Significance and Use
3.1 Fig. 1 is a sectional view of an example circular foil heat-flux transducer. It consists of a circular Constantan foil attached by a metallic bonding process to a heat sink of oxygen-free high conductivity copper (OFHC), with copper leads attached at the center of the circular foil and at any point on the heat-sink body. The transducer impedance is usually less than 1 V. To minimize current flow, the data acquisition system (DAS) should be a potentiometric system or have an input impedance of at least 100 000 Ω.
3.2 As noted in 2.3, an approximately linear output (versus heat flux) is produced when the body and center wire of the transducer are constructed of copper and the circular foil is constantan. Other metal combinations may be employed for use at higher temperatures, but most (4) are nonlinear.
3.3 Because the thermocouple junction at the edge of the foil is the reference for the center thermocouple, no cold junction compensation is required with this instrument. The wire leads used to convey the signal from the transducer to the readout device are normally made of stranded, tinned copper, insulated with TFE-fluorocarbon and shielded with a braid over-wrap that is also TFE-fluorocarbon-covered.
3.4 Transducers with a heat-sink thermocouple can be used to indicate the foil center temperature. Once the edge temperature is known, the temperature difference from the foil edge to its center may be directly read from the copper-constantan (Type T) thermocouple table. This temperature difference then is added to the body temperature, indicating the foil center temperature.
3.5 Water-Cooled Transducer:
3.5.1 A water-cooled transducer should be used in any application where the copper heat-sink would rise above 235°C (450°F) without cooling. Examples of cooled transducers are shown in Fig. 2. The coolant flow must be sufficient to prevent local boiling of the coolant inside the transducer body, with its characteristic pulsations (“chugging”) of the exit flow indicating that boiling is occurring. Water-cooled transducers can use brass water tubes and sides for better machinability and mechanical strength.
3.5.2 The water pressure required for a given transducer design and heat-flux level depends on the flow resistance and the shape of the internal passages. Rarely will a transducer require more than a few litres of water per minute. Most require only a fraction of litres per minute.
3.5.3 Heat fluxes in excess of 3400 W/cm2 (3000 Btu/ft2/s) may require transducers with thin internal shells for efficient transfer of heat from the foil/heat sink into a high-velocity water channel. Velocities of 15 to 30 m/s (49 to 98 ft/s) are produced by water at 3.4 to 6.9 MPa (500 to 1000 psi). For such thin shells, zirconium-copper may be used for its combination of strength and high thermal conductivity.
Note 1: Changing the heat sink from pure copper to zirconium copper may change the sensitivity and the linearity of the response.
3.6 Foil Coating:
3.6.1 High-absorptance coatings are used when radiant energy is to be measured. Ideally, the high-absorptance coating should provide a nearly diffuse absorbing surface, where absorption is independent of the angle of incidence of radiation on the coating. Such a coating is said to be Lambertian and the sensor output is proportional to the cosine of the angle of incidence with respect to normal. An ideal coating also would have no dependency of absorption with wavelength, approximating a gray-body. Only a few coatings approach these ideal characteristics.
3.6.2 Most high absorptivity coatings have different absorptivities when exposed to hemispherically-incident or narrower-angle, incident radiation. For five coatings, measurements by Alpert, et al, showed the near-normal absorptivity was 3 to 5 % higher than the hemispherical absorptivity (5). This work also showed that commercial heat flux gauge coatings generally maintain Lambertian (Cosine Law) behavior out to incidence angles 60° to 70º off-normal.
3.6.3 Acetylene soot (total absorptance αT = 0.99) and camphor soot (αT = 0.98) have the disadvantages (4) of low oxidation resistance and poor adhesion to the transducer surface. Colloidal graphite coatings dried from acetone or alcohol solutions (αT = 0.83) are commonly used because they adhere well to the transducer surface over a wide temperature range. Spray black lacquer paints (αT = 0.94 to 0.98), some of which may require baking, also are used. They are intermediate in oxidation resistance and adhesion between the colloidal graphites and soots. Colloidal graphite is commonly used as a primer for other, higher-absorptance coatings.
3.6.4 Low-absorptance metallic coatings, such as highly polished gold or nickel, may be used to reduce a transducer's response to radiant heat. Because these coatings effectively increase the foil thickness, they reduce the transducer sensitivity. Gold coating also makes the transducer response nonlinear because the thermal conductivity of this metal changes more rapidly with temperature than that of constantan or nickel; the coating must be thin to avoid changing the Seebeck Coefficient.
3.6.5 Exothermic reactions occurring at the foil surface will cause additional heating of the transducer. This effect may be highly dependent on the catalytic properties of the foil surface. Catalysis can be controlled by surface coatings (3).
Scope
1.1 This test method describes the measurement of radiative heat flux using a transducer whose sensing element (1, 2)2 is a thin circular metal foil. These sensors are often called Gardon Gauges.
1.2 The values stated in SI units are to be regarded as the standard. The values stated in parentheses are provided for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
