Data Acquisition

Analog and digital signals 25Look-up tables: With this technique, a table of temperatures versus all possiblemeasured voltages is stored, and the appropriate temperature is obtained via an indexingoperation. This is very fast, but requires large amounts of memory. Cold-junctioncompensation is also difficult to handle.Polynomial compensation: Using this technique, polynomial approximations are used toobtain temperature from voltage. The number of polynomial terms used depends on thetemperature range, and the type of thermocouple. For example, type J thermocouples canbe approximated to 0.1º over 0 to 760ºC with a fifth-order polynomial, but an F-typethermocouple requires a ninth-order equation for only 0.5º accuracy.For wide temperature ranges, several lower-order polynomials over narrower ranges areoften used. For example, there are thermocouple board drivers that use three eighth-orderpolynomials for voltage-to-temperature conversions. The range of each equation is optimizedfor each type of thermocouple. In addition, a second-order polynomial is used toconvert the cold-junction temperature to a thermocouple voltage for compensation.The use of a second-order polynomial is only possible because the terminal block temperaturevaries from 0º to 70ºC.2.6.4 Thermocouple types and standardsThermocouple standards specify the voltage vs temperature characteristics, color codes,error limits and composition of standard thermocouples. There are five standards for thermocouplesin general use, namely NBS/ANSI (American), BS (British), DIN (German),JIS (Japanese), and NF (French).Eight main types of thermocouples are general used in industry. These are divided intotwo main groups: base metal thermocouples (types J, K, N, E & T) and noble metalthermocouples (types R, S & B). Their composition and operating temperature rangeaccording to the NBS standard is shown in Table 2.1.In addition, there are several high temperature tungsten-based thermocouples (types G,C & D), which allow temperature measurements between 0ºC and 2320ºC. As thesethermocouples do not follow any official standards, manufacturers’ data sheets should beconsulted to ensure correct use. B Pt, 30% Rh Pt, 6% Rh +300 to 1700C W, 5% Re W, 26% Rh 0 to 2320D W, 3% Re W, 25% Re 0 to 2320E Ni, 10% Cr Cu, 45% Ni –200 to 900G W W, 26% Re 0 to 2320J Fe Cu, 45% Ni –200 to 750K Ni, 10% Cr Ni, 2% Mn, 2% Al –200 to 1250N Ni, 14% Cr, 1% Si Ni, 4% Si, 0.1% Mg –200 to 1350R Pt, 13% Rh Pt 0 to 1450S Pt, l0% Rh Pt 0 to 1450T Cu Cu, 45% Ni –200 to 350Table 2.1Thermocouple specifications (NBS Standard)