From Where Do Errors Stem?
Contact temperature sensors such as thermocouples, RTDs, thermistors and bimetal thermometers measure only their own temperatures. Thus, the object of the design engineer is to select the correct sensor design and install it such that it measures the temperature of interest.
Four thermocouples, each with a different style of thermal junction, were installed in the wall of the hot water tank and aligned flush with the inner surface of the tank.1 Thus, the thermocouples were in contact with the hot water. All thermocouples had iron/constantan elements, and their outputs were monitored by the same meter through a thermocouple selector switch. In addition, all thermocouples were made from the same lot of wire, and resistances were matched. The four thermal junction styles used were:
Thermocouple 2 (T2). The second thermocouple in the experiment used a grounded junction. In this junction system, the sheath is welded closed and the thermal junction is electrically in contact with the sheath.
Thermocouple 3 (T3). The third thermocouple in the experiment used an insulated or ungrounded junction. In this junction style, the sheath is welded closed but the thermal junction is electrically isolated from the sheath. All RTDs and thermistors are similar in design to this insulated junction. The sensing element is electrically insulated and then enclosed in a metallic sheath that protects the sensing element from environment abuse such as corrosion, oxidation, erosion and chemical attack.
Thermocouple 4 (T4). The fourth thermocouple in the experiment used a right-angle ribbon junction. In this configuration, the right-angle ribbon sensor has a junction thickness of 0.003".2 The extension leads in the vicinity of the junction also are ribbons and lie in the same plane as the sensing junction for a distance of at least 20 times the thickness of the junction. Thus, the sensing tip and the adjacent ribbons are parallel to the plane of heat flow, and because the ribbons on both sides of the junction are heated simultaneously with the junction, the error caused by the stem effect is eliminated (figure 3).
Applying this requirement to conventional temperature sensors dictates that the sensor be installed such that it is parallel to the plane of heat flow for a distance of at least 20 times its diameter. For example, a 0.125" round stainless steel probe must be positioned in the oven so that it lies parallel to the plane of heat flow for 2.5". This can be accomplished by one of two methods: Make a 90o bend in the probe 2.5" from its tip; or position the mounting so that the probe is parallel to the plane of heat flow.
When installing the sensor parallel to the plane of heat flow, it still must be immersed a distance of at least 20 diameters. For a 0.375" dia. probe, this distance is 7.5".
A simple test can be performed to determine if your sensor is producing a significant error caused by the stem effect. Replace your current temperature sensor with a similar sensor that has a much smaller diameter. Insert this sensor to the same depth as the original sensor. Compare the indicated temperatures of the two sensors. If the sensor with the smaller diameter indicates a higher temperature than the larger diameter, you have a significant error.
Minimizing errors caused by conduction along the stem of the sensor will greatly improve the accuracy of the measured temperatures.