Thin-film RTDs offer fast response, streamline calibration

Compact size and clever design of resistance temperature detectors improve OEE in sterilization applications.

By Paul Studebaker, editor in chief, Control

Thermocouples, RTDs and thermistors each have application areas where they excel, but none of the available versions seemed to work at a yogurt ingredient factory in Denmark. Fortunately, recent developments in thin-film technology, advanced manufacturing techniques and some clever engineering came to the rescue.

Chr. Hansen had a problem with temperature sensors at its plant in Avedoere Holme, Denmark. The sensors they were using didn’t respond fast enough, and the required calibrations were too frequent and took too long.

Chr. Hansen produces bacterial cultures used by food producers for making yogurt. During the ultra-high temperature (UHT) process used to sterilize the bacterial culture solution, temperature control is critical. The solution must be maintained at temperatures above 139 °C; if the temperature drops below that, the sterilization process must be repeated.

The process requires an accurate temperature sensor with a very fast response time. The various temperature sensors they'd been using for the past seven years suffered from poor accuracy, lack of stability and slow response. Chr. Hansen replaced those sensors with a small, thin-film platinum resistance thermometer.

The large sensing area in wire-wound RTDs typically results in a slow response time—in some cases, a wire-wound RTD might take a full minute to respond to a temperature change. But the thin-film sensor, an iTherm TM411 RTD from Endress+Hauser, offered a 3.5 sec. response speed and accuracy of 0.1 °C.

Complicating the problem was the need to insert a thermowell into the process fluid to protect the sensor. Thermowells tend to slow the response of any temperature sensor. TM411 is just 6 mm in diameter, so it can fit in a thermowell as small as 9 mm diameter that has very little effect on response time. This thin thermowell provides the necessary mechanical protection, while allowing fast heat transfer from the process fluid to the sensor, which is mounted at the tip of the thermowell.

Tests with and without a thermowell showed no measureable slowing of response time.

Chr. Hansen conducted tests of the TM411 with and without a thermowell. The tests showed no measureable slowing of response time, and a deviation in temperature measurement of only 0.045 ˚C.

Accuracy is also important in this process, so the RTD sensors have to be calibrated several times a year. Previously, calibration took about 45 minutes, often shutting down the process while the sensor was removed. About 20 minutes of this time was spent disconnecting wires and reconnecting them when the calibration was complete. TM411 RTDs have a QuickNeck feature that allows tool-free removal of the complete upper part of the sensor from the thermowell with just one twist. There’s no need to open the housing or to disconnect any wires, so a calibrated RTD can be quickly inserted to replace an RTD requiring calibration, or the sensor can be removed for a quick on-site calibration. With the Quick-Neck, an experienced technician can remove and replace the sensor in less than five minutes. This also results in no possibility of incorrectly rewiring the transmitter upon reinstallation.

The greatest benefit of quicker calibration is more process throughput. While calibration is being performed, the UHT process can't be monitored and controlled, and must therefore be shut down. Reducing calibration time by more than 20 minutes per sensor means more product can be sterilized on a daily basis. In addition to increasing plant availability, it also eliminates the need for clean in place (CIP) procedures after recalibrations.

The metrology department at Chr. Hansen performs calibrations using a temperature-controlled calibration bath that compares the measured values of the TM411 against a precise reference thermometer. Once the sensor has reached equilibrium and the technician has verified the sensor is within tolerance, they perform any necessary trimming of the output, remove the sensor from the temperature calibrator, insert it back into the thermowell, and secure it with a final clockwise quarter turn.

Chr. Hansen has installed 27 sensors so far on its UHT sterilization processes, and since then has yet to experience a bad batch caused by malfunctioning temperature sensors.

 

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