  12  disintegrates. However, inspection of paper  16  is difficult if not impossible because transformer  12  is sealed. When insulating paper  16  disintegrates, windings  14  of transformer  12  can short thereby causing failure of transformer  12 . Probe  10  is designed to detect this disintegration of paper  16 . Therefore, when probe  10  does not operate, meaning that it no longer detects a luminescent signal, it is probable that the paper  16  corresponding to probe  10  has disintegrated.  
         [0021]     Referring back to  FIG. 1 , probe  10  has outer layers of paper  20   a  and  20   b . In preferred embodiments paper  20   a  and  20   b  is a fine-grade electrically insulating paper such as, for example, rag paper (e.g., Copaco paper, Copaco-125 paper, Kraft paper). An exemplary source of such insulating paper is the Cottrell Paper Company (Rock City Falls, N.Y.). Copaco is made from one hundred percent cotton using new clippings from clothing and denim manufacturers. Paper  20   a ,  20   b  is similar to the paper that wraps windings  14  of transformer  12 . Disposed adjacent to an inner side of each of the outer layers of paper  20   a  and  20   b  is a sheet of material  22   a  and  22   b . In preferred embodiments material  22   a  and  22   b  is a sheet of GORETEX GR. Materials  22   a  and  22   b  sandwich spacer  26  (substrate) which includes a cutout  28  to receive an end of optical fiber  24 . In some embodiments spacer  26  is Nomex® (Dupont) pressboard or paper (e.g., type 992, 993, or 994 Nomex pressboard). Formed within cutout  28  of spacer  26  is a hole  30  about 1 millimeter in diameter and about 1 millimeter deep. Disposed within hole  30  is a photoluminescent material whose emission varies predictably with temperature. Photoluminescent material can be inserted into hole  30  in many ways, such as by coating the photoluminescent material suspended in powder form in a binder of resin or glass directly into the hole. An appropriate glass binder is potassium silicate or Corning sealing glass. An appropriate resin is silicone hard coating material.  
         [0022]     The end of fiber  24  is in optical communication with hole  30 . In some embodiments, the end of fiber  24 , bearing probe tip  32 , is between 1 and 3 millimeters away from hole  30 . In some embodiments fiber  24  is made of silica. In a particular embodiment, fiber  24  is 200μm silica fiber optic cable.  
         [0023]     During operation, light is emitted from the tip of fiber  24  toward the photoluminescent material in hole  30 . The wavelength range of this excitation radiation is appropriate for the particular photoluminescent material being utilized. Typically, the excitation radiation is visible or near visible light. Luminescent emission from the material is received by the tip of fiber  24  and transmitted to control electronics for processing and for determining the temperature of the material and hence transformer  12 . This resultant luminescent radiation, in a visible or near visible radiation band, is usually, but not necessarily, of longer wavelength than the excitation radiation. Spacer  26  is designed to degrade over time like corresponding paper  16 . When spacer  26  degrades, the photoluminescent material in hole  30  will shift or fall out resulting in a change in light intensity transmitted through fiber  24  to the controller. This failure to detect luminescence indicates that paper  16  of transformer  12  is also degrading. It is also possible to quantify such degradation based on the intensity of the light received from the photoluminescent material in hole  30 . As spacer  26  degrades, the intensity of the light therefrom will lessen due to the photoluminescent material falling off of spacer  26 .  
         [0024]     Referring to  FIG. 3 , an assembly for fiber  24 , including probe tip  32 , is shown. Fiber  24  has a probe tip  32  that is supported by spacer  26 . As disclosed in more detail below, a polymer outer jacket  41  circumferentially coats fiber  24  down the length of fiber  24 . In preferred embodiments, polymer outer jacket  41  is permeable to oil, vapor and gases. Typically, polymer outer jacket is rendered permeable to oil, vapor and gases by perforating the jacket  41  as illustrated in  FIG. 5 . A flexible overlap (spiral wrap)  50  circumferentially coats the polymer outer jacket  41  down the length of fiber  24  with the exception of probe tip  32 . In some embodiments, spiral wrap  50  is formed from convoluted or spiral cut fluoropolymer tubing that is wound spirally around polymer outer jacket  41 . Spiral wrap  50  is wound in such a manner that gaps or spaces are formed between polymer outer jacket  41  and spiral wrap  50 . Furthermore, spaces are formed between adjacent sides of polymer outer jacket  41  and spiral wrap  50  to allow oil from transformer  12  to enter the space formed between spiral wrap  50 , polymer outer jacket  41 , and fiber  24 . Because polymer outer jacket  41  is perforated, as described in more detail in conjunction with  FIG. 5 , below, such oil permeates through polymer outer jacket  41  as well. By allowing oil to flow through spiral wrap  50  and polymer outer jacket  41 , low dielectric strength air is displaced by high dielectric strength oil.  
