When Temperature Measurement Goes Awry
January 1, 2009
All contact temperature sensors such as thermocouples and RTDs, suffer from stem effect, which occurs when a probe is immersed in a heated gas, liquid or any hot zone. In these situations, heat is conducted away from the tip of the sensor (the portion that makes the measurement) along the sheath (or stem) of the probe to the outer atmosphere, resulting in a lower reading than reality.
NANMAC conducted several experiments to determine the magnitude of stem-effect errors by use of a heated chamber surrounded by a cooled, controlled chamber. A gradient of about 800F was maintained. Four thermocouples, each with a different style of thermal junction, were installed in the wall of the hot chamber, aligned flush with the inner surface. The outputs were monitored by the same high accuracy pyrometer.
Thermocouples 1, 2, and 3 were conventional thermocouples with the following junction designs: exposed, grounded, and ungrounded. Thermocouple 4 utilized a new thermal junction design, the Right-Angle Ribbon thermal element. After a series of 15 tests, conventional thermal junctions produced errors ranging from 10 to 66 percent while the Right-Angle thermocouple showed no measurable errors.
The patented Right-Angle thermocouple features a thermal junction at right angles to the longitudinal axis of the probe. The thermal elements consist of ribbons electrically welded together at the sensing tip, these ribbons at the junction are parallel to the plane of heat flow. As both the thermal junction and ribbons are heated simultaneously, conduction errors are eliminated. Ribbon elements also have response times in the low milliseconds to transients and are an excellent choice for any gas or liquid measurement.