ISO 8990 pdf download – Thermal insulation – Determination ofsteady-state thermal transmission properties -Calibrated and guarded hotbox.
1.5.3 Calibrated hot box
The calibrated hot box (see figure 2) is surrounded by a temperature-controlled space not necessarily at the same air temperature as that inside the metering box. The heat losses through the box walls, 0, are kept low by using a construction of high thermal resistance. The total power input, 0, shall be corrected for the wall losses, 03. and for the flanking losses, 04. The flanking heat flow path is illustrated in figure 3, which shows details of the specimen and specimen frame with the adjacent hot and cold side box walls. The correction for box wall losses and flanking losses are determined by tests on calibration specimens of known thermal resistance. For flanking loss calibration, the calibration specimens should cover the same thickness dnd tiwimal resistance range as the specimens to be measured and the temperature range of intended use.
1.6 Limitations and sources of errors
The operation of the apparatus, to a certain desired accuracy. is limited by a number of factors related to equipment design, calibration and operation and specimen poperties, e.g. thickness, thermal resistance and homogeneity.
1.6.1 Limitations and errors du. to apparatus
1.6.1.1 LimItations In Imbalanc. r.solutlon In a guarded hot box
In practice, even with homogeneous specimens, local surface coefficients of heat transfer are not uniform, especially close to the borders of the metering box. As a consequence, neither the specimen surface- temperature nor the air temperature are uniform close to the periphery of the metering box both inside and outside. This has two consequences:
a) It can be impossible to reduce to zero at the same time both the lateral heat flow, 02, through the specimen, and the heat flow, 03, through the metering box walls;
The apparatus shall be designed and operated in such a way as to obtain optimum heat flow balance as indicated in a) above, i.e. apparatus geometry and guard air space and air flow speed so that 03 does not exceed 10% of 0,
Inhomogeneities in the specimen will enhance nonuniformities in local surface coefficients and in specimen surface-temperatures. Heat flow imbalance through the metering box wall and in the specimen shall be evaluated, and when necessary corrected for. For this purpose the metering box walls shall be equipped to serve as a heat flowmeter. Additionally. a thermopile across the metering area periphery can be mounted on the specimen surfaces. In routine testing, imbalance detection can be simplified by calibration and calculation.
1.6.1.2 Size of metered area
The metering area is defined:
a) for a guarded hot box, as the centre-nose to centre-nose when the specimen is thicker or equal to the nose width, or if the specimen is thinner than the nose width, as the inner periphery of the nose;
b) for a calibrated hot box, as the inner periphery of the metering box.
The size of the metered area determines the rnaximum thickness of the specimen. The ratios of the metering area side to the specimen thickness and of the guard width to the specimen thickness are governed by principles similar to those for the guarded hot box.
The size of the specimen can also limit possibilities for a representative section of the construction to be tested and thus allow errors and difficulties in interpretation of the result.
Measurement errors in testing to the hot box methods are in part proportional to the length of the perimeter of the metering area. The relative influence of this diminishes as metering area is increased. In the guardeti hot box, the minimum sue of the metered area is 3 times specimen thickness or 1 m x 1 m, whichever is the greater.
ISO 8990 pdf download – Thermal insulation – Determination ofsteady-state thermal transmission properties -Calibrated and guarded hotbox
