Measuring Temperature in Small Diameter Lines- Direct Immersion RTD’s and Best Practices

April 18, 2014

Measuring and accurately monitoring process conditions is essential in high purity applications. In the over sixty years Holland has spent applying process equipment for the high purity industry, we’ve learned that the process is the product. If we can’t control the process, we can’t control the final product. A critical process parameter that needs to be measured is temperature. This post will highlight some challenges, best practices, and solutions for measuring temperature in a high purity application.

To begin, both the food and pharmaceutical industries rely and platinum RTD’s when measurement of process temperature is critical because these platinum resistance thermometers (PRT’s) offer superior accuracy, stability, and repeatability compared to other temperature measuring devices. Most RTD’s are classified in accordance to either the ASTM E1137 or IEC 60751 standard and are rated either Grade/Class A or Grade/Class B. Grade A sensors have temperature tolerances of +/-0.3 C at 100 C and Grade B sensors have tolerances of +/-0.67 C at 100 C. These tolerances only apply when measured under ideal lab conditions and do not take into account common process variables such as vibration and immersion depth.

The most accurate way to measure process temperature is with a sensor that is immersed directly into the flow. These direct immersion sensors are the preferred because they provide accurate measurement and quick response time. Most often, RTD’s are clamped in to a tee, perpendicular to flow. For small diameter lines, this RTD orientation may not be adequate to get an accurate temperature measurement.

RTD's Work Best When Inserted into the Middle of the Flow Path

RTD’s Work Best When Inserted into the Middle of the Flow Path

A general rule that is used for immersion RTD’s is that the minimum immersion length into the flow should be at least 10 times the sheath diameter plus the length for the sensing element. This immersion depth is required to minimize temperature conduction through the stem of the sensor. Known as stem conduction error, this is the error caused by heat transfer between the sensing element and the ambient conditions at the back of the sensor.

To put things in perspective, for a typical 0.25” diameter sensor with a 1” long element, the 10 times plus length would require 3.5” immersion into the process. You could make this work on a 4” process line, but good luck on a half inch line. While reduced diameter sensors are available, it’s tough to find one much thinner than 1/8” of an inch. One fix is to insert the sensor perpendicular to the flow. By inserting the sensor into the end of the run of a tee, as opposed to the outlet, we can increase insertion depth.

While this fix addresses stem conduction issues, flow blockage and pressure drop must also be considered. The other big drawback to direct immersion sensors is that in order to remove or replace the sensor from the process line, the line must be fully drained. This isn’t good if a sensor fails mid batch. Direct immersion sensors can also pose problems with sticky, viscous products that are prone to coating and difficult to clean or remove. High flow and turbulence in CIP can also cause drift and affect measurement accuracy. Fortunately, there are ways we can address these challenges, so stay tuned for future posts focusing on non-intrusive surface sensors and indirect immersion sensors. If you can’t wait, contact a Holland Sales Engineer today for help with your next temperature application.