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The precision and accuracy of guarded and calibrated hot box apparatus have seldom been evaluated, either during the design process or at the performance check stage. Even round robins can supply little information on errors if no theory of the apparatus is available. The purpose of this paper is to consider heat transfer in the apparatus, to supply some rough practical tips to aid in understanding the apparatus performance, even though much more has to be done to reach the same level of knowledge achieved for the guarded hot plate apparatus.
Convection heat transfer is considered first. Theoretical relationships are recalled to predict local surface coefficients in natural convection. These change along specimen surfaces, inducing temperature nonuniformities along the border of the metering box. Transitions from laminar to turbulent flow are also considered, which add hardly predictable temperature fluctuations. The problem of the position of the sensors that detect air temperatures in forced convection is then analyzed.
The analysis of coupled radiation and convection heat transfer allows a definition of the maximum expected sensitivity of the apparatus to imbalance conditions.
From the theoretical description of some sources of error, guidelines may be obtained for an apparatus performance check.
measuring techniques, hot box method, error analysis, imbalance detection, convection, radiation, thermal insulation
De Ponte, F
Professor, Università di Padova, Padua,