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Electrical resistance breakdown of Type N mineral-insulated metal-sheathed thermocouples above 800 °C.

Hastings, M W*; Pearce, J V; Machin, G (2012) Electrical resistance breakdown of Type N mineral-insulated metal-sheathed thermocouples above 800 °C. Meas. Tech., 55 (8). pp. 941-948.

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Abstract

Thermocouples are the most commonly used temperature sensor in industry for measurements above around 800 °C. However, at higher temperatures, the resistivity of the ceramic insulators employed to electrically isolate the thermoelements drops by many orders of magnitude. The breakdown in the electrical resistance of insulation and the discrepant temperature reading due to that breakdown has been measured for two mineral insulated metal sheathed (MIMS) type N thermocouples in a temperature range of 850 °C to 1100 °C. The two thermocouples had diameters of 1.5 mm and 3 mm respectively. The insulation breakdown was detected by applying a DC bias voltage between the thermocouple sheath and one of the thermoelements. Bias voltages of between -20 V and +20 V were used. It was found that the indicated temperature offset (difference between the indicated temperature with and without a bias voltage applied) due to the insulation breakdown varies approximately linearly with the magnitude and is also dependent upon the direction in which the bias voltage is applied. The resistance of the MgO insulation is broadly independent of bias voltage. The insulation leakage in a MIMS type N thermocouple with MgO insulation can give rise to a significant error in the thermocouple reading; here, the effect is quantified. The effect has a strong inverse dependence on thermocouple diameter. We present the results of this investigation.

Item Type: Article
Keywords: thermocouples, insulation resistance, high temperature
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1007/s11018-012-0065-1
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5679

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