Kee Hoon Kang

Thermoelectric properties of the Au/Pt thermocouple

An Au/Pt thermocouple having a gold-bridged junction instead of a Pt coil is fabricated and its thermoelectric properties are investigated at the Sn, Zn, Al, and Ag freezing points. Reproducibility is found to be excellent and standard deviation is as small as ±4 mK at the Ag fixed point. Immersion properties, which indicate homogeneity of the thermocouple wire, are also shown to be very good, with standard deviation estimated as about 10 mK. In order to investigate interchangeability, 20 sets of Au/Pt thermocouples are made and their emfs are measured at the same fixed points. Thermocouples from the same spool are reproducible within ±0.02 °C for the whole temperature range. For practical use of the Au/Pt thermocouple, an inverse function and a deviation function for calibration are proposed. The inverse function has two temperature ranges and is accurate within ±1 mK, while the deviation function is a third-order polynomial without a constant term. The Au/Pt thermocouple is stable at high temperatures for long periods. After 1500 h at 1000 °C, the thermal emf at the Ag fixed point is changed by about 0.9 μV (about 40 mK). After the other experiments, calibration uncertainties at the fixed points are evaluated. At the Ag fixed point, the maximum expanded uncertainty is calculated and is about ±30 mK at a confidence interval of 95%. It is shown that the gold-bridged Au/Pt thermocouple is reproducible, accurate, and stable. At present, the Korea Research Institute of Standards and Science (KRISS) uses Au/Pt thermocouples as secondary standard thermometers; they are supplied to industry as certified references.

First-order compensation for time lag in the dynamic calibration of industrial thermometers

A new calibration method that provides first-order compensation for the time-lag error of dynamic temperature comparisons is suggested. The calibration uses the ramp response of the first-order system, assuming that the temperature change of the system can be approximately linearized in a narrow region near the calibration point. The calibration is done by measuring the rate of temperature change and comparing the temperature readings of a reference thermometer and test thermometers while the temperature of the system is decreased and then increased. This calibration is useful for industrial thermometers at low temperatures without using a sophisticated temperature-control system. We demonstrate in this work that the deviation of the result of time-lag compensation from the calibration in a stable thermal enclosure was less than 20 mK. The result indicated that the time-lag compensation can be used for calibrations where the required uncertainties are around 40 mK, and the calibration procedure is much simpler than that for comparison in a stable enclosure. Furthermore, the compensation suggested here has wider applicability at higher temperature ranges where thermometers are calibrated dynamically in liquid baths or furnaces.

Measurement of the Seebeck coefficients of binary Cu-Ni alloys

In order to identify materials suitable as a compensating wire pair for the Pt/Pd thermocouple, binary Cu100−xNix (x = 3, 5, 8 and 10 wt%) alloys were prepared and their Seebeck coefficients were measured using the temperature gradient method from room temperature to approximately 120 °C using a pure platinum reference material. The Cu–Ni alloys showed negative Seebeck coefficients at all test temperatures. As the temperature increased, the Seebeck coefficients of the Cu–Ni alloys became more negative. At a fixed temperature, the Seebeck coefficients became more negative as the Ni content increased. Among the test alloys, Cu95Ni5 and Cu90Ni10 (designated as CuNi5/10) both showed a calculated thermal emf value that was very similar to that of the Pt/Pd thermocouple. It was concluded that the CuNi5/10 system could be a good compensating material for the Pt/Pd thermocouple, and by a small adjustment of the Ni content the temperature error could be reduced even further.

Self-testable industrial platinum resistance thermometers integrated with miniature mercury and indium fixed-point cells

Miniature temperature fixed-point industrial platinum resistance thermometers (IPRTs) have been constructed to investigate the feasibility of a self-testable IPRT integrated with a mercury or indium fixed-point cell. The miniature cell was constructed from stainless steel with a combined small PRT sensor element inside it, and was contained within an IPRT protection tube. The reproducibilities of the freezing and melting temperatures measured using the mercury miniature cell were ±0.08 °C and ±0.63 °C, respectively. In the case of indium, only the melting temperature was taken into account, and its reproducibility was ±0.01 °C. The performance of both miniature fixed-point IPRTs was good enough to keep track of the long-term stability of the IPRTs in the order of 0.1 °C.

An investigation of the thermoelectric properties of type S thermocouples from different manufacturers

An investigation into the differences between commercial thermocouple wires from five different manufacturers was carried out, including fixed-point emf, reproducibility, high temperature durability, cyclic properties, and interchangeability. All the thermocouples met the ASTM criteria, but, dependent on the wire source, a maximum temperature difference of 2°C was found at the Cu fixed-point temperature. The reproducibility of fresh type S thermocouples was within 40 mK at the Ag fixed-point temperature, regardless of the wire source. On exposure to high temperatures of 1400°C, the fixed-point emf values and immersion behaviours were changed, in that the emf decreased and the inhomogeneity increased. After about 300 thermal cycles between 600 and 1100°C, the fixed-point emf at the Ag fixed-point did not change, but the inhomogeneity increased by about 4 times. From the emf measurements on 19 thermocouples from a single manufacturer, the interchangeability was estimated to be about 0.16°C. This work can provide information either on the selection of a thermocouple and its proper usage for the precise measurement of temperature in industry, or for calibration.

Fabrication of Small Copper Sealed‐Cells for Thermocouple Calibration

Small, sealed Cu freezing cells (100 mm high and 27.5 mm diameter) were fabricated for use in calibrating thermocouples in industry. The cell volume was about 15.8 cm3 and contained about 125 g of Cu. These cells can be used to calibrate a thermocouple of 5.5 mm diameter. The cells were sealed in an argon atmosphere so as to be at a pressure of 1 atmosphere at the freezing temperature. The cells were tested with a Pt/Pd‐thermocouple as a reference thermometer and compared with a large, standard Cu freezing cell. The small cells showed a slightly lower temperature than the standard cell. The uncertainty of the cell temperature was calculated to be 0.1 °C within a confidence range of 95 %.

A New Cryostat for Measuring the Triple Point of e‐H2 in Sealed Cells at KRISS

A new form of cryostat that can realize the triple points defined in the International Temperature Scale of 1990 (ITS‐90) at less cost and within a short period of time was designed and manufactured. This cryostat combined a He‐gas closed‐cycle refrigerator, which has a low operating cost, for low‐temperature maintenance and an auxiliary gas‐cooled refrigerator, which has the capacity to attain low temperature quickly by using a small amount of liquid helium. After examining the cooling speed using the manufactured cryostat, the results showed that it reached 9 K within 9 hours. Our experience in realizing the e‐H2 triple point (13.8033 K) by using a new cryostat, demonstrated that achieving the triple point was easy and fast. Moreover, the repeatability of the measurement of the triple point was 0.07 mK. In addition, over 15 e‐H2 triple point measurement tests were possible in about three weeks time with 100 L of liquid helium. These results show that the newly manufactured cryostat realizes the e‐H2 tri...

Reproducibility of the gold melting point of the type S thermocouple by the wire-bridge method with change of the heating rate

Abstract To improve the calibration accuracy of the noble thermocouple, the reproducibility at the gold melting point of type S thermocouples using the wire-bridge method was investigated with the variation of the heating rate. By using a computer with an digital-to-analog converter system, the temperature and heating rates were very precisely controlled. It was found that emf for the gold melting point was not dependent on the heating rates, and the reproducibility ( l σ) of the gold melting point was within ±0.04°C. The overall combined standard uncertainty was evaluated as ±0.24°C. It was confirmed that a thermocouple can be simply calibrated at the gold melting point by the wire-bridge method with considerable accuracy.