Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 260
BRHalf3W, PRTCalc
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Section 7. Installation Example PRT specifications: • Alpha = 0.00385 (PRTType 1) The temperature measurement requirements in this example are the same as in PT100 in Four-Wire Half-Bridge (p. 257). In this case, a three-wire half-bridge and CRBasic instruction BRHalf3W() are used to measure the resistance of the PRT. The diagram of the PRT circuit is shown in figure PT100 in Three-Wire HalfBridge (p. 260). As in section PT100 in Four-Wire Half-Bridge (p. 257), the excitation voltage is calculated to be the maximum possible, yet allow the measurement to be made on the ±25-mV input range. The 10-kΩ resistor has a tolerance of ±1%; thus, the lowest resistance to expect from it is 9.9 kΩ. Solve for VX (the maximum excitation voltage) to keep the voltage drop across the PRT less than 25 mV: 0.025 V > (V * 115.54)/(9900+115.54) X V < 2.16 V X The excitation voltage used is 2.2 V. The multiplier used in BRHalf3W() is determined in the same manner as in PT100 in Four-Wire Half-Bridge (p. 257). In this example, the multiplier (Rf/R0) is assumed to be 100.93. The three-wire half-bridge compensates for lead wire resistance by assuming that the resistance of wire A is the same as the resistance of wire B. The maximum difference expected in wire resistance is 2%, but is more likely to be on the order of 1%. The resistance of RS calculated with BRHalf3W() is actually RS plus the difference in resistance of wires A and B. The average resistance of 22-AWG wire is 16.5 ohms per 1000 feet, which would give each 500-foot lead wire a nominal resistance of 8.3 ohms. Two percent of 8.3 ohms is 0.17 ohms. Assuming that the greater resistance is in wire B, the resistance measured for the PRT (R0 = 100 ohms) in the ice bath would be 100.17 ohms, and the resistance at 40°C would be 115.71. The measured ratio RS/R0 is 1.1551; the actual ratio is 115.54/100 = 1.1554. The temperature computed by PRTCalc() from the measured ratio will be about 0.1°C lower than the actual temperature of the PRT. This source of error does not exist in the example in PT100 in Four-Wire HalfBridge (p. 257) because a four-wire half-bridge is used to measure PRT resistance. A terminal input module can be used to complete the circuit in figure PT100 in Three-Wire Half-Bridge (p. 260). Refer to the appendix Signal Conditioners (p. 561) for information concerning available TIM modules. 260