Campbell Scientific CR3000 CR3000 Micrologger - Page 265
PT100 in Three-Wire Half-Bridge
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Section 7. Installation 7.8.18.2.3 PT100 in Three-Wire Half-Bridge Example shows: • How to measure a PRT in a three-wire half-bridge configuration. Advantages: • Uses half as many input channels as four-wire half-bridge. Disadvantages: • May not be as accurate as four-wire half-bridge. 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. 262). 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. 266). As in section PT100 in Four-Wire Half-Bridge (p. 262), the excitation voltage is calculated to be the maximum possible, yet allow the measurement to be made on the ±50-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 50 mV: 0.50 V > (V * 115.54)/(9900+115.54) X V < 4.33 V X The excitation voltage used is 4.3 V. The multiplier used in BRHalf3W() is determined in the same manner as in PT100 in Four-Wire Half-Bridge (p. 262). 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. 262) 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. 266). Refer to the appendix Signal Conditioners (p. 563) for information concerning available TIM modules. 265