Watch_Out_for_Those_The维修操作手册oelectric_Voltages_Cal_Lab_Journal_reprint_182_MB

METROLOGY 101Watch Out for ThoseThermoelectric Voltages!Martin L.KiddFluke CorporationWhat are Thermal EMFs?Paired MaterialsHVPC (Q)Copper-Copper<0.2Thermoelectric voltages(EMFs)are the most commonCopper-Cadmium/Tin Solder0.2source of errors in low-voltage measurements.ThermalCopper-Gold0.3EMFs result from temperature differences within aCopper-Silver0.3measuring circuit at junctions between conductors madeCopper-Brassof dissimilar materials.The magnitude of a thermalCopper-Lead-Tin Solder5EMF generated by a material junction depends on theCopper-Aluminum5thermoelectric coefficient of the two materials.ForCopper-Nickel10example,when connecting test leads to a voltmeter,atCopper-Kovar40the point where the leads connect to the measurementCopper-Copper Oxide>1000terminals is a natural point where a thermoelectric voltageis created.The leads are rarely at the same temperatureTable 1.Various Seebeck coefficients Q.for different materialsas the terminals when they are connected.Over time theythat might be connected to copper.Copper is the common materialused for electrical conductors,so the Seebeck coefficients relativeshould stabilize to a common temperature point.Thisto copper describe the most common conditions that are found intime might be seconds,a few minutes,or many minutes orelectrical test and measurement applications [1].longer.There are many influences over this stabilizationtime.You can see from Table 1,copper to copper has thelowestRegarding the amount of this voltage,it has two maincoefficient.Gold and silver are also low,so you often seeinfluences:the different materials that are in contact,whichgold flashed copper in connections.The advantage heredetermines the Seebeck coefficient;and the temperatureis gold does not oxidize as readily as copper.You candifference between the two materials.Mathematically,thesee oxidized copper has an extremely high coefficienttemperature between materials is shown in the equation(1000uV/degree)-so it is important to have clean copperbelowconnections.A common soldier connection uses tin/leadsolder.Its connection with copper is shown here.At5uV per degree it has about 25 times the effect as bettermaterials,so a lower thermal soldier such as cadmium-tinT2soldier isbetter,having effects similar to the best materials.Material A+Material BHow to Avoid EMF ErrorsVab=Qab (T1-T2)The best measurement practices to minimize thermalQ is the Seebeck Coefficient of material A with respectEMFs include zeroing the measurement device.It isto material B [1].important to zero the digital multimeter(DMM)toCAL LAB:THE INTERNATIONAL JOURNAL OF METROLOGY18APR·MAY·JUN2012