Product temperature, know what you're measuring

Measure and display a temperature seems trivial. ‘Numbers tell the tale’ you may think, but it doesn't stop there. There are quite a few aspects to consider. For example, what do you measure exactly, how do you carry out this measurement, and where? These are determining factors for the interpretation of your values. 

Microbial growth, the ripening of fruit, the spoilage of products, the speed of (bio)chemical processes: the food industry has to deal with these things on a daily basis. Processes that largely depend on temperature. Temperature is also an important parameter for physical properties such as viscosity, density and electrical conductivity. In order to control these processes in a controlled way and to properly estimate product properties, you have to be able to control and know the temperature. So measurements are taken continuously. But ‘numbers tell the tale’ does not equal 'the more you measure, the more you know'. To correctly interpret the temperature, it is essential to know what, where and how to measure. In practice, this is not so straightforward. 

Practical examples 

In a process such as High Pressure Processing (HPP), quite a lot happens in the thermal field. HPP often takes place at an elevated temperature and the temperature in the pressure vessel also rises due to the pressure build-up. As a result, the product as well as the pressure medium are warmer than the pressure vessel. This causes the temperature to drop again. When the pressure in the vessel is removed, the temperature drops further. When you report a temperature in such a process, it must be very clear exactly at what point in the process it was measured, and what temperature is being referred to. 

Suppose you want to measure the core temperature of a product. Then the sensor must reach the thermal center of the product. In other words, is the sensor long enough? It sounds logical, yet it often goes wrong. If the thermal center is difficult to determine, taking several measurements helps you to get a clear picture of the actual product temperature. It is also essential to wait until the temperature stabilizes on a display screen. When thicker sensors are used, the recording will take a little longer. If the same sensor is used to measure the core temperature of different products, be aware of the risk of cross-contamination via the sensor. And in the case of cleaning it in between: avoid getting detergent from the sensor into the product.

What and where do you measure?

In some countries, temperature is still expressed in degrees Fahrenheit, as in the US. In almost all other countries, on the contrary, degrees Celsius is common. And there is also the unit Kelvin, where zero K is the coldest possible temperature (0°C = 273.15K). So always explicitly state the correct unit of measurement.

The "where" of a measurement is also important: for example, are you measuring the temperature in a cold room near the cooling unit, or is the sensor hanging near the door? Quite important when making statements about the quality of cooling control.
When measuring temperature changes, the response time of the thermometer is important.

Further, realize that "THE" temperature of a food product such as an apple or a piece of meat does not exist: as are you talking about the core, surface or average temperature? Moreover, the temperature in a product varies over time, as a function of environmental temperature and/or the process it undergoes.
Tip: Measuring the temperature in a room is easier to conduct with a jar filled with glycerin or water left in the room. After some time, the temperature of the content takes the same value as the temperature of the products stored in the room.

Types of thermometers 

There are several types of thermometers. The first ever functioning thermometer was the mercury thermometer. The operation of this liquid thermometer relies on the thermal expansion of mercury when heat is supplied. Because of the toxic mercury and the fragility of the glass of these thermometers, they are hardly used today. The mercury has been replaced by red-colored alcohol.
Commonly used temperature sensors in the measurement and control technology and food industry, are the Pt100 and Pt1000 sensors as part of a resistance thermometer. Another name is RTD, for Resistance Temperature Detector, although this includes other types of sensors as well.

Thermocouples are also used in the food industry. This measures a temperature difference between two points. This is in contrast to a thermometer, which measures temperature relative to a certain standard. Thermocouples are less accurate and stable, but robust and flexible in use, and they have a very short response time. Temperature range and accuracy depend on the alloys or metals used.
Optical sensors such as Infrared (IR) are also used to measure surface temperatures. A risk with this type of measurement is that the temperature of another object may be detected due to unwanted reflection. Sensors consisting of a fiber optic also use IR. The "light beam" is transported in the fiberglass from the product to the sensor. Thanks to the thin glass fiber, measurements can be made in places that are inaccessible or difficult to reach due to, for example, high ambient temperature, chemicals or other specific circumstance.

Calibration

For the accuracy of measurements, it is important to calibrate the instrument and make adjustments when necessary.
Calibration is the determination of the value of the deviation of a measuring instrument from an applicable standard.
Adjustment is the performance of actions necessary to make the measuring instrument function accurately enough to make it suitable for its intended use.
Gauging is the determination whether the measuring instrument fully complies with the applicable regulations. This is often performed the responsibility of a national service such as the Calibration Department. With legal gauging, a certification mark of gauging is also applied.  

Melting ice (0°C) and boiling water (100°C) are often used for calibration. Use distilled or demineralized water for this calibration to avoid abnormalities caused by freezing point decrease or boiling point increase. When determining the temperature of boiling water, the influence of the prevailing atmospheric pressure must also be considered; as it can easily cause 1°C deviation. The sensor must also be given sufficient time to assume the correct temperature. Accuracy is required when representing temperature as a number: more decimal places suggest higher accuracy than can actually be achieved.

Metadata

Only by properly describing measured temperatures will they have meaning, and thus value. Therefore, always mention the measured unit: for example, in the column heading containing the values. In addition, an accompanying text document with a description of the measured temperature is essential. In this so-called 'metadata'-file you indicate for example in which room the measurements are taken and where in the room the sensor is located; which sensor was used and when it was last calibrated; This information increases the quality of the measured data, helps you to understand it better and makes it possible to repeat the experiment as accurately as possible.

Numbers tell the tale; but knowing and explaining what you are measuring is very important when determining the temperature of products and/or rooms. 

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