7.4 Unexposed-face Temperature Thermocouples

 The temperature of the unexposed surface should be measured by means of disc thermocouples of the type shown in Figure 6. Thermocouple wires, 0.5 mm in diameter, should be soldered to a 0.2 mm thick by 12 mm diameter copper disc. Each thermocouple should be covered with a 30 mm square x 2.0 ± 0.5 mm thick non-combustible insulating pad. The pad material should have a density of 900 ± 100 kg/m³.

Figure 6 Unexposed-surface thermocouple junction and insulating pad

 Connection to the recording instrument should be by wires of similar or appropriate compensating type.

  Steel - Surface finishes should be removed and the surface cleaned with a solvent. Loose rust and scale should be removed by wire brush.

  Irregular surfaces - A smooth surface, not greater than 2,500 mm², to provide adequate adhesive bond should be made for each thermocouple by smoothing the existing surface with a suitable abrasive paper. The material removed should be the minimum to provide adequate bonding surface. Where the surface cannot be smoothed, fillings should be used of minimum quantity to provide a suitable surface. The filling should comprise a ceramic cement and when the filled surface is dry it should be smoothed, if necessary, with abrasive paper.

  Steel - The insulating pad with the thermocouple fitted should be bonded to the cleaned surface of the steel using a 'water-based ceramic cement' produced by integrating the components to form a high-temperature resistant adhesive. The adhesive should be of such a consistency that no mechanical aid is necessary for retention purposes during the drying process, but, where difficulty in bonding is experienced, retention by adhesive tape may be employed provided that the tape is removed sufficiently long in advance of the test to allow complete drying of the adhesive. Care is required in the removal of the tape to ensure that the insulating pad is not damaged. If the thermocouple pad is damaged when the tape is removed then the thermocouple should be replaced.

  Mineral wool - The thermocouples with insulating pads fitted should be arranged in such a way that if a surface wire mesh is present it may aid retention, and in all cases the bond to the fibrous surface should be made using a "contact adhesive". The nature of the adhesive necessitates a drying time before mating surfaces are put together, thus obviating the need for external pressure.

  Mineral fibre spray - Thermocouples should not be fitted until the insulation has reached a stable moisture condition. In all cases the bonding technique for steel should be used and where a surface wire mesh is present the thermocouples should be affixed to the insulation in such a way that the wire mesh aids retention.

  Vermiculite/cement type spray - The technique specified for wet fibrous spray should be employed.

  Boards of fibrous or mineral aggregate composition - The bonding technique for steel should be used.

 In all cases of adhesive binding the adhesive should be applied in a thin film sufficient to give an adequate bond and there should be a sufficient lapse of time between the bonding of the thermocouples and the test for stable moisture conditions to be attained in the case of the ceramic adhesive and evaporation of the solvent in the case of the "contact adhesive".

 For "A" and "B" class divisions the insulation performance of a construction should be given by that part of the construction which is manufactured from non-combustible materials only. However, if a material or panel is only produced with a superimposed finish, or if the Administration considers that the addition of a superimposed finish may be detrimental to the performance of the division, the Administration may allow, or may require, the finish to be incorporated during the test. In these cases the superimposed finish should be removed locally over as small an area as possible to allow fixing of the thermocouples to the non-combustible part, e.g. a deck provided with overlayed non-combustible insulation (a floating floor) should have any combustible top surface finish removed locally to the thermocouples to allow them to be fixed to the insulation material.