The encased probe is the standard resistance thermometer configuration for industrial process control and machinery protection. Most probe cases are stainless steel or Inconel to withstand high temperatures, although other materials offer advantages at intermediate ranges.
For example, the tip-sensitive probe of Figure a copper-alloy tip which conducts heat 20 times better than stainless steel. This design improves thermal contact with sensed surfaces and reduces errors from conduction along the sheath.
Standard probe diameters range from 0.125. - 0.250. Smaller probes respond faster when directly immersed, but larger probes may fit more snugly in standard Thermowells. Probe lengths range from a few inches to ten feet or more.