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Chilled and frozen foods

Temperature Monitoring Tools

The European standard for thermography thermometers

Considering the diversity of national requirements and test methods for thermographs (temperature recorders) and thermometers used in the transport of quick-frozen products in EU member states under the 92 / 1 / EU Directive [7], and the growing importance of temperature control in chilled and frozen food products, the European The Center for Standards (European Center for Standards, CEN) has agreed on a standard for thermographs. The BSEN12830: 1999 [21] standard and the EM 13485 [22] draft standard concern thermographs used for transporting, storing and distributing chilled, frozen, quick-frozen food and ice cream, as well as thermometers used in the same field. There is a draft third standard prENl3486 [23], which establishes a procedure for the periodic calibration of thermographs and thermometers in previous standards.

Temperature recorders

The CEN [21] standard establishes electrical safety requirements, resistance to mechanical vibrations, and performance in specified climatic conditions. The standard also specifies the minimum requirements for accuracy, inertia, recording intervals and the maximum relative error recording time. In tab. 5.2 shows the climatic conditions in which thermographs and thermometers should work to measure air temperature (even during storage

5.2 Table. Climatic conditions, which have to work thermographs and thermometers for measuring the air temperature

The Registrar or a thermometer
for warehouses and distribution centers located outside the refrigerator in a heated or air-conditioned room; with external sensor for transport, positioned inside or outside the vehicle with an external sensor for warehouses and distribution centers located in the refrigerator; with internal or external sensor for transport, located in a refrigerator; with internal or external sensor
1 2 3 4

Ratings

terms

manual *

thermometer, recorder and display device

+ 5 ° С… + 40 ° С -30 ° С… + 65 ° С -30 ° С… + 30 ° с -30 ° С… + 30 ° С

Range

measurement **

thermometer, recorder and display device

0 ° C ... + 50 ° C -30 ° С… + 70 ° С -40 ° С… + 50 ° С -40 ° С… + 70 ° С

Storage or transportation conditions ***

thermometer, recorder and display device

-20 ° С… + 60 ° С -40 ° С… + 85 ° С -40 ° С… + 60 ° С -40 ° С… + 85 ° С

* The conditions under which the device operates in accordance with the specifications.

** The conditions that the device can withstand during operation so that later it worked in nominal operating conditions, in accordance with the specifications.

*** The conditions that the device can withstand in the idle state so that it could later work in nominal operating conditions, in accordance with the specifications.

or work in these conditions for a short time). Clearly, these conditions are different in the device in the refrigerating chamber and the outside, where they are exposed to changeable weather conditions outside and inside of buildings or vehicles. CEN standard [21] also sets the test conditions, which is determined by compliance with specified requirements thermographs.

Thermometers

The draft prEN13485 [22] standard defines the requirements for thermometers for measuring air temperature during transportation, storage and distribution, as well as measuring the temperature of chilled or frozen food. In tab. 5.2 shows environmental conditions in which thermometers for measuring air temperature should work in various cases of their use, and in Table. Xnumx 

5.3 Table. The reaction time for Thermography Sensor *

device type Transportation Storage All applications
External sensor 10 minutes max. 20 minutes max.
Internal sensor 60 minutes max.
anchored thermometers 10 minutes max. 20 minutes max.
Portable thermometers 3 minutes max.
Thermometer to measure the temperature of food 3 minutes max.

[1] Reaction time is the time required to reach the measured or recorded

90% true value changes applied temperature under test conditions corresponding inertia values ​​of these thermometers. In tab. 5.4 shows the environmental conditions in which portable thermometers should operate for measuring air and food temperature. For thermometers used for food products, there is also a limit for measuring the accuracy of -0,3 ° С when operating over the whole range of ambient temperatures (from -20 ° С to + 30 ° С). Accuracy classes for thermometers that measure the temperature of air and food are given in Table. 5.5.

The draft standard [22] also establishes test procedures for determining the error in measuring temperature and response time.

5.4 Table. Climatic conditions, which have to operate, portable thermometers and thermometers for measuring the temperature of the products

Thermometers for measuring the temperature of the products

Limit operating conditions

rated conditions

Storage conditions

-30 ° С + 50 °С

-20 ° С + 30 ° С *

 -30 ° С… + 70 ° С

* For measurements made in this range of ambient temperatures, the measurement error should not be more than 0,3 ° C.

