Discrepancies between readings of TPR-2075 thermoelectric thermometers

Abstract

Thermoelectric devices with disposable sensing components (measuring heads~ are increasingly used in temperature monitoring during steel melting [1-3]. The probe is a steel tube of length several meters enclosing a two-wire conductor. One end of the tube has a handle together with a connector leading to the secondary instrument, while the other end of the tube is fitted with the disposable part of the device. This is constituted by a thermocouple having wires of diameter 0.08-0.12 mm, in which the hot junction is placed in a quartz tube and the cold junction is placed in a ceramic or plastic sleeve pressed or cemented into a paper outer sleeve. This sleeve protects the steel tube from failure and prevents the cold junction from becoming heated during the measurement, which usually takes only 7-15 sec. The design of the device, particularly the disposable section, is constantly being improved, especially with the object of improving the metrological characteristics, reliability, and cost. The Termopribor production organization in L'vov has designed the TPR-2075 platinumrhodium thermoelectric thermometer fitted with PTPR disposable tips, which are manufactured by the Teplopribor instrument-manufacturing plant in Chelyabinsk. These are designed for rapid (5 sec) measurement of the temperature of molten steel in convertors, steel-melting furnaces, and intermediate vessels in continuous-casting plants. The range covered is 13001800~ calibration for PR 30/6, probability of failure-free operation not less than 0.92. Figure 1 gives a general view of the TPR-2075, where 1 is the PTPR disposable tip, 2 is the mounting ring, 3 is the tube, 4 and 5 are respectively the body~ and handle, and 6 is the connector. The PTPR tip (Fig. 2) consists of the paper sleeve 2 of length I000 r~m, internal diameter 20 mm, and wall thickness 6.5 mm, which itself encloses the ceramic sleeve 3. The plastic insert 12 is cemented into the latter and bears the quartz U-tube 4 of outsidediameter 3 mm and wall thickness 0.8 mm. The tube is fitted with the elastic cover 9, which separates the refractory section 8 from the cold junctions 10 and II of the thermocouple 5, whose wires of diameter 0.1 mm are made of alloys PR 30 and PR 6. The design of the hot junction 6 ensures reliable thermal contact of the junction with the quartz tube and prevents failure due to the thermal shock on immersion in the metal [4]. Electrical contact between the thermocouple and the holder is provided by the extender leads 1 and 13, which are copper wires of diameter 1.0 mm (in the case of the PR 30 wire) or 0.8 mm (for the negative electrode). The metal cover 7 protects the quartz tube from mechanical damage on immersing the tip in the molten steel. As the PTPR tips are intended for use once only, the aspect of main interest from the metrological viewpoint is interchangeability. The main characteristic of the TPR-2075 is therefore the range in the readings, which characterizes [5] the performance of the means of measurement and reflects any deviation from zero in the average random error. The spread in the readings of the TPR-2075 has been examined for the range 1300-1700~ Random errors arise in temperature determination on account of the following deviations of the thermo-emf from the standard calibration (error ~,), the error o2 due to change in the temperature of the cold end during use, ~s due to inhomogeneity in the wire, ~ due to inhomogeneity in the copper wire, ss arising from temperature differences between the precious metal leads, and ~ the same arising from temperature differences between the copper parts of the circuit.