Modern PRTD temperature sensors and high-resolution delta-sigma ADC enable high-precision temperature measurement over a wide range

Introduction

This article explains how to achieve high-performance precision, wide-range temperature measurement for platinum resistance temperature detectors (PRTDs), using a data-acquisition system (DAS) comprised of a delta-sigma analog-to-digital converter (ADC) and a modern processor. This DAS offers high performance, yet is cost effective. The development DAS presented here resolves design and mathematical challenges quickly and achieves precision temperature measurement in the PRTD's maximum range (-200°C to +850°C). Platinum resistance temperature detectors, or PRTDs, are absolute temperature-sensing devices that can assure repeatable measurements over temperature ranges of -200°C to +850°C. Platinum, moreover, is very stable and not affected by corrosion or oxidation. PRTDs thus provide optimal performance for precision industrial and medical applications that require precise temperature measurements.1 PRTDs are nearly linear devices. Depending on the temperature range and other criteria, you can make a linear approximation by calculating the PRTD resistance change over a temperature range of -20°C to +100°C.1 For a wider temperature range (-200°C to +850°C) and for higher accuracy, however, the temperature-measurement PRTD standard (EN 60751:2008) defines the behavior of platinum resistance versus temperature by a nonlinear mathematical model called the Callendar-Van Dusen equation. Years ago, implementation of such algorithms could present both technical and cost constraints in DAS design. Today's modern processors like the MAXQ2000 and affordable PCs can resolve these challenges quickly and cost effectively, while providing the user with a friendly graphical display. The Callendar-Van Dusen equation can be used in such a modern DAS to reduce errors to negligible levels for the wide -200°C to +850°C dynamic range. Accuracy will be ±0.3°C or better can be achieved.