5.5 Table. Accuracy classes for thermometers that measure temperature or food

Temperature
air product
Class 1 2 0,5 1

Maximum

permissible

error

emolliate

ability

± 1 ° C <0,5 ° C ± 2 ° C <1 ° C ± 0,5 ° C <0,1 ° C ± 1 ° C <0,5 ° C


Sensors Accuracy (error)

Regardless of the system for collecting or recording temperatures, they share a sensor or temperature sensitive part. The three main types of sensors commonly used are thermocouples, platinum resistance, and a semiconductor device (thermistor). The choice of the type of sensor depends on the requirements for accuracy and inertia, temperature range, strength and price.

Until recently, most universal thermometers and measuring systems used a thermocouple in the thermosensitive part of the system. A thermocouple is a pair of different metals connected to one side of the junction. The circuit is closed by another connection, which is maintained at a known temperature (sometimes this connection is called free or cold junction thermocouple). For measurements related to food products, whose temperatures are relatively close to the ambient, two types of thermocouples prevail: type K thermocouples using chromel wire (nickel-chrome alloy) and alumel (nickel-aluminum alloy), and type T thermocouples using wire copper and constantan (copper-nickel alloy). The advantages of thermocouples are their low cost, the ability to manufacture manually from wire and a very wide range of temperature measurements (from-184 ° С to 1600 ° С).

In tab. 5.6 lists possible tolerances for three types of sensors that, for thermocouples and platinum resistance sensors, meet standard requirements.

Uncertainties, ° С By Type type T Platinum resistive thermistor
sensor ± 1,5 * ± 0,5 ** + 0 2 *** ± 0,1
Appliance (instrumental error) * ± 0,3 ± 0,3 ± 0,2 ± 0,2
Системы ± 1,8 ± 0,8 ± 0,4 ± 0,3

* Standard UK BS 4937: Class A24.** BS 1904: Class A25.*** Includes precision cold junction compensation [26].

The difference in instrumental error arises from the fact that the electronic circuit must compensate for changes in the temperature of the reference or cold junction (usually the ambient temperature). This temperature is measured by an integrated semiconductor sensor, and changes in ambient temperature are automatically compensated.

The error in using thermocouples increases if the ambient temperature changes significantly, for example, when moving from a cold to a warm environment. Errors in measurement using thermocouples are also possible due to the induced voltages from the motors, moisture, and temperature gradients in other junctions. To improve the accuracy of measurement and control, it is necessary to limit the use of sensors based on type T thermocouples, which usually allows to satisfy the basic requirements for monitoring air temperature [21].

The resistance of the thermistor sensors varies with temperature, but can usually be used to measure only in a narrower temperature range compared to thermocouples (from - 40 ° C to 140 ° C). The use of such sensors for measuring the temperature of food products has expanded with the introduction of requirements for measuring systems for determining the temperature of food products to give an error of ± 1 ° C, which is supported by the draft CEN standard for thermometers [22]. These sensors are durable, provide good accuracy and reproducibility of results, and are also slightly affected by changes in ambient temperatures.

Platinum resistance thermometers also provide system accuracy that meets the requirements of the draft standard CEN [22]. They can be used over a wide range of temperatures (from -270 ° C to 850 ° C). Usually their inertia (tabl. 5.7) is greater if their designs are not protected by special precautions. Correction of wire resistance and self-heating effect should be performed. The higher cost has limited their use to cases where high accuracy is required in stationary process control systems.

5.7 Table. Typical lag (c) in the air and water [26]

still air Forced Air water *
Open thermocouple 20 5
Coated thermocouple 150 40 6
Outdoor thermistor 45 20
Coated thermistor 260 50 12
Coated platinum 365 65 15

* Sensor type probe mounted in a casing in water; time to change to 20 ° C to the level of 99%.

Calibration and periodic verification

During manufacture, each sensor and instrument is checked to ensure that it meets the requirements and ensures accuracy within the tolerance specified by the manufacturer and in accordance with 55 EY 12830: 1999 [21] and 13485: 1999 [22]. In many cases, different sensors are connected to the system for measurements, which are usually considered to be interchangeable, but if more accurate measurements are necessary, an individual calibration of the sensor is performed with the device (in the system).

At the same time determine the readings of the system in the range of attached temperatures. It should be possible to check these temperatures according to a state standard (for example, the State Physical Laboratory). The resulting table or graph in the verification certificate allows you to adjust the measurement results using the system to the true values ​​(within the calibration tolerances).

