Campbell Scientific CR3000 CR3000 Micrologger - Page 262

Section 7. Installation Table 52. PRTCalc() Type-Code-6 Sensor Standard ITS-90 SPRT, alpha = 0.003926 (Reference: Minco / Instrunet) Constant Coefficient a 3.9848000E-03 d -2.3480000E-06 e 1.8226630E-05 f -1.1740000E-06 g 1.6319630E+00 h -2.4709290E+00 i 8.8283240E+00 j 2.5091300E+02 7.8.18.2 Measuring PT100s (100-Ohm PRTs) PT100s (100-ohm PRTs) are readily available. The CR3000 can measure PT100s in several configurations, each with its own advantages. The CR3000 is also equipped to measure PRTs with current excitation. 7.8.18.2.1 Self-Heating and Resolution PRT measurements present a dichotomy. Excitation voltage should be maximized to maximize the measurement resolution. Conversely, excitation voltage should be minimized to minimize self-heating of the PRT. If the voltage drop across the PRT is ≤ 50 mV, self-heating should be less than 0.001°C in still air. To maximize measurement resolution, optimize the excitation voltage (Vx) such that the voltage drop spans, but does not exceed, the ±50-mV input range. 7.8.18.2.2 PT100 in Four-Wire Half-Bridge Example Shows: • How to measure a PRT in a four-wire half-bridge configuration • How to compensate for long leads Advantages: • High accuracy with long leads Example PRT Specifications: • Alpha = 0.00385 (PRT Type 1) A four-wire half-bridge, measured with BrHalf4W(), is the best configuration for accuracy in cases where the PRT is separated from bridge resistors by a lead length having more than a few thousandths of an ohm resistance. In this example, 262