The system includes a thermal probe with a heat-sensitive element, a fluid-tight plunger body, shaped to completely wrap the heat-sensitive element of the probe, a probe support including two orifices for joining the system to the duct, linked by an inner channel substantially axially extending in a main flow direction of the fluid in the support and including a radial opening through which the plunger body is configured to be at least partially submerged inside the inner channel. More particularly, the inner channel is delimited by a tubular wall having an inner surface with a depression located facing the opening in order to limit singular pressure losses of the fluid likely to be generated by the presence of the plunger body in the fluid by defining a flow pathway of the fluid around the plunger body.