An unsteady, three-dimensional numerical simulation model is developed to investigate conjugate heat transfer effects from a low temperature nitrogen gas ambient to a thermocouple of diameter ranging between 10 and 100 mu m. The response of the thermocouple to a sudden temperature jump of the ambient gas has been investigated. Results include the calculated time constant of the thermocouple, temperature distributions in the gas and the solid and velocity distributions in the gas as a function of the ambient conditions. The wire size is clearly the most important factor, while the ambient temperature and velocity also significantly affect the response time. Comparison with an experiment shows good agreement. (C) 2013 Elsevier Ltd. All rights reserved.