Calibration
Sanitary temperature sensor calibration
Sanitary temperature sensors are commonly used in many industries, such as food and beverage, dairy, pharmaceutical and life-science. Here, Beamex’s Heikki Laurila takes a look at this type of sensor and discusses the considerations that should be taken into account during calibration
T
emperature is one of the critical process parameters in many industries and the accurate temperature measurement in
processes is a crucial consideration. Food and beverage, dairy, pharmaceutical and life-
science industries have additional requirements for the temperature measurement sensors because of their processes. They require temperature sensors that are “sanitary”, meaning that these sensors need to be suitable to be installed in hygienic and aseptic process environments. These sensors need to be hygienic and designed
to be easy to clean, often supporting the clean-in- place (CIP) process (cleaning without disassembly). The mechanical design needs to be free from any cavities, dead-pockets, gaps or anything that would complicate the hygienic cleaning. Surface finishes of these sensors are hygienically
graded and need to meet the strict standards in these industries, such as the 3-AR or EHEDG (European Hygienic Engineering & Design Group). The material of the wetted parts in these
sensors is often high-grade stainless steel, suitable for these applications. For easy installation, the sanitary sensors are
often provided with the clamp installation. One very common feature in these sanitary
temperature sensors is that they are typically very short. This makes the calibration way more difficult than with normal temperature sensors. Another thing that makes the calibration difficult is the large metallic flange needed for the clamp installation. The temperature ranges typically go up to
around 150°C, or in some cases up to 200°C, so that is not very challenging.
THe roLe of caLiBraTion In any industry, it is vital that the process measurements do measure correctly and as accurately as designed. This
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can be achieved with the help of suitable process instruments and with a proper calibration program. Within the food and beverage, pharmaceutical
and life-science industries, calibration plays an even more important role than most other industries. In these industries, the consequences of a bad or a failed calibration can have a really dramatic effect, as we talk about consumer and patient health and safety. As failed calibration can be very costly in these industries, it has to be avoided by all means. These industries also have dedicated strict
regulations concerning calibration, such as various FDA regulations.
WHy are SaniTary SenSorS difficuLT To caLiBraTe? 1. The sensors are very short As mentioned earlier, these sanitary temperature sensors are typically very short. Most often less than 100mm, typically around 50mm, but can also be as short as 25mm. The outer dimension of the sensor typically is 3mm or 6mm. The commonly used practice in temperature
calibration (and a Euramet guideline recommendation) is that a temperature sensor should be immersed deep enough to achieve sufficient accuracy. The recommendation is to immerse into a depth that is 15 times the sensor diameter (plus the length of the sensor element). But with these short sanitary sensors, it is simply impossible to immerse the sensor into sufficient depth during the calibration, because the sensor is so short compared to the diameter. For example, a typical sanitary sensor with a
diameter of 6mm should be immersed (15 x 6mm) into at least 90mm depth during the calibration, to ensure accurate results. But if that 6mm sensor has a length of only 50mm, sufficient immersion is simply not possible. When not immersed deep enough, additional
error and uncertainty will be caused in the calibration. Beamex’s temperature calibration experts
provide these rules of thumb for the immersion depth (when calibrating in liquid bath):
one per cent accuracy - immerse five diameters + length of the actual sensing element inside the sensor
0.01 per cent accuracy - immerse 10 diameters + length of the sensing element
0.0001 per cent accuracy - immerse 15 diameters + length of the sensing element
The “accuracy” in the above rule is to be calculated from the temperature difference between the block temperature and the environment temperature.
This picture illustrates the commonly used relationship rule between thermometer immersion depth (in diameters) and the relative error of the temperature difference (of the temperature block and environment temperatures). So if you do not immerse at all, you naturally get a 100 per cent error, and if you immerse deep enough the error caused by immersion becomes insignificant. Somewhere around where the immersion is five times the dimension, the error is about one per cent of the temperature difference
For example, if the environment temperature is
20°C and the block temperature is 120°C, there is a 100°C difference. If you then immerse the probe only five times the dimension (plus the sensing
January 2021 Instrumentation Monthly
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