National Instruments AMUX-64T User Manual

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National Instruments AMUX-64T User Manual Manual pdf 34 page image

Chapter 3 Signal Connections© National Instruments Corporation 3-5 AMUX-64T User ManualTable 3-2. Thermocouple Voltage Output Extremes (mV) 1Thermocouple Low HighJ -8.095 at -210° C 2 69.553 at 1,200° C 2K -6.458 at -270° C 54.886 at 1,372° CE -9.835 at -270° C 76.373 at 1,000° CT -6.258 at -270° C 20.872 at 400° CS -0.236 at -50° C 18.693 at 1,768° CR -0.226 at -50° C 21.101 at 1,768° CB -0.000 at 0° C 13.820 at 1,820° C1 Source of information is NIST Monograph 175: Temperature-Electromotive Force Reference Functions andTables for the Letter-Designated Thermocouple Types Based on the ITS-90, National Institute of Standards andTechnology, 1993.2 All temperatures are the difference between the measuring end and the cold junction, or AMUX-64T screwterminals in this case.Linearizing the DataThermocouple output voltages are highly nonlinear. The Seebeck coefficient, or voltage changeper degree of temperature change, can vary by a factor of three or more over the operatingtemperature range of some thermocouples. For this reason, the temperature from thermocouplevoltages must either be approximated by often complex polynomials or matched against a look-up table. The polynomial approach is easier to use, but it trades measurement time for memoryusage. The polynomials are in the following form:T = a 0 + a 1 x + a 2 x2 + ... + a n xnwhere x is the thermocouple voltage in volts, T is the temperature difference between themeasuring end and the AMUX-64T screw terminals in degrees Celsius, and a0 through a n arecoefficients that are specific to each thermocouple type. To speed computation time, apolynomial should be computed in nested form. Consider the following fourth order polynomial:T = a 0 + a 1 x + a 2 x2 + a 3 x3 + a 4 x4If this polynomial is evaluated as it is written, then several extra multiplications will beperformed to raise x to the various powers. If the polynomial is instead written as follows:T = a 0 + x(a 1 + x(a 2 + x(a 3 + xa 4 )))and evaluated this way, then no powers are computed, and execution proceeds much faster.Table 3-3 lists the National Institute of Standards and Technology (NIST) polynomialcoefficients for several popular thermocouples.