To ensure the correct operation of the equipment and its compliance with the same conditions as for its purchase, as indicated in [23], after installing the temperature monitoring system, it is necessary to perform periodic calibrations. The frequency of calibration depends on the use of equipment. The manufacturer (or a competent laboratory) should conduct ongoing checks on the functioning of the equipment. Checks performed by the manufacturer are recommended at least once a year, as well as after a long period of inactivity or failure. The equipment is usually tested with a different thermometer that has been calibrated to a standard. Usually they also check the accuracy and operability of hours or the duration of the recording.

Skins sensors and probes

For use in temperature monitoring, the sensing element (sensor) must be protected from damage or breakage. Various methods are used for this - from epoxy coating to placement in a stainless steel casing. If fast response is needed, the heat flux should be as low as possible. It is important that sensors for measuring air temperature, installed in chambers or vehicles, are protected from damage during loading and unloading of products, but so as not to impede the movement of air.

Monitoring and measuring food temperature often requires sensors placed in handheld probes. The design of the probe depends on its application. The most common probe serves to introduce into food products and is therefore pointed (Fig. 5.10, a). If non-destructive temperature measurements are required, then a probe is needed, which can be inserted between packages or boxes of products. To minimize the errors of such measurements, good contact between the package and the probe is important, as well as an acceptable time to reach steady-state readings. Examples of probes for measurements between packages and boxes are shown in fig. Xnumx b and c.

indicating and recording systems

Systems with a single indication

The instrumentation of the first mercury thermometer and alcohol in a glass tube, showing one temperature value, has come a long way. CreatureHand-held temperature probes: a) different probes to measure

Fig. 5.10. Hand-held temperature probes: a) different probes to measure

temperature and products; b) probe for temperature measurement between packages; c) probe for temperature measurement between the boxes

pointer and rod thermometers with analog or digital display eliminate the risk of breakage, but their use may be limited by a large error (this applies especially to bimetallic plate based thermometers). The arrow thermometers used to indicate the temperature in the display cases are replaced mainly by digital thermometers.

Thermochromic liquid crystals change their orientation and transparency depending on their composition and temperature. Placed in strips, they show the corresponding temperatures printed under them. Their accuracy is limited, but can reach ± ​​1 ° C.

Digital display electronic devices powered by batteries are more common. The resolution and intervals of the displayed temperature vary depending on the model and type of sensor. The temperature can be memorized and even printed, and when the temperature exceeds the specified limit, an alarm may be generated.

chart recorder

Historically, recording on a moving tape was the only existing method of recording the history of temperature changes. The use of chart recorders is now less common, and they are giving way to electronic devices, but some are still present in systems such as cold rooms and vehicles. Charts can be circular or mounted on a coil (to get a rectangular graph), and records are left in ink (due to pressure) or heat-sensitive paper. The advantage of recorders with circular diagrams is that the temperature history and dramatic changes are visible, as well as the fact that the diagram can be easily saved for future use. The time scale of a chart is usually more than a day, a week, or a month, but some marine recorders may run 6-8 weeks. The clock generator and the electronic circuit can operate on battery power, which gives mobility, or from the power grid (for some stationary applications). The error in determining the recording duration in accordance with BS ЕN12830: 1999 [21] should be 0,2% of the recording time, if it is less than 31 of the day, and 0,1% of the recording time, if it exceeds the 31 day. The system error depends on the sensor, but more modern chart recorders have an error in the range of 0-25 ° С below 0,5 ° С. Often the limitation for them is the degree of resolution on chart paper and the thickness of the record. Some chart recording systems are complex instruments that allow you to record 30 and more channels in different colors and types of printing.

Chart recorders installed on vehicles (more often on trailers) must have a stronger structure and withstand the difficulties of the road in any terrain and in all weather conditions. There are recorders, giving two or more records that can be added to the event indicators (for example, marking the opening of the door).

Stationary system for cooling chambers

The complexity of interpreting a large number of different records and the rapid development of microelectronics and computer technology contributed to the replacement of chart recorders with data recording systems that allow not only storing large amounts of data, but also processing and analyzing them, which makes it possible to use them in control systems.

In a cold store, where a lot of temperature measurements are taken every day throughout the year, computerized data processing systems are increasingly being installed. A digital temperature indicator may be located on the control unit next to the refrigeration system, but more often the information can be displayed on the display located in the control room. When any monitored parameters go beyond the specified limits, a signal can be given to the alarm system. An emergency signal can be transmitted via a communication system to service personnel located on or off the site.

temperature recording systems in transport

Some companies have developed specialized systems for temperature control (monitoring) in vehicles. These systems are designed to withstand the vibration and harsh conditions that arise in transport and are specified in BS EN12830: 1999 [21]. Data is collected during the entire journey from loading to unloading, and alarms are given if temperatures are outside specified limits. This equipment can be installed both in the vehicle cabin (often it is the size of a car radio system) and outside (sometimes next to the cooling control unit).

