Apparatus and method for the calibration of thermal paint

Abstract: A method and apparatus for calibrating the temperature/colour correspondence of thermal paint. A strip of metal (6) is heated at both ends independently to achieve a predetermined temperature differential measured by thermocouples (8, 9). Heat flow from the hot end to the cooler end is constrained to the test piece (6) by enclosing it in insulation material (7). A linear temperature gradient is thus achieved, so providing a more accurate and flexible calibrating arrangement.

Description: This invention relates to the calibration of thermal paints, that is, temperature-indicating paints which progressively change colour with temperature increase and which are commonly used to ascertain the working temperature of components otherwise difficult to monitor. The change in paint colour is not reversed on cooling so that a record of the temperature reached is effectively achieved. A typical application for such paint might be for those components in a gas turbine engine subject to the hot gas flow. Such paints may vary in performance from one manufacturer to another and between batches from the same manufacturer. Where accurate results are important it is necessary to test each batch for temperature to colour relationship and it is this aspect which is the subject of the invention. One method of calibrating paint colour is described in British Standard 1041. Briefly, this is an electrical resistance method where a test piece of the material is painted and an electrical heating current passed through it. The piece is of tapered section so the temperature (measured by thermocouples) is highest at its narrowest point and lowest at its widest point. A temperature gradient is thus created along which the changing colours can be observed and plotted against corresponding temperature change. Methods such as this suffer at least from the disadvantage that different shaped test pieces are required for different temperature gradient ranges. A further prior art method is to take a test piece and bring to a selected temperature in an oven for a prescribed time before removal. This process is then repeated for a number of test pieces with different heating times to cover the temperature range required. Obviously for a range of temperatures this is a time consuming and expensive method. An infrared temperature analyser for temperature calibration is known from Japanese patent abstract number JP5093655, in which a heat conducting material is heated at one end of its longitudinal extent to establish a linear thermal gradient therealong. Temperature sensors are spaced apart along the material to measure the thermal gradient. According to one aspect of the invention, temperature control apparatus comprises: a heat conducting test piece having a heat flow axis extending from a heat source and first and second temperature sensing means disposed in temperature sensing relationship to the test piece, the heat source being arranged to establish a temperature gradient along the heat flow axis of the test piece between the first and second temperature sensing means, wherein the first and second heat sources are disposed at opposing ends of the test piece, the first and second temperature sensing means are disposed at points between a centre-point of the test piece and the first and second heat sources respectively, and control means are provided to control heat input from the heat sources to the ends of the test piece in response to temperatures sensed by the temperature sensing means thereby to establish a different reference temperature at each temperature sensing means and a resulting predetermined temperature gradient between the first and second temperature sensing means. The test piece may be of uniform cross-section and may be enclosed in insulation material between the first and second temperature sensing means. The insulation material may be relieved around the test piece between the temperature sensing means in a central region of the test piece intended to receive a coating of thermal paint. Each end of the test piece preferably extends into a respective chimney section and terminates in the respective chimney section, the heat sources being contained within the respective chimney sections. The test piece may be a rigid strip of metal, each end of the test piece extending into a respective chimney section and terminating in the chimney section, the heat sources being contained within the respective chimney sections beneath the respective test piece ends, the chimney sections being of rectangular section and joined by a rectangular housing which encloses the insulation material, at least one chimney section having an aperture therein to permit insertion of the test piece into its operative position. The heat sources may be gas burners. At least one surface of the test piece in the portion of the test piece between the first and second temperature sensing means is provided with a coating of thermal paint. The test piece may be coated with a thermally responsive paint in a region between the first and second temperature-sensing means, and the control means may be arranged to control the heat sources to maintain temperatures sensed by the temperature sensing means such that temperatures within the paint-coated region include the range of temperatures to which the thermally responsive paint is responsive. In a further aspect, the invention provides a method of temperature calibration in which