Fire Research Station had a requirement in the 1960’s for a heat flux meter to measure the rate of heat transfer to a surface, both conductive and radiative. This was in the range of 0 to 10 W/cm2 to an accuracy of at least 0.1 W/cm2. The specification also required that it should be of robust construction, fit for outdoor use, capable of brief immersion in flame and unharmed by a smoky atmosphere.
The instrument is constructed in two parts comprising the heat flux meter and the cooling jacket which slides close round it. The heat flux meter comprises a 9.5 mm copper tube 90 mm long, across one end is soldered to a constantan disc. A 0.12 mm dia. (40 sWG) copper wire is soldered to the centre of the disc. The wire passes down the central axis of the copper tube and is secured to an insulated terminal at the opposite end of the tube. Another terminal is mounted on the tube side near the first one.
The water jacket is made from 50 mm overall diameter brass in the form of a cylinder 50 mm long, Figure 1. The axial tunnel is 9.5 mm wide to accept a heat flux meter. Entry and exit cooling water tubes project behind the meter’s rear face. The constantan receiving disc is painted with heat resistant black paint and coated with soot from a candle flame.
Heat transfer to the constantan disc results in the centre becoming warmer than the circumference since the latter is in close thermal contact with the water-cooled jacket. Thus a radial temperature gradient is set up with the central copper wire, constantan disc and the copper tube acting as a differential thermocouple, the positive terminal being connected to the wire and the negative terminal to the tube.
Testing confirmed that both radiation and convection heat fluxes could be measured. The instrument is not sensitive to wind, or changes in cooling water flow. It obeys Lambert’s Cosine Law of Absorption. While less sensitive than a field radiometer, readings can be made accurately and recorded using data-logging equipment.
H Wraight, The development of a foil heat flux meter, Fire Research Note No 790, November 1969