《肉制品冷冻技术》（英文版） Part 14 Temperature measurement

Temperature measurement It is often stated by those in the meat and refrigeration industries that 'anyone can measure a temperature. Many millions of measurements are made of both meat and environmental temperatures in the meat industry. However, in many cases the measurements made are an unreliable guide to the effectiveness of the refrigeration process. Even when the correct tem- peratures have been obtained the data are often poorly analysed and rarely acted upon If a group of people are asked to measure the temperature of a beef carcass. the number of values obtained is often the same as the number of people in the group. Few initially ask the obvious question, what is meant by the temperature of the carcass? Is it the average temperature, the ighest temperature, the lowest surface temperature, the average surface temperature.? The increase in temperature legislation and the desire of meat produc ers and retailers to maintain the organoleptic and microbiological quality of meat throughout the chilled and frozen distribution chain has cre ated an increased demand for equipment and expertise on temperature measurement The industry needs to measure temperatures accurately, reliably, mean ingfully, simply and cheaply. It needs to be able to analyse the data and respond when required. It needs the correct instrumentation and the exper- tise to collect and interpret the temperature data

14 Temperature measurement It is often stated by those in the meat and refrigeration industries that ‘anyone can measure a temperature’. Many millions of measurements are made of both meat and environmental temperatures in the meat industry. However, in many cases the measurements made are an unreliable guide to the effectiveness of the refrigeration process. Even when the correct tem￾peratures have been obtained the data are often poorly analysed and rarely acted upon. If a group of people are asked to measure the temperature of a beef carcass, the number of values obtained is often the same as the number of people in the group. Few initially ask the obvious question, ‘what is meant by the temperature of the carcass?’ Is it the average temperature, the highest temperature, the lowest surface temperature, the average surface temperature...? The increase in temperature legislation and the desire of meat produc￾ers and retailers to maintain the organoleptic and microbiological quality of meat throughout the chilled and frozen distribution chain has cre￾ated an increased demand for equipment and expertise on temperature measurement. The industry needs to measure temperatures accurately, reliably, mean￾ingfully, simply and cheaply. It needs to be able to analyse the data and respond when required. It needs the correct instrumentation and the exper￾tise to collect and interpret the temperature data

84 Meat refrigeration 14.1 Instrumentation The first consideration is the range of temperatures to be measured. For the meat industry, a range from -40 to +150C would cope with the tempera tures found in freezers, chillers, storage rooms, retail display cabinets and in water used for cleaning or scalding tanks in the abattoir. If they produce cooked meat products then the upper temperature may rise to 250-300C. As well as the measuring range, the range of ambient temperatures over hich the instrument will work needs to be considered The electronics of many temperature measurement instruments are designed to work to the pecified accuracy only within certain ambient temperature ranges, usually 0-40C. If temperatures in a cold store are to be measured the instrument itself may need to be kept warm until it is used. 14.1.1 Hand-held digital thermometers Purely from a cost consideration many small producers and retailers rely on spot temperature checks obtained using hand-held thermometers to produce the temperature records they require. The main tasks they carry ut with such equipment is the measurement of air temperature, between pack or product temperature, and the temperature of the meat itself. They require thermometers that are accurate, easy to use, react quickly and ar robust. Ease of use is a personal judgement and best answered by trying out a range of instruments. Most modern electronic thermometers are reli- able if handled with reasonable care. However, in general, the more robust the sensor the slower the response. There are three types of digital thermometer generally available: ther- mocouple, platinum resistance or semi-conductor(thermistor). The name refers to the type of temperature sensor used. Type T(copper-constantan) thermocouple thermometers with a wide range of interchangeable sensors are the most widely used because of their wide temperature range and rea- sonable accuracy. The accuracy of the temperature measurement of a digita thermometer will depend on how accurate the instrument and the sensor It can be seen from Table 14.1 that only thermometers based on ther mistor or platinum resistance sensors can be guaranteed to provide better than +0.5C accuracy. However, it is possible to calibrate any thermometer at known temperatures and use the calibration curve obtained to correct errors in measured values. In many cases the supplier of the instru- ment can provide a calibration curve for a particular instrument/sensor combination Sensors do not immediately in which they are positioned. Their ure the temperature of the meat or air onse rate depends on the sensor itself and the environment in which it is used. a thin sensor in a wet/solid food will respond rapidly, and a thick sensor in still air very slowly. When