a longitudinally extending test piece having first and second ends is heated at an end thereof to provide a temperature gradient over a portion of the test piece intermediate the ends thereof, and in which test piece temperature is monitored at each end of the intermediate portion, wherein the test piece is coated with thermal paint over at least a central part of the intermediate portion, the thermal paint being adapted to assume a range of colours progressively and irreversibly with increasing temperature, the method further comprising the steps of heating the test piece at both ends thereof, and controlling the heating of the test piece ends differentially in dependence upon the monitored temperatures to provide a predetermined temperature gradient over the intermediate portion. Apparatus and a method for the calibration of thermal paint will now be described, by way of example, with reference to the accompanying drawing, showing a diagrammatic cross-section. A test piece 6 is a metal strip conveniently 5 millimetres thick, 15 millimetres wide (dimension not shown) and 100 millimetres long. The strip is supported between two chimney sections 1 and 2 which are rectangular in section and joined by a housing 5 also of rectangular section. The inner walls of the chimneys have apertures 11 through which the test piece protrudes into the chimneys 1 and 2. The opposite face of each chimney also has an aperture 12 through which the test piece can be inserted. In each chimney is a heat source 3 or 4 which is a controlled gas burner, the control means being indicated at 10. The burners are mounted underneath the test piece end and heat it by convection and radiation. By controlling the burners differentially, a temperature difference is imposed between the thermocouples 8 and 9 by causing heat to travel from (say) the left hand end of the test piece to the right hand end. If no heat is lost transversely from the test piece and the cross-section is constant, there will be a linear temperature gradient between the thermocouples. Monitoring of the test piece temperature is achieved by two thermocouples 8 and 9 positioned at each end of a test region and just within the housing 5. The housing 5 is lined on all faces with a thick wall of insulation material which is relieved in the central test region so as to permit room for the passage of a gas if required. A completely linear temperature gradient along the heat flow axis requires zero heat loss from the sides and edges of the test piece in the test region so that all heat input at the 'hot end' emerges at the 'cool end'. It also requires a constant cross-section of the test piece along the heat flow axis. A degree of transverse heat loss could be compensated by a corresponding decreasing gradation of cross-section. The ideal arrangement for zero heat loss is therefore one in which the insulation 7 is in contact with the test piece 6 over the whole of the test region. A cavity 13 and a thick wall of insulation 7 is, however, a close approximation to the ideal arrangement. In operation the test piece is painted, on one or both surfaces, over the central region between the thermocouples. The burners are then turned up under the control of their respective thermocouples until the temperatures have stabilised for a predetermined period. The plate is then removed, a linear temperature scale fixed to it in alignment with the reference temperatures at 8 and 9 so providing a range of colours and their corresponding temperatures. The apparatus may be used to calibrate paints for use on gas turbines where the component in question may reach a temperature of 1200°C. The calibration apparatus must therefore be capable of achieving a temperature gradient extending up to this temperature and down to, say, 600°C. Selected smaller ranges may also be imposed on paints capable of finer discrimination, e.g. where a colour change occurs with a temperature change of, say, 20°C. The controlled temperatures required at 8 and 9 are entered into the control means 10 which in turn controls the burners 3 and 4 to achieve these temperatures. It is found that the material of the test piece can in some cases affect the paint colouration. The material, eg, Nimonic 75, may therefore be chosen to simulate the material of the turbine component. Again, the paint colour may be affected by the working gas (exhausted combustion products) in the turbine. This factor may be taken into account by heating the test piece in the presence of exhaust gases pumped slowly through the cavity 13. The attached illustration is only diagrammatic and it will be clear that the thermocouples cannot be attached as shown prior to insertion of the test piece. An end wall of the housing 5 may be removable together with its insulation wall to provide access once the test piece is inserted. The advantages of the embodiment described are as follows: 1. Greater accuracy. Test piece material, paint and thermocouples are at same temperature so there is less cause for error and also the linear temperature profile is perpendicular to heat flow. 2. Just one test piece required for a range of temperature. 3. A relatively quick and cheap method. 4. With minor alterations to the rig it is possible to surround the test piece with a gas environment similar to that which the actual component will experience in operation. If for some reason the paint is affected differently by such gas as opposed to normal air environment, this cause for error can thus be eliminated.

