Chr. Hansen had issues with slow temperature sensor response times and frequent calibrations that took too long, interrupting their sterilization process. They replaced the sensors with thin-film platinum resistance temperature detectors (RTDs) that responded 3.5 seconds faster and were more accurate. The compact size of the RTDs allowed them to fit into small thermowells without slowing response time. The RTDs also had a quick release mechanism, allowing calibration to be completed in under 5 minutes instead of 45 minutes, increasing process uptime and throughput. The new sensors provided faster, more accurate temperature measurements without process interruptions, improving plant efficiency.

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Home / Articles / 2016 / 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
Mar 03, 2016
Paul Studebaker is chief editor of Control. He earned a master's degree in
metallurgical engineering and gathered 12 years experience in manufacturing
before becoming an award-winning writer and editor for publications including
Control and Plant Services.
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.
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2. “Tests with
and without a
thermowell
showed no
measureable
slowing of
response
time.”
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.
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.