Measuring the Temperature of Yogurt Bacterial Cultures
The sterilization process for yogurt bacterial cultures requires accurate temperature measurement with fast response times, necessitating use of a specialized thin-film RTD sensor.
At its production plant in Avedoere Holme, south of Copenhagen, Chr. Hansen — a Denmark bioscience company that develops natural ingredient solutions for the food, nutritional and pharmaceutical industries — produces bacterial cultures used by food producers such as Dannon and Nestlé to make yogurt and other sour milk products. To eliminate any contaminants that could pollute or destroy the final product, the nutrient solutions are sterilized by ultra high temperature (UHT) treatment before being transferred to a fermenter.
In the UHT process, the solution is exposed to temperatures above 282°F (139°C) for several seconds. A safe sterilization can only be guaranteed if the temperature does not fall below the minimum temperature at any time. Thus, if the temperature drops and approaches the minimum required temperature, a quick reaction by the control system is necessary to counteract the temperature decrease and heat up the system again. This requires an accurate temperature sensor with a fast response time.
Chr. Hansen had been using the UHT process for more than seven years, but the temperature sensor used in its existing system had poor accuracy, lack of stability and slow response. These problems forced the company to repeat the UHT process in some cases to ensure final product quality.
To alleviate these issues, Chr. Hansen opted to replace the existing sensor with an iTherm TM411 RTD sensor from Endress+Hauser (figure 1). This sensor is connected to a TMT82 temperature transmitter, also from Endress+Hauser. The temperature transmitter translates the raw RTD signal to a temperature value and transmits this reading via a 4 to 20 mA Hart output to the control system.
The RTD used at Chr. Hansen has a measuring range of -58 to 392°F (-50 to 200°C). A thin-film platinum resistance thermometer, the RTD offers advantages over wire-wound RTDs, including smaller size, better vibration resistance and faster response. Thin-film RTDs typically are used at temperatures below 752°F (400°C).
Accuracy and speed are important for sterilizer monitoring. The thin-film RTD offers accuracy of 0.18°F (0.1°C) with a 3.5 second speed of response. A thermowell houses the sensor and protects it from the process conditions, but thermowells can slow the response of the temperature sensor. For Chr. Hansen, a thinner, 0.24” (6 mm) dia. thermowell was used. The thin thermowell provides the necessary mechanical protection while allowing fast heat transfer from the process fluid to the sensor, mounted at the tip of the thermowell.
Once the new sensor was installed, Chr. Hansen conducted tests of the RTD with and without a thermowell. The results (table 1) showed no slowing of response time and a deviation in temperature measurement of only 0.081°F (0.045°C) for the (UHT) treatment process.
The expected negative impact of the thermowell on overall accuracy was negligible. This allows Chr. Hansen to use thermowells without negative effects on temperature measurement accuracy.
Response time is just one half of the story. Being able to use a thermowell meant Chr. Hansen could keep the process closed during calibration. Instead, the thermowell remains in the process while the sensor itself is removed to perform a calibration.
Accuracy and Fast Recalibrations
To ensure temperature measurement accuracy, each RTD is calibrated several times a year.
The iTherm TM411 RTDs have a feature that allows tool-free removal of the upper part of the sensor from the thermowell with just one twist (figure 2). The housing does not need to be opened, and any wires do not need to be disconnected. As a result, an already-calibrated RTD can be inserted quickly to replace an RTD requiring calibration. This increases plant availability because the process does not have to be shut down for a calibration. It also eliminates the need for clean-in-place (CIP) procedures after recalibrations.
The metrology department at Chr. Hansen performs the calibrations using a temperature-controlled calibration bath (figure 3) that compares the measured values of the RTD against a precise reference thermometer. To calibrate the RTD, the technician removes the sensor/transmitter assembly from the thermowell and opens the housing to the transmitter. He hooks up the PC/temperature calibrator — which is carried to the sensor location — to the transmitter terminals and immerses the sensor tip in the temperature calibrator. Once the sensor come to equilibrium (usually less than two minutes), the technician verifies the sensor is within tolerance. (Calibration at Chr. Hansen is typically three points within the process range.) If needed, he performs any necessary trimming of the output. Then, the sensor is removed from the temperature calibrator, inserted back in the thermo-well and secured to the thermowell with a final clockwise quarter turn.
For an experienced technician, a typical a “standard” thermowell-sensor connection takes about an hour. Typically, 25 minutes is spent disconnecting sensor wires and reconnecting them when the calibration is completed. With the quick-connect design of the TM411, disconnecting or reconnecting the wires is not necessary. While this design feature does not reduce the calibration time of 30 minutes, the handling time is reduced. This also eliminates the possibility of incorrectly rewiring the transmitter.
The main benefit to a customer such as Chr. Hansen is greater process throughput. As long as a calibration is being performed, the UHT process cannot be monitored and controlled. Reducing calibration time by more than 25 minutes per sensor means that more product can be sterilized on a daily basis.
In the end, Chr. Hansen installed 27 of the RTD/thermowells on its UHT sterilizers as weld-in installations. Since the new sensors were installed, Chr. Hansen has not experienced any bad batches caused by slow response from the temperature sensors.