         [0025]     A coupling sleeve  34  is disposed on an end of fiber  24  opposite probe tip  32 . Coupling sleeve  34  fits onto a connector that has an O-ring  36  and protective cap  38 . Coupling sleeve  34  is designed to position and hold this sealing optical connector such that the optical fiber within the connector may convey light to a second optical fiber positioned to optically communicate with the probe. Light conveyed in this way ultimately reaches the appropriate signal processing electronics.  
         [0026]     In some embodiments the appropriate signal processing electronics coupled to the probes of the present invention are configured to detect a change in the intensity of reflected light and/or the intensity of the reflected light. Such information is used by the controller to detect localized degradation in the electronic device (e.g., transformer) under observation and/or the localized temperature within the electronic device. In some embodiments, all inputs and outputs to the controller meet the requirements of the surge test of IEEE C37.90.1-2002 in which a 3000V surge is applied to all inputs and all outputs without permanent damage to the equipment.  
         [0027]      FIG. 4  illustrates another embodiment of probe tip  32 . In this embodiment, photoluminescent material  46  is applied directly to end  44  of optical fiber  24  instead of to the hole  30  of spacer  26  that is illustrated in  FIG. 1 . In the embodiment illustrated in  FIG. 4 , inner jacket  42  circumferentially coats fiber  24  and serves as a protective coating. In some embodiments this inner jacket is made of polyimide. Polymer buffer (not shown) circumferentially coats inner jacket  42 . In some embodiments this polymer buffer is a fluoropolymer such as PFA. A layer of Kevlar  40  circumferentially coats the polymer buffer thereby providing strength. Polymer outer jacket  41  circumferentially coats Kevlar layer  40 . As illustrated in  FIG. 4 , polymer outer jacket  41  extends past optical fiber  24  in order to mechanically protect photoluminescent material  46 . In some embodiments, polymer outer jacket  41  extends past the position of photoluminescent material  46  by 1-2 millimeters. The probe tip design illustrated in  FIG. 4  is particularly advantageous because it keeps the probe tip open thereby allowing for the purging of air during probe tip installation. In preferred embodiments, polymer outer jacket  41  is rendered permeable to oil, vapor and gases by perforations  60 .  
         [0028]     The embodiment of probe tip  32  illustrated in  FIG. 4  is particularly adept at measuring the temperature of an electronic device in which the probe tip is inserted. Excitation light is transmitted through optical fiber  24  and absorbed by photoluminescent material  46 . In response to the excitation light, photoluminescent material  46  emits light characteristic of its temperature. This emitted light is conveyed through fiber  24  to the controller. The light emitted by photoluminescent material  46  has a different wavelength relative to that of the excitation light. Furthermore, the light emitted by photoluminescent material  46  decays overtime in a known manner as a function of the temperature of the photoluminescent material  46 . Thus, by measuring the decay time of the emission light, the temperature in the vicinity of photoluminescent material  46  within an electronic device can be determined. In some embodiments, temperatures in the range of −30° C. to +200° C. can be measured using the apparatus of the present invention. As such, in some embodiments, the probes of the present invention work when completely immersed in hot transformer oil. Furthermore, in some embodiments, the temperature probes of the present invention can withstand exposure to hot kerosene vapor during the transformer insulation drying process. In some embodiments, the accuracy of such measurements is ±2° C. without calibration.  