14.1 Instrumentation The first consideration is the range of temperatures to be measured. For the meat industry, a range from -40 to +150 °C would cope with the tempera￾tures found in freezers, chillers, storage rooms, retail display cabinets and in water used for cleaning or scalding tanks in the abattoir. If they produce cooked meat products then the upper temperature may rise to 250–300 °C. As well as the measuring range, the range of ambient temperatures over which the instrument will work needs to be considered. The electronics of many temperature measurement instruments are designed to work to the specified accuracy only within certain ambient temperature ranges, usually 0–40 °C. If temperatures in a cold store are to be measured the instrument itself may need to be kept warm until it is used. 14.1.1 Hand-held digital thermometers Purely from a cost consideration many small producers and retailers rely on spot temperature checks obtained using hand-held thermometers to produce the temperature records they require. The main tasks they carry out with such equipment is the measurement of air temperature, between pack or product temperature, and the temperature of the meat itself. They require thermometers that are accurate, easy to use, react quickly and are robust. Ease of use is a personal judgement and best answered by trying out a range of instruments. Most modern electronic thermometers are reli￾able if handled with reasonable care. However, in general, the more robust the sensor the slower the response. There are three types of digital thermometer generally available: ther￾mocouple, platinum resistance or semi-conductor (thermistor). The name refers to the type of temperature sensor used. Type T (copper–constantan) thermocouple thermometers with a wide range of interchangeable sensors are the most widely used because of their wide temperature range and rea￾sonable accuracy.The accuracy of the temperature measurement of a digital thermometer will depend on how accurate the instrument and the sensor are. It can be seen from Table 14.1 that only thermometers based on ther￾mistor or platinum resistance sensors can be guaranteed to provide better than ±0.5 °C accuracy. However, it is possible to calibrate any thermometer at known temperatures and use the calibration curve obtained to correct errors in measured values. In many cases the supplier of the instru￾ment can provide a calibration curve for a particular instrument/sensor combination. Sensors do not immediately measure the temperature of the meat or air in which they are positioned. Their response rate depends on the sensor itself and the environment in which it is used. A thin sensor in a wet/solid food will respond rapidly, and a thick sensor in still air very slowly. When 284 Meat refrigeration

Temperature measurement 285 Table 14.1 Accuracy of digital instrument, temperature sensor and overal temperature accuracy of combined thermometer Instrument(°C) Sensor(C)Both(°C) Type k thermocouple Tvpe T thermocouple latinum ±0.4 hermit 0.3 Table 14.2 Response times(s)of sensors in air ensor Air condition Still M Bare thermocouple Bare thermistor Shrouded thermocouple Shrouded thermistor 260 Shrouded platinu 365 sensors are shrouded to improve their robustness their response times increase substantially (Table 14.2) Most manufacturers supply a range of cased sensors(probes)normally made of stainless steel suitable for most applications. These include blunt ended probes for general purpose use, needle-point or hypodermic probes for inserting into solid or semi-solid food, probes with spring-loaded ends for measuring surface temperatures or very robust probes that can be screwed or hammered into frozen meat Probe length is not important for measuring air and liquid temperatures. However, to check the deep leg temperature of a side of beef the probe needs to be at least 15 cm long to measure the temperature at the deepest 14.1.2 Temperature recorders There may often be a requirement to measure the values of temperatures at many different positions at the same time. The simplest solution to this problem is often to attach a multipole switch to a digital thermometer A number of temperature sensors can then be connected to the switch and their temperatures monitored in succession. This procedure is com- monly used in central plant rooms where an operator can routinely look at and record the temperatures at many different locations. A hand-held digital thermometer will provide information on one temperature at one

sensors are shrouded to improve their robustness their response times increase substantially (Table 14.2). Most manufacturers supply a range of cased sensors (probes) normally made of stainless steel suitable for most applications. These include blunt￾ended probes for general purpose use, needle-point or hypodermic probes for inserting into solid or semi-solid food, probes with spring-loaded ends for measuring surface temperatures or very robust probes that can be screwed or hammered into frozen meat. Probe length is not important for measuring air and liquid temperatures. However, to check the deep leg temperature of a side of beef the probe needs to be at least 15 cm long to measure the temperature at the deepest point. 14.1.2 Temperature recorders There may often be a requirement to measure the values of temperatures at many different positions at the same time. The simplest solution to this problem is often to attach a multipole switch to a digital thermometer. A number of temperature sensors can then be connected to the switch and their temperatures monitored in succession. This procedure is com￾monly used in central plant rooms where an operator can routinely look at and record the temperatures at many different locations. A hand-held digital thermometer will provide information on one temperature at one Temperature measurement 285 Table 14.1 Accuracy of digital instrument, temperature sensor and overall temperature accuracy of combined thermometer Instrument (°C) Sensor (°C) Both (°C) Type K thermocouple ±0.3 ±1.5 ±1.8 Type T thermocouple ±0.3 ±0.5 ±0.8 Platinum ±0.2 ±0.2 ±0.4 Thermistor ±0.2 ±0.1 ±0.3 Table 14.2 Response times (s) of sensors in air Sensor Air condition Still Moving Bare thermocouple 20 5 Bare thermistor 45 20 Shrouded thermocouple 150 40 Shrouded thermistor 260 50 Shrouded platinum 365 65