In addition, customers to whom goods are delivered, increasingly require the recording of the “temperature history” of the food they receive. Systems have been developed for inclusion in the delivery documentation that provide an immediate listing of temperatures to the point of discharge. Other possible characteristics of such systems are the ability to register for different periods of time, memory with the ability to store data for up to one year, event recorders for recording defrosting and opening doors, as well as channels for monitoring the status of different compartments. The selection of information is being improved, and it has become possible to load data received via radio, infrared or satellite communications into the memory of office personal computers.

Portable data recording system

The miniaturization of electronic circuits has led to the creation of very compact and capacious data recording systems, some of which are small enough to “travel” with boxes of food or pallets and constantly record the temperature of food. Such devices can also be used in systems permanently installed in storages and vehicles. This is convenient if the position of the fixed sensors should change from time to time (for example, in temporary storage of refrigerated products or when moving partitions in vehicles with several compartments). The choice of system depends on the type of specific application, ease of use and price. [21] described two such devices that were used in large catering systems and showed their usefulness for registration at critical control points.

Another type of data logger is useful for monitoring storefronts. The registrar is placed on the shelf and registers the temperature of the product model, which is in the registrar and has the same thermophysical properties as the food product displayed on the shelf. Such a recorder is equipped with an emergency indicator light, which makes it easy to identify and eliminate problems that arise. The infrared data of the recorder is transmitted for display and analysis.

The characteristics of manufactured systems with the development of microelectronics are changing, and it is obvious that the miniaturization of recorders will continue. Modern systems are generally still too large to be placed in boxes with products, without removing one package from them; Devices that become significantly smaller and thinner can be placed between product packages.

Remote Sensors - Non-Contact Thermometers

All objects at temperatures above absolute zero emit energy in the form of infrared radiation. With increasing temperature, the radiation intensity increases, but its wavelength decreases. In the temperature range of the cooled products, infrared radiation to determine the temperature can be measured. With increasing temperature, its intensity increases, and the peak of energy shifts towards shorter wavelengths. Therefore, most commercially available low-temperature infrared thermometers filter radiation in the infrared region of the spectrum (in the 8-14 μm range) and measure its intensity. The use of this range reduces the remote sensitivity of the device due to atmospheric absorption (water vapor, carbon dioxide). Very narrow ranges (2,2; 5,2 and 7,9 μm) can be used to obtain greater accuracy at very high temperatures, but the signals are very small and require expensive amplifiers with high gain.

Not all materials emit the same energy at the same temperature. The ratio of energy emitted by a material to an ideal emitter (an absolutely black body) is known as the emissivity (emissivity). Radiation coefficients range from 0 to 1,0, while for most organic substances the emissivity is about 0,95. Substances differ in the amount of energy they absorb, reflect and emit. Infrared thermometers have emissivity compensators, which must be set to different values ​​(0,1-1,0) to account for these differences. The size of the object is also important. The device averages all temperatures in its field of view. If the object does not close the entire field of view of the device, the temperature reading will be the average temperature of the object and its environment. The focal length varies depending on the device, measurement is possible from a very close distance (up to 50 m). The greater the distance, the more difficult it is to accurately aim at the object, and in many models laser sights (sights) are used.

There are two main types of equipment for remote measurement. One type is a gun-shaped device that is aimed at an object; at the same time, readings of temperature are read from a digital indicator in the back of the device. The laser sight can be built into the gun for through aiming (through the lens) to determine the target, and instruments for measuring at a long distance are often equipped with optical sighting devices. The accuracy of this type of instrument is about ± 1 ° С.

A study of nine industrially produced infrared thermometers performed at the University of Bristol [28] showed that their readings should be treated with caution. Surface temperature can vary greatly from internal product temperature. This problem is most acute for frozen products, in which the difference between surface temperature and internal temperature can be quite large (especially when the product is transported at ambient temperatures above -18 ° C). The infrared sensor measures not only the radiation of the surface, depending on its temperature, but also the radiation due to reflection from objects surrounding the product (for example, lighting).