         [0029]     An advantage of the probe tip  32  illustrated in  FIG. 4  as well as the probe tip illustrated in  FIG. 1  is that no air is entrapped within the probes. For example, referring to  FIG. 4 , polymer outer jacket  41  includes slits and/or perforations  60  to facilitate movement of gas and fluid in and out of the assembly. Thus, for example, when probe  32  is immersed in hot oil while in vacuum, as is the case in the interior of a transformer, oil displaces air within the probe.  
         [0030]     An end  44  of fiber  24  is highly polished and a layer of photoluminescent material  46  is applied on this end. Surrounding photoluminescent material  46  is a non-conducting optically reflective layer  48 . In some embodiments, optically reflective layer  48  comprises titanium dioxide. In order to secure photoluminescent material  46  and non-conducting optically reflective layer  48  to end  44  of fiber  24 , a layer of epoxy  90  is applied over both materials  46  and  48 , as seen in  FIG. 4 .  
         [0031]     It is desirable to fix the position of probe tip  32  relative to the end of spiral wrap  50  so that the spiral wrap will not interfere with the probe tip despite the elastic properties of the spiral wrap. One method for fixing the relative position is to weld spiral wrap  50  onto the polymer outer jacket  41  of probe tip  32 . However, this is undesirable because of the risks of creating pockets of air when the probe tip is immersed in a fluid. Thus, the present invention provides alternative methods for fixing the position of probe tip  32  relative to the end of spiral wrap  50  that advantageously remove the threat of developing pockets of air when the probe tip is immersed in fluids, as in the case when the probe tip is installed in a transformer. Referring to  FIG. 5 , probe tip  32  may be positioned relative to the end of spiral wrap  50  at a set position beyond the end of spiral wrap  50  such that it will remain at this set position despite the elastic properties of spiral wrap  50 . In the embodiment illustrated in  FIG. 5 , probe tip  32  is fixedly held with respect to spiral wrap  50  through the use of a reduced diameter at the end of spiral wrap  50 . The reduced diameter of the spiral wrap where probe  32  emerges from spiral wrap acts as a collet to hold the probe at the desired set position.  FIG. 5  illustrates how the reduced diameter is slit in such a way as to allow the reduced diameter to accept a slightly larger diameter probe  32  by allowing the diameter to expand elastically.  
         [0032]      FIG. 6  illustrates another apparatus and method for fixedly holding probe  32  to a set position relative to spiral wrap  50 . A polymer bushing  11  is placed between the outer surface of the probe&#39;s polymer outer jacket  41  and the inner surface of spiral wrap  50 . In this embodiment, the elastic properties of the spiral wrap  50  diameter and polymer bushing  11  act as collets on probe  32 . In some embodiments, bushing  11  is attached to spiral wrap  50 , e.g. fused. In some embodiments, bushing  11  is not attached to spiral wrap  50 .  
         [0033]     In preferred embodiments of the present invention, probe tip  32  is advantageously open ended, thereby allowing movement of gasses and fluids throughout assembly. In this way, high-dielectric strength transformer oil can permeate the assembly. In some embodiments, sprial wrap  50  is made of a bright color to improve visibility when handling. The construction of spiral wrap  50  allows sufficient bend radius while adding a protective layer of crush resistance to the fiber optic cable. Spiral wrap  50  may stretch a bit with adjustment of tip position. However, the collet action of the spiral wrap is strong enough to overcome elastic forces of the spiral wrap. Thus, the position of probe tip  32  advantageously remains fixed relative to spiral wrap  50 . The end of optical fiber  24  may also be positioned relative to the end of spiral wrap  50  using the same mechanics illustrated in  FIGS. 5 and 6 .  
         [0034]     The present invention has a number of advantageous features. For instance, when used in transformers, the probes of the present invention have the advantage of increased dielectric strength because probe&#39;s polymer outer jacket  41  with slits  60  allows high dielectric transformer oil to flow between the fiber optic cable and the spriral wrap  50 . Such high dielectric strength prevents the probe from creating any air pockets that can reduce dielectric strength and harm the transformer. Another advantage of the probes of the present invention is mechanical strength. Spiral wrap  50  increases protection of optical fiber  24 . Optical fiber  24  is employed in a harsh environment with heavy sheet metals and larger mechanical structures. Spiral wrap  50  prevents such elements from damaging optical fiber  24 . Yet another advantage is the collet of spiral wrap  50  near the distal end of optical fiber  24  because it serves as a strain relief thereby preventing probe  32  from breaking during installation of probe tip  32  into a spacer of transformer  12 . Still another advantage is that the collet of spiral wrap  50  helps to hold the wrap  50  in a set position with optical fiber  24 . Since about  0 . 5  inch of spiral wrap  50  is placed inside spacer  26  ( FIG. 1 ), spiral wrap  50  helps to protect the distal end of probe  32  tip and helps prevent the mistake of installing the wrong end of optical fiber  24  (the proximal end of the probe) into spacer  26 .  
