Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 259

Section 7. Installation Figure PT100 in Four-Wire Half-Bridge (p. 260) shows the circuit used to measure a 100-Ω PRT. The 10-kΩ resistor allows the use of a high excitation voltage and a low input range. This ensures that noise in the excitation does not have an effect on signal noise. Because the fixed resistor (Rf) and the PRT (RS) have approximately the same resistance, the differential measurement of the voltage drop across the PRT can be made on the same range as the differential measurement of the voltage drop across Rf. The use of the same range eliminates range translation errors that can arise from the 0.01% tolerance of the range translation resistors internal to the CR800. Calculating the Excitation Voltage The voltage drop across the PRT is equal to VX multiplied by the ratio of RS to the total resistance, and is greatest when RS is greatest (RS = 115.54 Ω at 40°C). To find the maximum excitation voltage that can be used on the ±25-mV input range, assume V2 is equal to 25 mV and use Ohm's Law to solve for the resulting current, I. I = 25 mV/RS = 25 mV/115. 54 ohms = 0.216 mA Next solve for VX: VX = I*(R1 + RS + Rf) = 2.21 V If the actual resistances were the nominal values, the CR800 would not over range with VX = 2.2 V. However, to allow for the tolerance in actual resistors, set VX equal to 2.1 V (e.g., if the 10-kΩ resistor is 5% low, i.e., RS/(R1+RS+Rf)=115.54 / 9715.54, and VX must be 2.102 V to keep VS less than 25 mV). Calculating the BrHalf4W() Multiplier The result of BrHalf4W() is equivalent to RS/Rf. X = RS/Rf PRTCalc() computes the temperature (°C) for a DIN 43760 standard PRT from the ratio of the PRT resistance to its resistance at 0°C (RS/R0). Thus, a multiplier of Rf/R0 is used in BrHalf4W() to obtain the desired intermediate, RS/R0 (=RS/Rf * Rf/R0). If RS and R0 were each exactly 100 ohms, the multiplier would be 1. However, neither resistance is likely to be exact. The correct multiplier is found by connecting the PRT to the CR800 and entering BrHalf4W() with a multiplier of 1. The PRT is then placed in an ice bath (0°C), and the result of the bridge measurement is read. The reading is RS/Rf, which is equal to R0/Rf since RS=R0 at 0°C. The correct value of the multiplier, Rf/R0, is the reciprocal of this reading. The initial reading assumed for this example was 0.9890. The correct multiplier is: Rf/R0 = 1/0.9890 = 1.0111. Choosing Rf The fixed 100-Ω resistor must be thermally stable. Its precision is not important because the exact resistance is incorporated, along with that of the PRT, into the calibrated multiplier. The 10 ppm/°C temperature coefficient of the fixed resistor will limit the error due to its change in resistance with temperature to less than 0.15°C over the -10 to 40°C temperature range. Because the measurement is ratiometric (RS/Rf), the properties of the 10-kΩ resistor do not affect the result. 259