Newthermalconductivityprobedesignbasedontheanalysisoferror sources D. Elustondo, M.P. Elustondo, M.J. Urbicain * Planta Piloto de Ingenier õa Qu õmica UNS±CONICET), Camino La Carrindanga Km 7 ± C.C. 717, 8000 Bah õa Blanca, Argentina Received 8 August 2000; accepted 14 October 2000 Abstract The thermal conductivity determination by means of the thermal probe is a classical non-stationary method, suitable for foodstusbecauseoftherelativelyshortperiodoftimeandthesmalltemperatureriserequiredduringexperimentalmeasurements. Eventhoughitssimplicity,thismethodhastheoreticalandpracticalimpliciterrors,theirmainsourcesbeingevaluatedinthispaper. The theoretical analysis shows that experimental errors can be kept within acceptable values, if the testing time period is within certain limits, out of which the errors grow quite rapidly. The error arising from the calculation of the thermal conductivity by means of an approximate equation is studied and a new way to express it is presented. Also, testing times were calculated for dierentprobedimensions.Finally,onthebasisoftwoformerprobewhichexhibitssomeadvantagesuponeachother,anewprobe design is proposed which recoveries those features. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermal conductivity; Error analysis; Probe design 1. Introduction The linear heat source method is a non-stationary method to measure thermal conductivity in solids and liquids that has been developed and reviewed by dier- ent researchers as Stalhane and Pyk 1931), Van der Held and Van Drunen 1949), Hooper and Lepper 1950), Sweat and Haugh 1974). It is based on the mathematical solution of an ideal system composed by an in®nite linear heat source of zero mass, immersed in an in®nite solid medium. To measure thermal conduc- tivity with this method, the power input and the probe temperature rise during a relatively short period of time are the only data required, so the technique results particularly useful in food applications. Due to the theoretical approximations and practical considerations, there are certain experimental errors that must be considered. This problems have been studied previously Vos, 1955; Nieto de Castro, Taxis, Roder,&Wakeham,1988;Murakami,Sweat,Sastry,& Kolbe, 1993), and speci®c criteria are given for partic- ular problems. They have been found that experimental error is very high at small times because of the trunca- tion of the series derived from the dierential equation solution and of the instrument ®nite radius, but these errorsdecreasewithtimesomeasurementcanbestarted after some minimum testing time. When time increases, otherkindoferrorsduetothenon-in®niteprobelength and the onset of convective heat transfer start to be important, so there also exists a maximum testing time in which measure must be ended. Sinceresearchersusuallyprefertocalibratetheprobe experimentally Van der Held & Van Drunen, 1949; Murakami & Okos, 1986), usual design criteria are ambiguousandtherearesomecommonproceduresthat make errors undervalued. Hence, main features of the probe theory are reviewed here, and a criterion to cal- culate the measurable period of time is proposed. The numerical analysis shows that the smaller the probe radius, the better for testing foodstus, and, considering this, there are two types of probes that can be used for this purpose. The ®rst one consists of a thermocouple and a heating wire placed inside an hy- podermic tubing, while in the second one the thermo- couple and the heating wire are glued outside a sewing needle. We found that these two models show advantages thatcanbecombinedintoanewimproveddesignthatis presented in this work. Journal of Food Engineering 48 2001) 325±333 www.elsevier.com/locate/jfoodeng * Corresponding author. Fax: +54-291-486-1600. E-mail address: urbicain@criba.edu.ar M.J. Urbicain). 0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0260-877400)00174-6