88 Meat refrigeration Most data logging systems that will measure over 20 temperatures are physically too big to be considered as truly portable even though some can be battery powered. Some have a built in display and keyboard but the majority are operated using a video display unit a basic system would consist of a number of input cards to which the temperature sensors are connec a card-based voltmeter to measure the output from sensors when instructed a microcomputer to provide the instructions and convert voltages into temperature measurements, a storage system that could be floppy or hard disc, and a video display lany systems can be expanded to hundreds and in some cases thousands of temperature sensors by the addition of extra input cards, some of which can be up to a mile away from the central system The temperature measurement possibilities of large logging systems are nly limited by the ingenuity of the programmer/operator. Different com binations of temperature sensors can be monitored at varying time inter- vals and the data displayed, analysed, used to control processes or set off alarms or be transmitted to central control rooms hundreds of miles away All three types of temperature sensor thermocouple, thermistor and platinum resistance are commonly used for multi-point temperature measurement Thermocouples are cheap, especially if the wire is purchased in bulk, and very small sensors can be manufactured Thermistors are more expensive, slightly larger but more accurate over limited temperature ranges. Platinum resistance sensors are typically 2-3 times the cost of ther mistors, but are capable of better than 0. 1C accuracy. Thin wire and thin film platinum resistance sensors can be very small Commercial sensors are often enclosed in stainless steel sheaths which makes them more robust but increases their response time. 14.1.3 Time-temperature indicators There are many different types of temperature or time-temperature indi- cators. Almost anything that undergoes a sensibly detectable change with temperature can be used Liquid crystal devices change colour to indicate he temperature at the time they are observed and time-temperature indi cators change irreversibly after a time dependent upon the temperature history or when a temperature threshold is exceeded Temperature indicators are already used as cheap, safe and hygienic ther- mometers in the food chain. Several types have been developed to the point where they have been introduced on some chilled and frozen foods in the USA and on chilled foods in france
Most data logging systems that will measure over 20 temperatures are physically too big to be considered as truly portable even though some can be battery powered. Some have a built in display and keyboard but the majority are operated using a video display unit. A basic system would consist of: • a number of input cards to which the temperature sensors are connected, • a card-based voltmeter to measure the output from sensors when instructed, • a microcomputer to provide the instructions and convert voltages into temperature measurements, • a storage system that could be floppy or hard disc, and a video display unit. Many systems can be expanded to hundreds and in some cases thousands of temperature sensors by the addition of extra input cards, some of which can be up to a mile away from the central system. The temperature measurement possibilities of large logging systems are only limited by the ingenuity of the programmer/operator. Different combinations of temperature sensors can be monitored at varying time intervals and the data displayed, analysed, used to control processes or set off alarms or be transmitted to central control rooms hundreds of miles away. All three types of temperature sensor – thermocouple, thermistor and platinum resistance – are commonly used for multi-point temperature measurement. Thermocouples are cheap, especially if the wire is purchased in bulk, and very small sensors can be manufactured. Thermistors are more expensive, slightly larger but more accurate over limited temperature ranges. Platinum resistance sensors are typically 2–3 times the cost of thermistors, but are capable of better than 0.1 °C accuracy. Thin wire and thin film platinum resistance sensors can be very small. Commercial sensors are often enclosed in stainless steel sheaths, which makes them more robust, but increases their response time. 14.1.3 Time–temperature indicators There are many different types of temperature or time–temperature indicators. Almost anything that undergoes a sensibly detectable change with temperature can be used. Liquid crystal devices change colour to indicate the temperature at the time they are observed and time–temperature indicators change irreversibly after a time dependent upon the temperature history or when a temperature threshold is exceeded. Temperature indicators are already used as cheap, safe and hygienic thermometers in the food chain. Several types have been developed to the point where they have been introduced on some chilled and frozen foods in the USA and on chilled foods in France. 288 Meat refrigeration