         [0035]     Probe  10  can be used with two optical fibers  24  to detect both degradation and temperature. For instance, the probe  10  shown in  FIG. 1  could be used to detect degradation, while the fiber  24  shown in  FIG. 4  can still be used to measure temperature after the probe  10  from  FIG. 1  fails. It is also possible to use probe  10  to measure arcing of transformer  12 . Any light produced from arcing can be transmitted by optical fiber  24  to the controlling electronics. This light can be analyzed and reported to the operator to show that the transformer is arcing.  
         [0036]     In some embodiments, the excitation radiation that is used to excite the photoluminescent material of the embodiments shown in  FIG. 1  and  4  is pulsed in the manner described in U.S. Pat. No. 4,652,143, which is hereby incorporated herein by reference in its entirety. After the excitation pulse has ended, a specific characteristic of the decaying luminescent intensity such as its decay time is measured in the manner described in U.S. Pat. No. 4,652,143. With this technique, only one wavelength band needs to be measured. In some embodiments, the entire emission band of the photoluminescent material is measured. In some embodiments, a narrower band selected from the total emission is measured. In any event, only one optical path and one spectral band need be involved for the returning signal and only one detector and one signal processing channel is required for each sensor to detect and analyze the transient data. The only requirements of such a set up is that: (1) that the decay time is truly characteristic of the sensor material and is not affected by either the intensity of excitation (within bounds) or the thermal or illumination history of the sensor, and (2) that there are no extraneous time dependent signal changes, as from stray light, which occur during the brief interval of the measurement and which alter the detected temperature signal. In some embodiments, the photoluminescent material disposed in hole  30  ( FIG. 1 ) and/or applied directly to end  44  of optical fiber  24  as layer  46  ( FIG. 4 ) is a phosphor made of a host of either magnesium germanate or magnesium fluorogermanate, activated with tetravalent manganese. In some embodiments, the concentration of the activator, based on starting materials, is within the range of from 0.05 to 5.0 mole percent, approximately one mole percent being preferable. The concentration of the activator controls the decay time and the intensity of luminescence. Magnesium fluorogermanate is sold commercially for use in lamps as a red color corrector in high pressure mercury lamps. A composition of a manganese activated magnesium germanate phosphor for use in the photoluminescent materials of on embodiment of the present invention is Mg 28 Ge 10 O 48  (1 mole % Mn +4 ). A composition of a manganese activated magnesium fluorogermanate phosphor for such use is Mg 28 Ge 7.5 O 38 F 10 (1 mole % Mn +4 ). The decay time of the latter phosphor as a function of its temperature is shown in  FIG. 7 , using an apparatus described in U.S. Pat. No. 4,652,143 hereby disclosed herein by reference, over a wide temperature range throughout which the material is useful as a temperature sensor. It will be noted that the measured decay times vary from about five milliseconds for the lower temperature of this range (about −200° C.) to about one millisecond for the higher temperature (about +400° C.), decay times which are easily measured to high accuracy by electronic techniques. The photoluminescent material disposed in hole  30  ( FIG. 1 ) and/or applied directly to end  44  of optical fiber  24  as layer  46  ( FIG. 4 ) in such embodiments is made up of a powder of such a phosphor. That is, rather than one or a few crystals, there are hundreds, or even thousands, of individual grains or crystallites of the size of a few microns, typically from one to ten microns, held together by an inert, transparent binder. Each grain has a temperature dependent luminescence that contributes to the total observed luminescence although the variation from cystallite to cystallite is small. These phosphor grains are preferably manufactured by a well-known dry process. A mixture of particles of the desired resulting phosphor component compounds is thoroughly mixed and blended. Any aggregates of such particles are also broken up without fracturing the particles themselves. The resulting mixture is then fired in a controlled atmosphere at a certain temperature for a set time. A description of this process is given in Butler,  Fluorescent Lamp Phosphors,  The Pennsylvania State University Press, particularly Sections 1.1, 1.2, and Chap. 4, particularly Section 4.6 on pp. 54-55, which is hereby incorporated herein by reference in its entirety. The growing of phosphor crystals from a liquid starting compound is not suitable for this application since the resulting crystals are not homogenous throughout. Primarily, the activator concentration is not uniform throughout such a crystal, and this results in significantly different luminescent decay times from different parts of the crystal. The luminescent decay time varies significantly as the activator concentration varies, for the same temperature. This is undesirable, so the making of the phosphor to have uniform activator concentration is important for a system that gives repeatable, accurate results in temperature measurement.  
