Thermocouple probes are available with one of three main junction types: grounded, insulated or exposed as pictured below.
• In an ungrounded or insulated probe the thermocouple junction is isolated from the outer sheath material, this reduces any electrical interference and provides a stable reading but results in a slower response time and is often more expensive than grounded or exposed alternatives.
• At the tip of a grounded thermocouple, the thermocouple junction is physically attached and is an integral part of the outer sheath. Response time is faster than that of an ungrounded junction as a result of good heat transfer from the sheath to the thermocouple junction; however readings can sometimes be unstable.
• An exposed junction either has no outer sheath or the thermocouple junction extends beyond the sheath. They are not recommended for use in corrosive environments but offer a very fast response time at a low cost in the right application.
Reference : sterlingsensors