86 Meat refrigeration time. However, in many cases there is a need to measure temperatures over a long time period. In these situations a temperature recorder is required Historically, the temperature history of a point has been obtained usin temperature sensor connected to a moving chart. In its simplest form this is a stylus on the end of a bimetallic strip that bends in response to tem perature changes and scratches a continuous trace on a carbon chart moved by a clockwork motor. More sophisticated devices use electrical tempera ture sensors attached to a small chart recorder. The recorders can be driven from batteries or direct from the mains and the chart can be circular or rec angular and mounted on a drum or on continuous rolls. Typically instru ments will provide a continuous trace for up to a week, but some specially developed for long distance shipboard transportation of meat can operate for 6-8 weeks Increasingly, solid state electronic devices are being used to obtain the temperature history of a point. In most solid state devices the output from an integral electronic sensor is measured at set time intervals, converted to a temperature measurement and stored in a computer memory chip. In a small number of devices the interval between recordings can be adjusted and recordings started using buttons or switches on the instrument and the temperatures examined on an in-built display. a development is the use of small printers that can either be used to print out the temperatures as they are measured or attached after data collection is finished and the whole temperature history printed out. However, with the majority of instruments a small computer is required to set up start times, logging intervals and so on and recover the temperature recordin With many systems it is difficult to look at the temperature while it is being recorded or even check that the required information has been obtained before leaving the recording site. Some of the newest instruments are totally encapsulated in waterproof plastic and can be placed in direct contact with solid or even within liquid foods. Solid state devices can be very small, effectively tamper proof, and the value of the temperature at set times easily obtained, but the requirement for an associated processing facility substantially increases their cost. In many cases moving chart instruments may still provide the most economic and convenient solution to a monitoring problem. If precise temperature values at certain times are not required, then a quick examination of the chart may be sufficient to show that the temperature of the display cabinet, store room or transport vehicle has kept within the prescribed limits. However, obtaining tempera ure values from a small chart can be time consuming and inaccurate. In some situations the actual or relative position of the sensing points is important, whilst in others the position of maximum or minimum temper ature is required. There are few, if any, commercial sources of multi-point temperature probes and most have to be specially constructed. The sensors are attached to basic probes constructed from composite fibre or wood of

time. However, in many cases there is a need to measure temperatures over a long time period. In these situations a temperature recorder is required. Historically, the temperature history of a point has been obtained using a temperature sensor connected to a moving chart. In its simplest form this is a stylus on the end of a bimetallic strip that bends in response to tem￾perature changes and scratches a continuous trace on a carbon chart moved by a clockwork motor. More sophisticated devices use electrical tempera￾ture sensors attached to a small chart recorder. The recorders can be driven from batteries or direct from the mains and the chart can be circular or rec￾tangular and mounted on a drum or on continuous rolls. Typically instru￾ments will provide a continuous trace for up to a week, but some specially developed for long distance shipboard transportation of meat can operate for 6–8 weeks. Increasingly, solid state electronic devices are being used to obtain the temperature history of a point. In most solid state devices the output from an integral electronic sensor is measured at set time intervals, converted to a temperature measurement and stored in a computer memory chip. In a small number of devices the interval between recordings can be adjusted and recordings started using buttons or switches on the instrument and the temperatures examined on an in-built display. A development is the use of small printers that can either be used to print out the temperatures as they are measured or attached after data collection is finished and the whole temperature history printed out. However, with the majority of instruments a small computer is required to set up start times, logging intervals and so on and recover the temperature recordings. With many systems it is difficult to look at the temperature while it is being recorded or even check that the required information has been obtained before leaving the recording site. Some of the newest instruments are totally encapsulated in waterproof plastic and can be placed in direct contact with solid or even within liquid foods. Solid state devices can be very small, effectively tamper proof, and the value of the temperature at set times easily obtained, but the requirement for an associated processing facility substantially increases their cost. In many cases moving chart instruments may still provide the most economic and convenient solution to a monitoring problem. If precise temperature values at certain times are not required, then a quick examination of the chart may be sufficient to show that the temperature of the display cabinet, store room or transport vehicle has kept within the prescribed limits. However, obtaining tempera￾ture values from a small chart can be time consuming and inaccurate. In some situations the actual or relative position of the sensing points is important, whilst in others the position of maximum or minimum temper￾ature is required. There are few, if any, commercial sources of multi-point temperature probes and most have to be specially constructed. The sensors are attached to basic probes constructed from composite fibre or wood of 286 Meat refrigeration

