ISO 8990:1994 pdf download – Thermal insulation — Determination ofsteady-state thermal transmission properties -Calibrated and guarded hotbox.
For the calibrated hot box, minimum specimen size is1.5 m× 1.5 m.
The perimeter error in the guarded hot box is due tothe heat flow rate, along the surface of thespecimen,due to imbalance between metering andguarded area, or by inhomogeneities. The perimetererrors in the calibrated hot box are due to the flankingheat flow,P4, which includes the distortion of theheat flow rate at the edges of the specimen. Minimum power input
Total power input,n to the metering box is the sumof the power supplied to heaters,fans, transducers,actuators,etc. Some of these cannot be reduced tozero thus defining a minimum heat flow which has topass through the specimen.
This limit can be lowered by cooling the hot chamber,but that will cause further uncertainty connected withthe measurement accuracy of the cooling rate.
The minimum power is also limited by the uncertaintyof total power input to the metering box including
All the above factors set a lower limit for the ratio(Ts-Tse)/R- Maximum power input
Maximum power input is limited by required tem-perature uniformity and surface coefficients. Largeheat flowrates imply large air mass flow across thespecimen surface if a high degree of air temperatureuniformity is to be maintained; this will affect the heattransfer mechanism of the surface.ln the case of theguarded hot box decreasing the specimen resistance,this imposes stricter requirements on the equivalenceof convective and radiative heat transfer in the me-tering and guard box to obtain a given accuracy.
1.6.2Limitations and errors due to specimen thickness and thermal
For a given apparatus design, specimen thickness canbe limited for reasons depending upon specimenproperties and boundary conditions,an upper limit forthe thickness is due to edge losses,5, or flankinglosses,awhich,although decreasing with increas-ing specimen thickness,can become significant incomparison to and degrade moasurement accuracy. inhomogeneity
Most test specimens representative of building andindustrial components will generally be inhomogene-ous. Inhomogeneities in the test specimen will affectthe pattern of the density of heat flowrate in such amanner that it is neither one-dimensional nor uniform.Also variations of the thickness throughout the speci-men can cause significant local modifications of thepattern of the density of heat fiowrate. The effectsof these are nonuniformities in temperatures and localtransfer coefficients making the following more diffi-cult or even impossible:
athe definition of a mean surface temperature;b)the detection of imbalance in the guarded hot box
cthe definition of the metering area:
the error analysis of test results for a given in-
homogeneous specimen.
Specific examples include:
afacings having a high thermal conductivity.These
form easy paths for imbalance heat flow rate,z.and flanking heat losses,Pa- lt can help to cut thefacing along the metering box periphery. Whenlayers are homogeneous,an alternative solution isto run independent tests on each layer with testmethods using a guarded hot plate or a heat flowvmeter;
b)horizontal and vertical structural members like
studs.Their effect is in most cases symmetrical;csections of the specimen made of different ma-
terials.The temperature differences through thematerials are not the same.A heat flow existsclose to the interface of the different materials.When this interface is not far from the meteringbox periphery, this implies a temperature nonuni-formity that affects both imbalance detection andthe ambiguity in the definition of the meteringarea. Also.local heat transfer coefficients are af-fected by these inhomogeneities;
dcavities within the specimen. Natural convection
can create an unknown imbalance heat flow rate.z.The effect of installing barriers shall be evalu-ated.
It is not possible to provide immediate solutions to alltypes of problems. The operator is advised to be fullyaware of te effects of anomalies.