Claims: Temperature control apparatus comprising: a heat conducting test piece (6) having a heat flow axis extending from a heat source (3, 4) and first and second temperature sensing means (8, 9) disposed in temperature sensing relationship to said test piece, the heat source being arranged to establish a temperature gradient along said heat flow axis of the test piece between said first and second temperature sensing means, characterised in that first and second heat sources (3, 4) are disposed at opposing ends of said test piece, said first and second temperature sensing means (8, 9) are disposed at points between a centre-point of said test piece and said first and second heat sources respectively, and control means (10) are provided to control heat input from said heat sources to said ends of said test piece in response to temperatures sensed by said temperature sensing means thereby to establish a different reference temperature at each temperature sensing means and a resulting predetermined temperature gradient between said first and second temperature sensing means. Temperature control apparatus according to claim 1, wherein said test piece (6) is of uniform cross-section. Temperature control apparatus according to claim 1 or claim 2, wherein said test piece (6) is enclosed in insulation material (7) between said first and second temperature sensing means (8, 9). Temperature control apparatus according to claim 3, wherein said insulation material (7) is relieved around said test piece (6) between said temperature sensing means (8, 9) in a central region of said test piece intended to receive a coating of thermal paint. Temperature control apparatus according to any preceding claim, wherein each end of said test piece (6) extends into a respective chimney section (1, 2) and terminates in said respective chimney section, said heat sources (3, 4) being contained within said respective chimney sections. Temperature control apparatus according to Claim 3 or Claim 4, wherein said test piece (6) is a rigid strip of metal and each end of said test piece (6) extends into a respective chimney section (1, 2) and terminates in said chimney section, said heat sources (3, 4) being contained within the respective chimney sections beneath the respective test piece ends, said chimney sections being of rectangular section and joined by a rectangular housing (5) which encloses said insulation material (7), at least one said chimney section having an aperture (12) therein to permit insertion of said test piece into its operative position. Temperature control apparatus according to any preceding claim, wherein said heat sources are gas burners (3, 4). Temperature control apparatus according to any preceding claim, wherein at least one surface of the test piece in the portion of the test piece between said first and second temperature sensing means (8, 9) is provided with a coating of thermal paint. A thermal paint calibration apparatus comprising a temperature control apparatus according to Claim 1, wherein said test piece (6) is coated with a thermally responsive paint in a region between said first and second temperature-sensing means (8, 9), and said control means (10) is arranged to control said heat sources (3, 4) to maintain temperatures sensed by said temperature sensing means such that temperatures within said paint-coated region include the range of temperatures to which said thermally responsive paint is responsive. A method of temperature calibration in which a longitudinally extending test piece having first and second ends is heated at an end thereof to provide a temperature gradient over a portion of said test piece intermediate said ends thereof, and in which test piece temperature is monitored at each end of said intermediate portion, characterised in that said test piece is coated with thermal paint over at least a central part of said intermediate portion, said thermal paint being adapted to assume a range of colours progressively and irreversibly with increasing temperature, said method further comprising the steps of heating said test piece at both ends thereof, and controlling said heating of said test piece ends differentially in dependence upon said monitored temperatures to provide a predetermined temperature gradient over said intermediate portion.

Inventors: MARRIOT DAVID G, SMITH MICHAEL K D, MARRIOT, DAVID G., SMITH, MICHAEL K.D.
Applicants: ALSTOM POWER UK LTD, ALSTOM POWER UK LTD.
Application year: 1995
Main classification: G01K15
IPC codes: G01K15