Temperature measurement 287 the smallest cross-sectional area that can be used whilst maintaining the required robustness, to minimise heat conduction and achieve a rapid tem- perature response For hygienic reasons, probes are thinly coated with inert epoxy resin. Currently there is no real alternative to the use of an array of individual temperature sensors if data on temperature distribution are required For over 50 years temperature sensors attached to multi-point chart recorders have been used to obtain the temperature history of up to 24 positions and these systems are still common in many processing plants. To differentiate between the sensors on the charts, a range of methods including different colours, line types or numbers have been used As the number of sensors creases it becomes more and more difficult to identify individual sensors and/or ten perature values. If the sole purpose of the recordings is to show that all the temperatures remain within upper and lower limits then chart recording systems are more than adequate. In situations where a more detailed analysis of the data is required then they are being increasingly replaced by microprocessor-controlled data logging systems. Data loggers range from multisensor( typically 2-16 temperatures)ver- sions of the solid state instruments already mentioned, to sophisticated processing systems with thousands of measurement points Two types of portable logging systems are available. The larger type (approximately the size of a large paperback novel) has built in displays and buttons or switches to set start times time intervals between measure ments and to scan through, using the display, the temperatures, that have been measured Instruments can be purchased with between 2 and 16 plug in temperature sensors, and some will display the maximum, minimum and mean temperature recorded by a particular sensor. For more detailed analy- sis of the temperatures, the recorded data are transferred to a personal com puter. The PC is required to program the start time, recording interval and so on, and analyse the data with the smaller loggers. These instruments usually have a maximum capacity of 8 temperature sensors, l of which is often built into the instrument Developments in computer storage chips are continually extending the number of temperature values that can be held in both types of logger Modern instruments would typically be able to take readings of 4 tem- perature sensors at 5min intervals over a 2-3-week period. Further developments in electronics are extending the temperature range over which the instruments will operate. Some instruments will record accu- ately inside blast and spiral freezing systems whilst others can operate ambient temperatures up to 70-80C For extended use at sub-zero tem peratures special batteries are required. Logging systems have been devel- oped which use insulated heat resistant cases to allow operation for several hours at temperatures up to 300C. This allows measurement of product and processing temperatures in batch and continuous baking/cooking operations

the smallest cross-sectional area that can be used, whilst maintaining the required robustness, to minimise heat conduction and achieve a rapid tem￾perature response. For hygienic reasons, probes are thinly coated with inert epoxy resin. Currently there is no real alternative to the use of an array of individual temperature sensors if data on temperature distribution are required. For over 50 years temperature sensors attached to multi-point chart recorders have been used to obtain the temperature history of up to 24 positions and these systems are still common in many processing plants. To differentiate between the sensors on the charts, a range of methods including different colours, line types or numbers have been used. As the number of sensors increases it becomes more and more difficult to identify individual sensors and/or temperature values. If the sole purpose of the recordings is to show that all the temperatures remain within upper and lower limits then chart recording systems are more than adequate. In situations where a more detailed analysis of the data is required then they are being increasingly replaced by microprocessor-controlled data logging systems. Data loggers range from multisensor (typically 2–16 temperatures) ver￾sions of the solid state instruments already mentioned, to sophisticated processing systems with thousands of measurement points. Two types of portable logging systems are available. The larger type (approximately the size of a large paperback novel) has built in displays and buttons or switches to set start times, time intervals between measure￾ments and to scan through, using the display, the temperatures, that have been measured. Instruments can be purchased with between 2 and 16 plug￾in temperature sensors, and some will display the maximum, minimum and mean temperature recorded by a particular sensor. For more detailed analy￾sis of the temperatures, the recorded data are transferred to a personal com￾puter. The PC is required to program the start time, recording interval and so on, and analyse the data with the smaller loggers. These instruments usually have a maximum capacity of 8 temperature sensors, 1 of which is often built into the instrument. Developments in computer storage chips are continually extending the number of temperature values that can be held in both types of logger. Modern instruments would typically be able to take readings of 4 tem￾perature sensors at 5 min intervals over a 2–3-week period. Further developments in electronics are extending the temperature range over which the instruments will operate. Some instruments will record accu￾rately inside blast and spiral freezing systems whilst others can operate in ambient temperatures up to 70–80 °C. For extended use at sub-zero tem￾peratures special batteries are required. Logging systems have been devel￾oped which use insulated heat resistant cases to allow operation for several hours at temperatures up to 300 °C. This allows measurement of product and processing temperatures in batch and continuous baking/cooking operations. Temperature measurement 287