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Measurement Performance within an Elbow Thermowell
Measurement Performance within an Elbow Thermowell: The Impact of Sensor Construction on Time Response
Introduction
Time response is a significant source of measurement error in many installations. It becomes especially important in thermowell applications, as the increased mass around the sensing element can cause delays of several minutes before a temperature change is fully realized by a sensing device in a typical thermowell.
The Burns Sanitary Well Elbow (SWE) was designed to reduce the mass around the sensing element while still ensuring the proper sensor immersion required for an accurate measurement. This evaluation will investigate the impact of sensor construction on time response when a sensor is used with a typical SWE.
Figure 1: Product Image and Cross Section View for Burns SWE Thermowell
Determining Sensor Constructions to Evaluate
Commercially available sensors used with the Burns SWE thermowell were selected for testing. This included spring loaded sensors with 6 mm and ¼” sheath diameters, in both thin film and coil element designs. Multiple sensors were evaluated for each sensor type.
Time Response Performance
The direct immersion time response for each sensor was recorded per ASTM E644-11. To test the SWE time response, each sensor was attached to the thermowell and ice water was pumped through the process line at approximately 15 ft/s until full response was achieved. The pump was then changed to room temperature water and the time response was recorded. The average 63.2% response times for each sensor type are given below, measured in the thermowell and in direct immersion. Tests were randomly repeated in order to validate the testing process, and the measurements made had a margin of error of less than 5%.
Table 1: Average Time Response for Each Sensor Construction
Figure 2: Sample Time Response Curve for Each Construction with an SWE Thermowell
The 63.2% time response was up to 60% slower for the 6 mm sensors than the corresponding construction 1/4” sensors when used in the SWE thermowell.
Figure 3: Sample Time Response Curve for Each Construction in Direct Immersion
Summary
The results show that ¼” diameter sensors should be used for SWE thermowell applications, as the time response improvement of up to 60% will provide a more consistent, reliable and accurate temperature measurement. If time response is especially critical in a given application, there may be an additional performance benefit to using ¼” sensors with a thin film design, assuming that a thin film sensor can meet the other application requirements.
Other factors that could further negatively influence the measurement accuracy include:
- Lower flow rates
- Larger difference between process and ambient temperatures
- Sensor constructions with lower thermal conductivity around the sensing element
Additional research would be necessary to quantify additional application variables.