Temperature measurement 291 Thermopile detectors consist of a collection of rods that act as thermocou ples to sense emitted thermal radiation. Pyroelectric detectors contain a crystal which exhibits temperature-dependent polarisation and requires the ncident radiation to be cut by a 'chopping device to prevent currents building up within the crystal that nullify this charge A certain amount of knowledge is needed in order to interpret the values that such instruments give(Evans et al., 1994; James and Evans, 1994). The first point to bear in mind when using infrared thermometry is that the tem- perature measured is the surface temperature. If the meat has been in surroundings that have not changed in temperature for a long period of time, then it is likely that the surface temperature will be very close to that of the meat beneath the surface. However, if the temperature of the sur roundings is changing or has changed over the previous 24 h, then it is likely that the surface temperature will not be the same as the temperature deep within the meat whe or example, transfer a carton of frozen meat from a refrigerated lorry where the temperature has been maintained at -18C to a refrigerated oading bay at 4C, and the surface of the carton will warm very rapidly Therefore, if an infrared thermometer is to be used to check the meat tem- perature the value must be taken before it is removed from the lorry if any ccuracy is to be obtained. Even temperature fluctuations of +2C caused by normal control fluctuations will mean that the surface tem- perature will differ significantly from the deep meat temperature. If the neat is still cooling down, its surface temperature will be warmer than the air temperature and, in turn, the interior of the meat will be warmer still These problems can be overcome if the operator is aware of them. For example, the temperature of the carton of frozen meat could be measured in the lorry, or if removed the carton could be opened and the temperature of an inner surface of one of the packs inside immediately read with the infrared thermometer. However, in doing this two of the principle advan- tages of using infrared have been removed: namely, being quick and totally non-destructive It is also necessary for the operator to know how much of the surface is 'seenby the infrared instrument, as it will measure the average'tempera ture over the whole of this area. The target area can vary significantly from instrument to instrument and with the distance between the instrument and the surface A further complication in the use of infrared thermometers is reflected radiation. The instruments will 'see the radiation emitted from a surface and also an amount of radiation from the surroundings that is reflected by that surface. The reflected radiation will therefore constitute an error. For warm objects at a temperature greater than their surroundings, the amount of reflected radiation will be small in relation to that from the surface and consequentially the error will be small. With frozen meat, the temperature of the meat is no warmer than, and often colder than, the temperature ofThermopile detectors consist of a collection of rods that act as thermocou￾ples to sense emitted thermal radiation. Pyroelectric detectors contain a crystal which exhibits temperature-dependent polarisation and requires the incident radiation to be ‘cut’ by a ‘chopping device’ to prevent currents building up within the crystal that nullify this charge. A certain amount of knowledge is needed in order to interpret the values that such instruments give (Evans et al., 1994; James and Evans, 1994). The first point to bear in mind when using infrared thermometry is that the tem￾perature measured is the surface temperature. If the meat has been in surroundings that have not changed in temperature for a long period of time, then it is likely that the surface temperature will be very close to that of the meat beneath the surface. However, if the temperature of the sur￾roundings is changing or has changed over the previous 24 h, then it is likely that the surface temperature will not be the same as the temperature deep within the meat. For example, transfer a carton of frozen meat from a refrigerated lorry where the temperature has been maintained at -18 °C to a refrigerated loading bay at 4 °C, and the surface of the carton will warm very rapidly. Therefore, if an infrared thermometer is to be used to check the meat tem￾perature the value must be taken before it is removed from the lorry if any degree of accuracy is to be obtained. Even temperature fluctuations of ±2 °C caused by normal control fluctuations will mean that the surface tem￾perature will differ significantly from the deep meat temperature. If the meat is still cooling down, its surface temperature will be warmer than the air temperature and, in turn, the interior of the meat will be warmer still. These problems can be overcome if the operator is aware of them. For example, the temperature of the carton of frozen meat could be measured in the lorry, or if removed the carton could be opened and the temperature of an inner surface of one of the packs inside immediately read with the infrared thermometer. However, in doing this two of the principle advan￾tages of using infrared have been removed: namely, being quick and totally non-destructive. It is also necessary for the operator to know how much of the surface is ‘seen’ by the infrared instrument, as it will measure the ‘average’ tempera￾ture over the whole of this area. The target area can vary significantly from instrument to instrument and with the distance between the instrument and the surface. A further complication in the use of infrared thermometers is reflected radiation. The instruments will ‘see’ the radiation emitted from a surface and also an amount of radiation from the surroundings that is reflected by that surface. The reflected radiation will therefore constitute an error. For warm objects at a temperature greater than their surroundings, the amount of reflected radiation will be small in relation to that from the surface and consequentially the error will be small. With frozen meat, the temperature of the meat is no warmer than, and often colder than, the temperature of Temperature measurement 291