Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 263

Section 7. Installation where X' = X / 1000 + R3/(R2+R3) Thus, to obtain the value RS/R0, (R0 = RS @ 0°C) for the temperature calculating instruction PRTCalc(), the multiplier and offset used in BRFull() are 0.001 and R3/(R2+R3), respectively. The multiplier (Rf) used in the bridge transform algorithm (X = Rf (X/(X-1)) to obtain RS/R0 is R1/R0 or (5000/100 = 50). The application requires control of the temperature bath at 50°C with as little variation as possible. High resolution is desired so the control algorithm will respond to minute changes in temperature. The highest resolution is obtained when the temperature range results in an output-voltage (VS) range, which fills the measurement range selected in BRFull(). The full-bridge configuration allows the bridge to be balanced (VS = 0 V) at or near the control temperature. Thus, the output voltage can go both positive and negative as the bath temperature changes, allowing the full use of the measurement range. The resistance of the PRT is approximately 119.7 ohms at 50°C. The 120-Ω fixed resistor balances the bridge at approximately 51°C. The output voltage is: VS = VX [RS/(RS+R1) ‐ R3/(R2+R3)] = VX [RS/(RS+5000) ‐ 0.023438] The temperature range to be covered is 50°C ±10°C. At 40°C, RS is approximately 115.8 ohms, or: VS = ‐802.24E‐6 VX. Even with an excitation voltage (VX) equal to 2500 mV, VS can be measured on the ±2.5 mV scale (40°C/115.8 Ω/-2.006 mV, 60°C/115.8 Ω/1.714 mV). There is a change of approximately 2 mV from the output at 40°C to the output at 51°C, or 181 µV/°C. With a resolution of 0.33 µV on the ±2.5-mV range, this means that the temperature resolution is 0.0009°C. The ±5 ppm per °C temperature coefficient of the fixed resistors was chosen because the ±0.01%-accuracy tolerance would hold over the desired temperature range. Figure 81: PT100 in four-wire full-bridge 263