A ratio instrument for resistance thermometers

RESISTANCE thermometers have several advantages over other types of temperature-measuring equipment especially in the − 150-to + 150-degree centigrade range. They are inherently stable and accurate, and the end device may be located several hundred feet from the primary detector without affecting calibration. There are no cold-junction errors requiring compensation, such as are encountered in thermocouple thermometers. Although resistance thermometers have been used for many years in standardizing work in thermometry and for aircraft temperature measurements, they have not been used extensively in industrial applications. The primary reason for this is that instruments which combine high sensitivity and simplicity with sturdy construction have not been available to make practical an industrial resistance thermometer which consists essentially of a resistance temperature detector, a power supply, and a ratio instrument.