The standard EN ISO 8990 Thermal insulation - Determination of steady-state thermal conductivity - Calibrated and protected hot box, which has its basis in a European standard and was later published by the International Organization for Standardization (ISO), describes the principles of apparatus design and minimum requirements that must be met for the determination of laboratory steady-state thermal conductivity of building components and similar components for industrial use. However, it does not specify a specific design, as the requirements vary, particularly in terms of size and, to a lesser extent, in terms of operating conditions.
This standard also describes the device, the measurement technique and the required data reporting. Special components, such as windows, require additional procedures not included in this standard. Measurements of the effect of moisture transfer or redistribution on heat flow are also excluded, but the possible effect of moisture transfer on the accuracy and relevance of test results should be taken into account in the design and operation of the equipment. The properties that can be measured are thermal transmittance and thermal resistance.
This standard includes two alternative methods: the calibrated hot box method and the protected hot box method. Both methods are suitable for vertical samples such as walls and horizontal samples such as ceilings and floors. The apparatus can be large enough to inspect full-scale components. These methods are primarily designed for laboratory measurements of large, inhomogeneous samples, but homogeneous samples can of course also be tested and are required for calibration and verification.
When testing homogeneous samples in accordance with this standard, experience has shown that an accuracy of plus or minus 5 percent can generally be obtained. However, the accuracy of each instrument is estimated by reference to homogeneous samples having thermal conductivity beyond the range to be measured using the instrument. Estimation of accuracy for nonhomogeneous samples is more complex and involves analysis of the heat flow mechanism in the particular type of nonhomogeneous sample being tested. Such analysis is beyond the scope of this standard.
This standard does not provide guidelines for measurements involving mass transfer from the sample during testing.
For the purposes of this standard, the following definitions apply:
Both types of devices, the protected hot box and the calibrated hot box, are generally designed to reproduce the conventional boundary conditions of a sample between two atmospheric fluids of uniform temperature. The sample is placed between a hot and a cold chamber of known ambient temperatures. Measurements are made at steady state for air and surface temperatures and for power delivered to the hot side chamber. From these measurements the thermal transfer characteristics of the sample are calculated. The heat exchange at the surfaces of the test sample components is both convective and radiative. The former depends on the air temperature and air velocity, and the latter depends on the temperatures of the sample surfaces and the surfaces seen by the test sample surface and on the total hemispherical emittances. The effects of heat transfer by convection and radiation are combined in the concept of an ambient temperature and a surface heat transfer coefficient.
Thermal transmittance is defined between two ambient temperatures and therefore appropriate temperature measurements are required to enable its determination. This is particularly important for test specimens with low thermal resistance, where the surface coefficients of heat transfer constitute a significant proportion of the total resistance. In the case of test specimens with medium to high thermal resistance, it may be sufficient to record only the air temperatures during the test, if it can be shown that the difference between the air and radiant temperatures on either side of the test specimen is small enough to meet the accuracy requirements.
A special case of the heat source occurs when the hot box has a radiant panel close to the hot side of the sample. In this case, the radiant component dominates the heat transfer to the sample surface. This method with a radiant panel can be used to measure the thermal resistance of the sample, but is not suitable for direct measurements of thermal transmittance at conventional surface coefficients.
In the protected hot box, the measurement box is surrounded by a protection box in which the environment is controlled to minimize lateral heat flow in the sample and heat flow through the measurement box walls. Ideally, when a homogeneous sample is mounted in the instrument and the temperatures are uniform both inside and outside the measurement box, and also when the cold side temperatures and surface heat transfer coefficients are uniform, a temperature balance for the air both inside and outside the measurement box implies an equilibrium at the sample surface and vice versa. The total heat flow through the sample is equal to the heat input to the measurement box.
The calibrated hot box is surrounded by a temperature-controlled area that is not the same temperature as the air temperature inside the measurement box. Heat losses through the box walls are kept low by using a structure with high thermal resistance. The total power input should be corrected for wall losses and side losses. The side heat flow path is shown in the figure showing details of the sample and sample frame with adjacent hot and cold side box walls. Correction for box wall losses and side losses is determined by testing on calibration samples of known thermal resistance. For side loss calibration, the calibration samples should cover the same thickness and thermal resistance range as the samples to be measured and the intended temperature range of use.
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