Depending on the type of package, the reflected radiation can be very significant and, therefore, distort the results of measuring the surface temperature.

The work of these nine devices when using them in a serial retail storefront in a retail store was very different. In tab. 5.8 shows the results of using these devices for six different types of packaging. The results of infrared radiation measurements were compared with those obtained using a calibrated thermocouple placed under the package. Two of the five instruments (b and g) gave an error of less than 1 ° С, five devices gave less than 2,5 ° С and two more gave unacceptable errors. The biggest errors were given by all the instruments on a foil package with a stamp printed on it, the reflected radiation of which was the largest. It is recommended not to carry out infrared measurements on packages with brightly lit surfaces arranged at an angle, but to choose horizontally and vertically arranged packages in the display case so that the device is located vertically to the upper surface. To increase accuracy, the illumination should be as low as possible, the distance for measurements should be as small as possible, and the product should be as smooth as possible.

If the thermometer is moved from a medium with one temperature to a medium with another (for example, from a room with room temperature to a refrigeration chamber), it is recommended to withstand the instrument at a new ambient temperature of at least 30 min for the best reproducibility of measurements. The thermometer should also be regularly checked on surfaces with a known temperature. You can make a relatively cheap camera for calibration on an absolutely black body with

5.8 table. The average error in ° C with a standard deviation (in parentheses) for different packaging materials

instrument Transparent packaging in CWG *

Glossy

cardboard

Polyethylene

Lenovo

package

Laminated foil printed Packaging CSG * with printed Vacuum packaging with printed

Average

module

а 0,6 (0,1) 1,7 (0,1) 1,1 (0,6) 6,6 (0,6) 1,9 (0,5) 1,3 (0,1) 2,2
b -0,3 (0,0) 0,7 (0,0) 0,8 (0,6) 5,3 (0,6) 0,6 (0,1) 1,4 (0,1) 1,5
с 0,7 (0,1) 0,6 (0,0) 0,5 (0,0) 6,0 (0,1) 0,4 (0,6) 0,4 (0,0) 1,4
d -3,3 (0,3) -4,5 (0,5) -5,1 (0,4) 7,0 (0,2) -9,1 (1,0) -7,2 (0,2) 6,0
е -1,9 (0,6) -2,3 (0,1) -2,5 (0,0) 4,1 (0,0) 1,8 (0,1) -0,6 (0,1) 2,2
f 0,8 (0,1) 0,9 (0,4) 1,0 (0,5) 4,2 (3,0) 2,9 (0,3) 2,3 (0,1) 2,2
g -0,1 (0,1) -0,5 (0,6) 0,4 (0,6) 6,2 (0,5) 0,2 (0,2) 0,6 (0,1) 1,3
h 0,5 (0,4) 3,8 (0,3) 6,4 (0,7) 10,4 (0,9) 6,1 (0,8) 4,0 (1,4) 5,5
i -2,2 (0,0) -1,2 (0,0) -0,8 (0,6) 3,2 (0,0) -0,9 (0,6) -1,0 (0,6) 1,5

Average

module

1,4 1,3 2,1 5,9 2,7 2,1

* RGS - packaging in a controlled gaseous environment.

using black PVC pipe and copper ingot. There are also industrially manufactured devices.

Another type of instrument is based on devices like an infrared video camera. Thermal images are displayed on a color or monochrome display, the temperature scale of which gives the temperature corresponding to a particular color or shade. Devices can vary from low-resolution devices, often used to detect casualties littered in destroyed buildings, to complex high-resolution systems that allow for computerized data processing. It was found that infrared systems are very useful for industrial monitoring and control of energy efficiency, as they can reveal overheated components and heat losses. In addition, manual or special models are increasingly used for continuous operation in the food industry. For example, it is possible to monitor sealing rollers for plastic trays subjected to microwave treatment to ensure uniform heating; It is also possible to check the uniformity of heating or cooling of food products leaving tunnel dryers or cold rooms. In these cases, the object is constant and relative temperatures are more important than accurate ones. Results are issued instantly and information can be transmitted directly to regulatory systems.

Infrared temperature measurements will never replace electrical temperature measurements for accurate determinations in accordance with the requirements of temperature regulation legislation. Nevertheless, there are excellent opportunities to use such measurements in daily monitoring and temperature control, where relative temperature changes are very important and caution is needed in interpreting the results. Handheld devices can be used to monitor the surface temperature of boxes discharged from a vehicle for accepting or abandoning a load, or for scanning a display case to detect warmer areas in it.

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