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 franceMost 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 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 0.1 °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 indi￾cators. 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 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. 288 Meat refrigeration