牛奶中导热系数与脂肪含量的关系检测方案(导热仪)

Application note nr. 2From Hot Disk Applications Low Viscosity Liquids andthe Hot Disk thermalconstants analyser The Hot Disk thermalconstants analyser can be usedfor many applications includingmeasurements on low viscosityliquids. Measurements carriedout on water yields very goodresults (typically 0.61W/mK atRT)， but measurements havealso been carried out onMethanol which is a liquid witheven lower viscosity than water. When measuring on lowviscosity liquids， threeparameters are more importantthan when measuring solids.First， the power given by thesensor spiral must be low toavoid convection in the sample.Second， the measurement mustoften be restricted to last for 1second roughly. Third， thesensor choice is crucial for theaccuracy of the measurements.Sensors with radii 2-3 mm areusually a good choice since themeasurement time can be keptshort. Measurements on moreviscous liquids (different oils，paint samples etc.) can usuallybe measured with slightlylarger sensors and also longermeasurement times. Hot Disk AB has for severalyears successfully carried outmeasurements on low viscosityliquids. Introduction In this Application note four different milk products with fat content (given by themanufacturer) <0.1%，0.5%，1.5 % and 3 % were compared in order to investigate if the HotDisk Thermal Constants Anlyser can be used to determine fat content in diary products. Measurements and Results In order to make sure that the measurements were performed at the same temperature， acylinder made of Aluminium (diameter 80 mm and height 60 mm)， in which 4 holes (diameter20 mm and depth 50 mm) were drilled， was used. These holes were then filled with thedifferent qualities of milk. Fig. 1. (Left) The 3 milk samples with highest fat content and(Right)the sensor used for the measurements (C7577). A Kapton insulated probe (design 7577， Fig. 1) was successively dipped into the differentmilk samples (Fig. 1). The diameter of the sensing spiral in the probe was about 4 mm and theKapton insulation on both sides of the spiral had a thickness of 13 um and the thermalconductivity of this insulation was 0.12WmK. The experiments were performed with a standard Hot Disk Bridge Thermal ConstantsAnalyser and the transient recordings were limited to 2.5 seconds. The analysis was thenperformed with the assumption that the heat capacity per unit volume was 4.17 MJmKPoints no. 17 to 77 out of a total of 200 collected during the transient recording were used.This means that data points recorded beyond 0.96 seconds were not considered and as aconsequence no sign of natural convection could be seen. The total output of power during thetransient was 0.15 W resulting in a temperature increase， over the time window used forevaluating the results， of around 1.7 K. The probing depth was 0.8 mm and the mean deviationaround the fitted line was less than 0.004 K. About the Hot DiskinstrumentThe Hot Disk ThermalConstants Analyser is a systemdesigned to convenientlymeasure the thermal transportproperties of a sample： thermalconductivity， thermaldiffusivity and specific heat.The system is based on apatented Transient PlaneSource (TPS) technique， whichcan be used to study materialswith thermal conductivitiesfrom 0.005 to 500 W/mK andcovering a temperature rangefrom 30 to 1000K. The following modes ofoperation are available with theHot Disk instrument 1) Basic method： The sensoris sandwiched between 2sample. This method alsofeatures a single sidedoption. 2)Thin Film method： Aspecial extremely sensitivesensor is sandwichedbetween2 pieces of thefilm (10-500pm). 3) Slab method： For veryconducting materials (>10W/mK like SiC， Cuetc.). 4) Anisotropic method： Thismethod measures theanisotropic thermalconductivity anddiffusivity of a uni-axialsample. 5)Cp-determination：Determines Cp of solidsamples. For more information， pleasevisit www.hotdisk.se orcontact Hot Disk AB inSweden. Fat concentration Thermal Conductivity (%) (WmK) <0.1 0.5634 +/-0.0003 0.5563 +/- 0.0008 0.5485+/-0.0007 3 0.5369 +/-0.0008 Fig. 2. The thermal conductivity dependence on fat content in milk isperfectly linear. The deviating point for the skimmed milk sample (<0.1%fat) can possibly be due to even lower fat content. Two measurements were made on each milk sample and the probe was moved from one holeto another between the measurements. First the four samples were measured with only a fewminutes between each measurement. Later a similar second series of measurements were madeafter about one hour. The three last concentrations indicate a perfectly linear dependence (Fig. 2.) of the thermalconductivity on the fat concentration. Comparing the change in thermal conductivity (with amean deviation of 0.0007 WmK'as in these experiments) with the change in fatconcentration indicates that it is possible to determine the fat concentration within about 0.09%. Conclusive remarks It has been shown that the thermal conductivity of low viscosity liquids can be measured withthe Hot Disk thermal constants analyser and also that the sensitivity of the instrument issufficient when it comes to determining the fat content of milk. Other examples of applications for low viscosity liquid measurements are for instance water-anti freezer mixtures， where the anti-freezer content can be measured. C Hot Disk AB CHot Disk AB our Local Distributor：