Temperature measurement 289 14.2 Calibration Any temperature measuring system should be tested over the operating range at regular intervals to ensure accuracy and should also have a current calibration certificate from its manufacturer or official standards laboratory The system can be checked by means of a calibration instrument, or against a reference thermometer that is known to be accurate Melting ice(which if made from distilled water should read 0C, or-006C if made from tap water with 0. 1% salt)may be used to check sensor accuracy. The ice should be broken up into small pieces and placed in a wide-necked vacuum flask with a depth of more than 50mm. The system should be agitated frequently and the temperature read after a few minutes when stable If differences of more than 0.5C are found, the instrument should either be very carefully adjusted or sent for calibration. Other simple calibration systems are available. These consist of a small stirred tank that can be filled with water or oil. The temperature of the stirred liquid is measured using a standard calibrated platinum resistance thermometer. The temperature sensors to be calibrated are placed in the liquid and compared with the standard measurement. The temperature of the liquid can be raised or lowered to different values by the addition ice, cold liquid or hot liquid 14.3 Measuring temperature data Accurately determining the temperature of chilled meat throughout the cold chain is difficult. Training and experience are required to locate posi tions of maximum and minimum temperature in abattoirs, stores, vehicles and display cabinets. The problem is further exaggerated by changes in posi tion with time caused by loading patterns and the cycling of the refrigera tion plants. Obtaining a relationship between environmental temperatures that can be measured relatively easily) and internal meat temperatures is not a simple process. Relating temperatures obtained in a non-destructive manner with internal meat temperatures again poses problems. Determin- ing the temperature of cuts of meat with regular shapes is quite simpl doing so for irregular cuts of meat is more difficult All the temperature measurement problems associated with chill foods will equally apply to quick-frozen foods. In addition, there are a number of other problems. Many instruments have sensors that will accurately measure temperatures of-20.C and below, but the instruments themselves become inaccurate or fail to operate at low temperatures. If frozen foods tre removed from their low temperature environment to one suitable for the instrument the surface temperature rises very rapidly. However, the main problem is that of actually inserting a temperature sensor into frozen meat
14.2 Calibration Any temperature measuring system should be tested over the operating range at regular intervals to ensure accuracy and should also have a current calibration certificate from its manufacturer or official standards laboratory. The system can be checked by means of a calibration instrument, or against a reference thermometer that is known to be accurate. Melting ice (which if made from distilled water should read 0°C, or -0.06 °C if made from tap water with 0.1% salt) may be used to check sensor accuracy. The ice should be broken up into small pieces and placed in a wide-necked vacuum flask with a depth of more than 50 mm. The system should be agitated frequently and the temperature read after a few minutes when stable. If differences of more than 0.5 °C are found, the instrument should either be very carefully adjusted or sent for calibration. Other simple calibration systems are available. These consist of a small stirred tank that can be filled with water or oil. The temperature of the stirred liquid is measured using a standard calibrated platinum resistance thermometer. The temperature sensors to be calibrated are placed in the liquid and compared with the standard measurement. The temperature of the liquid can be raised or lowered to different values by the addition of ice, cold liquid or hot liquid. 14.3 Measuring temperature data Accurately determining the temperature of chilled meat throughout the cold chain is difficult. Training and experience are required to locate positions of maximum and minimum temperature in abattoirs, stores, vehicles and display cabinets.The problem is further exaggerated by changes in position with time caused by loading patterns and the cycling of the refrigeration plants. Obtaining a relationship between environmental temperatures (that can be measured relatively easily) and internal meat temperatures is not a simple process. Relating temperatures obtained in a non-destructive manner with internal meat temperatures again poses problems. Determining the temperature of cuts of meat with regular shapes is quite simple but doing so for irregular cuts of meat is more difficult. All the temperature measurement problems associated with chill foods will equally apply to quick-frozen foods. In addition, there are a number of other problems. Many instruments have sensors that will accurately measure temperatures of -20 °C and below, but the instruments themselves become inaccurate or fail to operate at low temperatures. If frozen foods are removed from their low temperature environment to one suitable for the instrument the surface temperature rises very rapidly. However, the main problem is that of actually inserting a temperature sensor into frozen meat. Temperature measurement 289