         [0037]     In some embodiments, the excitation radiation that is used to excite the photoluminescent material of the embodiments shown in  FIG. 1  and  4  is used, and the resultant luminescent light measured, in the manner described in U.S. Pat. 4,560,286, which is hereby incorporated herein by reference in its entirety. The composition of the photoluminescent materials having suitable characteristics for hole  30  ( FIG. 1 ) and/or applied directly to end  44  of optical fiber  24  as layer  46  ( FIG. 4 ) in such embodiments may be represented by the generic chemical compound description A x B y C z , where A represents one or more cations, B represents one or more anions, A and B together form an appropriate non-metallic host compound, and C represents one or more activator elements that are compatable with the host material. Here, x and y are small integers and z is typically in the range of a few hundredths or less. There are a large number of known existing phosphor compounds that may be selected by a trial and error process for used in such embodiments. A preferred group of elements from which the activator element C is chosen is any of the rare earth ions having an unfilled f-electron shell, all of which have sharp isolatable fluorescent emission lines of 10 angstroms bandwidth or less. Certain of these rare earth ions having comparatively strong visible or near visible emission are preferred for convenience of detecting, and they are typically in the trivalent form: praseodymium (Pr), samarium (Sm), europium (Eu), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er) and thulium (Tm). Other activators such as neodymium (Nd) and ytterbium (Yb) might also be useful if infra-red sensitive detectors are used. Other non-rare earth activators having a characteristic of sharp line emission which might be potentially useful in the present invention would include uranium (U) and chromium (Cr 3+ ). The activator ion is combined with a compatible host material with a concentration of something less than 10 atom percent relative to the other cations present, and more usually less than 1 atom percent, depending on the particular activator elements and host compounds chosen. A specific class of compositions that might be included in the photoluminescent materials of the present invention is a rare earth phosphor having the composition RE 2 O 2 S:X, where RE is one element selected from the group consisting of lanthanum (La), gadolinium (Gd) and yttrium (Y), and X is one doping element selected from the group of rare earth elements listed above having a concentration in the range of 0.01 to 10.0 atom percent as a substitute for the RE element. A more typical portion of that concentration range will be a few atom percent and in some cases less than 0.1 atom percent. The concentration is selected for the particular emission characteristics desired for a given application. Such a phosphor compound may be suspended in an organic binder, a silicone resin binder or a potassium silicate binder. Certain of these binders may be the vehicle for a paint which can be maintained in a liquid state until thinly spread over a surface whose temperature is to be measured where it will dry and thus hold the phosphor on the surface in heat conductive contact with it. A specific example of such a material that is very good for many applications is europium-doped lanthanum oxysulfide (La 2 O 2 S:Eu) where europium is present in the range of a few atom percent down to 0.01 atom percent as a substitute for lanthanum.  
         [0038]     In some embodiments, the phosphor measurement techniques disclosed in U.S. Pat. Nos. 4,448,547; 4,215,275; and/or 4,075493, each of which is hereby incorporated by reference in its entirety, can be used in accordance with the present invention.  
         [0039]     It will be appreciated by those of ordinary skill in the art that the concepts and techniques described here can be embodied in various specific forms without departing from the essential characteristics thereof. The presently disclosed embodiments are considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalence thereof are intended to be embraced.