Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.

In this paper the influence of the resistor temperature coefficient to the uncertainty of the temperature measurement is presented. At the level of highest accuracy of the temperature measurements the standard platinum resistance thermometers (SPRT) in combination with an automatic resistance bridge and a standard resistor is used. The electrical resistance is like any other quantity more or less dependent on the temperature. This dependence is usually specified using the temperature coefficient, which gives the relative change of resistance, when the temperature changes for one °C. A temperature coefficient is also temperature dependent, so the relation between an electrical resistance and a temperature is given using the polynomial equations. These equations have an inflection point at a certain temperature, where the derivative of the resistance change on temperature (temperature coefficient) is equal to zero. The knowledge of the temperature of the inflection point is very important, because the use of the standard resistor at this temperature greatly reduces the influence of temperature on the electrical resistance, even if the thermal conditions are not optimal, and thus enabling low uncertainty and accurate measurements of the temperature using a SPRT. In the scope of this paper, measurements of the temperature coefficient are performed on series of a standard resistors in a wide temperature range and the temperature of the inflection point was determined for each standard resistor. All the measurements were performed by placing the resistors in an oil bath, where temperatures in the range from 19 °C to 37 °C could be precisely set. Electrical resistance of each resistor was measured using the precise automatic resistance bridge, which had its reference resistor placed in a separate thermal enclosure at constant temperature. Thus the resistor temperature coefficient of each standard resistor has been measured. It is influence to the uncertainty of the temperature measurement has been evaluated and uncertainty model presented.